Further Researches Concerning
Atomic Weights

of

Potassium, Silver, Chlorine, Bromine,
Nitrogen, and Sulphur

BY

THEODORE WILLIAM RICHARDS

IN COLLABORATION WITH

ARTHUR STAEHLER, GEORGE SHANNON FORBES,
EDWARD MUELLER, AND GRINNELL JONES

WASHINGTON, D. C.:

Published by the Carnegie Institution of Washington
APRIL, 1907

CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION No. 69

CONTRIBUTIONS FROM THE CHEMICAL LABORATORY
OF HARVARD COLLEGE

'

T

PSESS OF BYRON S. ADAMS

WASHINGTON, D. C.

CONTENTS

A REVISION OF THE ATOMIC WEIGHT OF POTASSIUM ; THE ANALYSIS OF POTAS-
sic CHLORIDE. BY T. W. RICHARDS AND ARTHUR STAEHLER.

Page

Introduction 7

Preparation of the Material • • • 9-14

Potassic Chloride

Argentic Nitrate

Silver 13

Nitric and Hydrochloric Acids 14

Water 14

The Drying and Weighing of the Potassic Chloride 14-16

The Precipitation and Weighing of Argentic Chloride 17-21

The Ratio of Potassic to Argentic Chloride

The Ratio of Potassic Chloride to Silver

Discussion of Final Results

Summary

A REVISION OF THE ATOMIC WEIGHT OF POTASSIUM ; THE ANALYSIS OF POTAS-
SIC BROMIDE. BY T. W. RICHARDS AND EDWARD MUELLER.

Introduction

The Preparation of Materials

The Source of the Potassium 28-30

Bromine

Potassic Bromide 31-34

Silver 34-35

Nitric acid 35

Water 35

The Laboratory

Utensils 35

The Drying and Weighing of Potassic Bromide 36-38

The Precipitation and Weighing of Argentic Bromide 38-40

The Ratio of Argentic Bromide to Potassic Bromide 40

The Determination of the Silver Needed for Precipitation 41-42

The Ratio of Potassic Bromide to Silver 42

The Atomic Weight of Potassium 43

Summary 44

THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE AND THE ATOMIC
WEIGHTS OF NITROGEN AND SILVER. BY T. W. RICHARDS AND G. S. FORBES.

Introduction 47

Preparation of Pure Materials 47-49

Nitric Acid 47

Silver 48

Water 49

Air 49

The Synthesis of Argentic Nitrate 50-54

3

4 CONTENTS.

Page

The Purity of the Fused Argentic Nitrate 55-63

'I < -ts for Dissolved Air 55-57

Tests for Retained Water 57-60

Tests for Insoluble Impurities 60

Tests for Ammonic Nitrate 61

Tests for Nitrite 62

Tests for Free Acid 62

The Final Result and Its Relations to the Values of Atomic Weights of Nitro-
gen and Silver 64-65

Summary 65

THE MOLECULAR WEIGHT OF ARGENTIC SULPHATE AND THE ATOMIC WEIGHT OF
SULPHUR. BY T. W. RICHARDS AND GRINNELL JONES.

Introduction 69

Preliminary Experiments 72-74

The Preparation of Pure Materials 74-75

Sulphuric Acid 74

Argentic Sulphate 74

Hydrochloric Acid 75

The Fusion of Argentic Sulphate 75-80

The Conversion of Argentic Sulphate into Chloride 80-85

The Final Results 86-87

The Atomic Weight of Sulphur 87-88

Summary 88

I

A REVISION OF THE ATOMIC WEIGHT OF POTASSIUM

THE ANALYSIS OF POTASSIC CHLORIDE

BY THEODORE WILLIAM RICHARDS AND ARTHUR STAEHLER

CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF HARVARD COLLEGE

A REVISION OF THE ATOMIC WEIGHT OF POTASSIUM.

THE ANALYSIS OF POTASSIC CHLORIDE.

INTRODUCTION.

The atomic weight of potassium is a chemical constant of unusual inter-
est and significance. Standing, as it does, in the middle of the series of
five atomic weights of the most electro-positive metals -- substances
which exhibit in a highly marked degree both similarity and progressive
change in properties - - this number must be of unusual importance in the
search for that undiscovered mathematical relationship which undoubtedly
exists between these constants. Moreover, the atomic weight of potassium
is essentially bound up with the atomic weights of oxygen, chlorine, and
silver, this group forming a fundamental basis in the calculation of nearly
all the other atomic weights ; so that a change in the atomic weight of
potassium has a widely ramified effect on the whole table of atomic
weights. If any further evidence of this fact were needed, the recent
paper of R. W. Gray1 would furnish it.

The recent investigation upon the atomic weight of sodium by one of
us in conjunction with Dr. R. C. Wells2 showed conclusively that Stas's
work, upon which rested earlier knowledge, was somewhat at fault in
several respects. The most serious of these errors were, first, the existence
of impurity in Stas's silver ; secondly, the fact that in his work solid salt
was dropped into the argentic solution, causing occlusion of salt ; and
thirdly, inadequate knowledge concerning solutions of argentic chloride.
In view of these errors it seemed not impossible that a similar revision of
potassium might likewise yield slightly lower results for potassium than
had been found by this master of exact analysis. For this reason the
present investigation was undertaken.

Before describing the present work a brief historical review of previous
investigations may not be out of place. The determinations may be
divided into two groups — the first group including all of these deter-
minations in which the molecular weight of a potassic halide was found
by decomposition of a chlorate, bromate, or iodate, and the second group,
those in which data for determination of the relation of potassium to

!R. W. Gray, Journ. Chem. Soc. (Lond.), Trans., 89, 1175 (1906).
2Richards and Wells, Carnegie Inst. of Washington Publication 28; Journ. Am.
Chem. Soc., 27, 459 (1905); Zeits. anorg. Chem., 47, 56 (1905).

7

8 THE ANALYSIS OF POTASSIC CHLORIDE.

chlorine, bromine, or iodine were obtained. The second group alone is
concerned with the present work.

The early work of Berzelius, Pelouze, and Marignac need scarcely be
considered here.1 It dealt for the most part with the precipitation of
argentic chloride from a solution of potassic chloride and gave very
widely discrepant results. Even the later work of Marignac on chloride,
bromide, and iodide of silver was of doubtful value, especially the last;
and accordingly until the present research was undertaken, the atomic
weight of potassium rested chiefly on the analyses of potassic chloride
made by Stas, by Richards and Archibald, and by Archibald alone, and
upon the analysis of potassic bromide made by Stas.

As in the case of sodium, Stas2 made several series of determinations
of the amount of silver needed to precipitate a known amount of potassic
chloride. One of these series was made in 1865, a later one in 1881, and
yet another shortly before his death. The earliest series, although in
some respects more careful and thorough than the later ones, was greatly
at fault, because in it he overlooked the solubility of argentic chloride.
Accordingly of this work only the later need be considered. The mean
of seventeen analyses indicated that 100 parts of silver were equivalent
to 69.122 parts of potassic chloride, corresponding to an atomic weight
for potassium of 39.130, if silver is taken as 107.93 and chlorine as 35.473.
Later, Richards and Archibald3 found as a side issue of a research upon
the atomic weight of caesium that 100 parts of silver correspond to 69.115
parts of potassic chloride, and also that 100 parts of argentic chloride
correspond to 52.022 parts of potassic chloride ; and still more recently,
Archibald4 found results not very different. With the values given above
for silver and chlorine these results indicate a value for potassium of
39.123 and 39.128, respectively, in the case of Richards and Archibald,
and 39.122 and 39.135, respectively, in Archibald's research.

In the case of the bromide Stas found in an unusually varying series
of experiments that 100 parts of silver needed 110.346 parts of potassic
bromide for complete precipitation, a result not very far from that of the
less precise early work of Marignac. If Baxter's value for bromine is
accepted (79.953),5 this corresponds to an atomic weight for potassium
of 39.143, noticeably higher than the results from the chloride. At that
time the discrepancy did not appear, because the atomic weight of chlorine

1For a brief discussion of this work Clark's Recalculation of the Atomic Weights
(1897) p. 41, may be consulted.

2Stas, Mem. Acad. Roy. Belg., 43 (1880) ; also Oeuvres Posthumes, edited by W.
Spring. See Clarke's Recalculation (1897), p. 42.

3Richards and Archibald (1903), Proc. Am. Acad., 38, 443 (1903). Zeit. anorg.
Chem., 34, 353, 1903.

Archibald, Trans. Roy. Soc. Canada [2], 10, III. 47 (1904).

5Baxter, Journ. Am. Chem. Soc., 28, 1322 (1906).

INTRODUCTION. 9

was not properly evaluated, Stas believing that his work on the chloride
also indicated an atomic weight above 39.14.

In view of the fact that these results differ among one another to an
extent beyond a reasonable limit of experimental error, it seemed advis-
able to investigate once more the atomic weight of potassium in order to
detect the cause of the discrepancy. This appeared to be especially desir-
able on account of the recent gain in knowledge concerning the peculiari-
ties of argentic chloride in solution. Accordingly, the present research
was begun, and simultaneously another concerning potassic bromide. A
description of the latter investigation follows immediately after this one.

An investigation upon atomic weights naturally resolves itself into
several different portions --first, the preparation of the pure material;
second, the method of drying and weighing this material ; third, the
details of the analysis and the calculation of the results. These will be
considered in order in the following pages.1

PREPARATION OF THE MATERIAL.
POTASSIC CHLORIDE.

Several methods of preparing potassic chloride were tried, the most
efficacious being adopted. The problem, of course, was to effect the
elimination of other metals and other acids, especially those of the same
groups. Both ends are most quickly and completely attained by incor-
porating into the process of preparation successive crystallization of
different salts, in order to eliminate by the crystallization of one salt an
isomorphous substance which might have been retained during the crys-
tallization of another salt. The commonest salts of potassium were
therefore studied in relation to their fitness for the elimination of impur-
ities in this way.

In the first place, on account of the good results which Richards and
Wells had attained in preparing pure sodic chloride from recrystallized
sodic sulphate by precipitation with gaseous hydrochloric acid, sulphate
of potassium also was early considered. This method was, however, soon
abandoned, because the relative solubilities of potassic sulphate and
chloride are far less favorable for the purpose than those of the sodium
salts. The method of Archibald, who converted sulphate into chloride
through precipitation with baric chloride, is complicated and involves the
double work of preparing pure baric chloride as well as pure potassic
sulphate. Moreover, it is not easy to eliminate the last trace of baric
sulphate from the resulting salt. This also was rejected.

1A somewhat less detailed statement of this work has appeared in the Berichte d.
deutsch, ch. Gesel., 39, 3611 (1906).

10

THE ANALYSIS OF POTASSIC CHLORIDE.

Next potassic chlorate was studied — a salt which had already been
used by Stas for the preparation of pure potassium material. Both in
crystalline form and in solubility this salt is very suitable for the separa-
tion of sodium from potassium material. Sodic chlorate crystallizes in
regular crystals which are six times as soluble in hot water and sixteen
times as soluble in cold water as the monoclinic crystals of potassic
chlorate. On the other hand, the data concerning the crystalline form
and solubility of the chlorates of rubidium and caesium are not suffi-
cient to enable one to make any certain prediction with regard to their
behavior, and therefore experiments were made in order to discover
whether or not these salts could easily be separated from potassic chlorate
by recrystallization. The four salts of sodium, potassium, rubidium, and
caesium were mixed together and qualitatively separated by fractional
crystallization. The sodium was effectively separated from the crystals ;
but after three careful crystallizations the caesium line, although stronger
in the mother liquor than at first, was still not absent from the crystals.
Indeed, neither rubidium nor caesium lines had considerably diminished
therein. Accordingly, this method was also abandoned.

Potassic nitrate showed itself to be much more satisfactory in its
behavior. Its usefulness did not seem at first so certain as to be a fore-
gone conclusion, especially as Stas had not succeeded in preparing a
particularly pure material from this salt. Probably this failure was due
to the circumstance that he carried out the crystallization in glass vessels,
which of course continually introduced sodium and silica. The rather
inadequate data concerning the four nitrates may be well put together
as follows :

Solubility in 100 parts of water.

Crystalline form.

Cold.

Warmer.

Temperature.

Parts.

Temperature.

Parts.

CsNO3

o

3
0
14

15

12

20
25

84

o

60
10
114

114

Very many
43
337

200

Tetragonal.
Needles.
Rhombohedral
and rhombic.
Rhombohedral.

RbNOg

KNO,

NaNO3

Because this series of results is not entirely conclusive, although
promising, a series of crystallizations was carried out here also. This
series showed that an admixture of the three other nitrates with a
large excess of potassic nitrate was nearly all eliminated by as few as
two crystallizations. The separation of the sodium was especially marked

PREPARATION OF THE MATERIAL. 11

in the spectroscopic tests. This method, therefore, appeared very well
suited for the preparation of pure potassium material, which might easily
be converted into chloride after the other metals had been eliminated.

Pure potassic nitrate of commerce, obtained from Germany, was dis-
solved in a little water, freed from solid substances by nitration, and
crystallized twice in Jena glass flasks. The salt was each time freed
from the mother liquor in a porcelain centrifuge.1 The resulting material
was four times subsequently recrystallized in platinum vessels, and whirled
in a platinum centrifuge. The pure crystals were now divided into two
parts; one part was dried in a vacuum desiccator, while the other part
was subjected to yet six more crystallizations and whirlings in platinum
vessels.

It now became a question how this nitrate could best be converted into
chloride. The method recommended by Stas of beating the salt with
pure ammonium chloride involves not only the preparation of this latter
salt in a pure state, but also great danger of contamination from the
vessel, whether of platinum or porcelain. Platinum is especially attacked
because of the action of the oxychlorides of nitrogen and of chlorine. The
more favorable method appeared to be to convert the nitrate into chloride
by repeated evaporation with hydrochloric acid in quartz dishes. This
method was found to have two objections. In the first place, dishes
large enough for the raw material were not to be had. Moreover, the
tendency of the branching crystals to grow over the edge of the dish
caused serious loss of material and danger of impurity. Neither could
potassic nitrate be heated to a high temperature with ammonic chloride,
or in the steam of hydrochloric acid in quartz dishes ; for there is
always danger that the reaction 4KNO3 -f 2SiO2 = 2K2SiO3 + 4NO2 -f O2
would occur.2

Because Stas had never been able to obtain a salt free from silica when
working in glass vessels, we desired to avoid completely the use of glass.
Therefore, only one way remained, namely, to convert the potassic nitrate
into chloride by precipitation with hydrochloric acid in a platinum dish.
The resulting aqua regia must naturally attack the platinum, but we pre-
ferred this impurity to that of silica, and found, moreover, that the amount
of platinum dissolved is very small. In the presence of large masses
of potassic salts, aqua regia, especially in the cold, dissolves platinum
but little. For example, in one experiment in which about half a kilo-
gram of potassic nitrate was treated with hydrochloric acid gas, only
three-tenths of a gram of platinum was converted into the form of
potassic chlorplatinate. In subsequent crystallizations the amount was

JRichards, Journ. Am. Chem. Soc., 27, 105 (1905).
2Richards and Archibald, loc. cit.

12 THE ANALYSIS OF POTASSIC CHLORIDE.

very much less, because the great mass of the nitric acid was eliminated
by the first crystallization. After the third recrystallization, the diphe-
nylamin reaction failed to reveal the presence of any retained nitrate.

The precipitation was carried out in the following manner : The often
recrystallized, very pure potassic nitrate was dissolved in the least possible
quantity of cold water, contained in a platinum dish, packed in ice.
Slowly, in order to prevent warming, hydrochloric acid gas was run into
this solution. The gas was supplied by warming a pure concentrated
solution, and was passed into the nitrate solution through an inverted
platinum funnel. The potassic chloride, which appeared somewhat yellow
from its slight platinic impurity, was whirled in the platinum centrifuge,
and dissolved in purest water. Potassic chlorplatinate is practically insol-
uble in a concentrated solution of the chloride, as the law of concen-
tration effect and the theory of ionization predicts ; hence the impurity
was easily separated upon a small, pure filter. The clear, colorless solution
was again saturated with hydrochloric acid, and the same round of oper-
ations repeated. Upon a third repetition of this process, the nitric acid
was eliminated and the salt, as a rule, was wholly colorless in consequence.
On no occasion, even with large amounts of materials, did perceptible
color remain after five treatments. No specimen was subjected to less
than seven such recrystallizations, while some was passed yet five times
more through the precipitation and centrifugal drying. Assuming that
each time about three-quarters of the ordinarily adhering mother liquor
was removed by the centrifugal separation, these crystals must have been
as free from heteromorphous substances as if they had been recrystal-
lized many thousand times in the ordinary fashion. It will be shown that
all the samples gave the same results upon analysis, hence further puri-
fication according to this method was needless. The salt was found to
be wholly free from sodium by a sensitive spectroscopic test. It was in
every case freed from acid by a single recrystallization from the purest
water, and by fusion in the manner to be described later.

Seven different samples were prepared after this fashion from varying
raw material. These are given in the list below.

(a) The pure potassic material ("C. P." of German preparation) was
recrystallized as nitrate six times, and precipitated as chloride seven times.
This salt after the final recrystallization from pure water was dried in a
platinum dish at 120° in an electric oven. The salt was snow-white, and
was as clear as water when fused, as indeed, were all the other samples.

(&) This sample was similar to a, but obtained by the evaporation of
the final aqueous mother liquor decanted from a. The mother liquor from
b was rejected.

PREPARATION OF THE MATERIAL. 13

(c) Five extra precipitations with hydrochloric gas (or twelve in all)
yielded this salt, which was prepared for analysis in the same way as a.

(d) This salt bore the same relation to c that b bore to a, being essen-
tially identical with c.

(e) For this sample and the next one the nitrate was recrystallized
twelve times, and the chloride precipitated seven times.

(/) From the final mother liquor of e, f was prepared, as b followed
after a.

(2) This sample was exactly like a, except that the raw material came
from an entirely different source.

ARGENTIC NITRATE.

The nitrate of silver used for precipitating chlorine in the analysis was
crystallized three times from pure dilute nitric acid solution and each
time centrifugally freed from mother liquor. Before each analysis the
aqueous solution of the salt was tested in the nephelometer1 for a possible
trace of argentic chloride, and no material was ever used which showed
to this exceedingly sensitive test the least trace of impurity. For preser-
vation in a pure state it was kept in a tight desiccator over potash.

SILVER.

The metallic silver used in the research was made from argentic nitrate
which had been six times recrystallized. The metal was precipitated as
fine powder by ammonic formate,2 melted on pure lime, and further
purified by electrolysis.3 The beautiful crystals thus prepared were fused
in a stream of pure electrolytic hydrogen and finally in a vacuum of
0.1 mm. The metal was supported on a boat of the purest lime, prepared
from calcic carbonate precipitated from the nitrate for this purpose.
The boat was provided with several compartments, each of which held
enough silver for one analysis, in order to avoid the possible introduc-
tion of impurity into the metal by subsequent cutting. It was inclosed
in a stout porcelain tube, provided with Hempel water-cooled stoppers,
and was heated electrically in a Heraeus tube-furnace.4 The vacuum
was maintained by a motor-driven Geryk oil air-pump. Except for these
minor improvements, the preparation was essentially similar to the best
methods employed by Richards and Wells in their often-cited work, to
which the reader is referred for further details.

JRichards, Proc. Am. Acacl., 30, 385 (1894) ; Zeit. anorg. Chem., 8, 269 (1895).
'Richards and Wells, Journ. Am. Chem. Soc., 27, 475 (1905).
3J. L. Hoskyns Abrahall, Journ. Chem. Soc. Trans. (1892), 660; also Richards.
Proc. Am. Acad., 28, 22 (1893).

4Heraeus, Zeit. f. Elektrochemie, 8, 201 (1902).

14 THE ANALYSIS OF POTASSIC CHLORIDE.

NITRIC AND HYDROCHLORIC ACIDS.

Very carefully tested preparations of commerce were used as the raw
materials in the preparation of these acids. They were redistilled with
platinum condensers until they were fully pure enough for the purposes
for which they were needed. The hydrochloric acid had been shown
in the research on sodium to be free from bromine or iodine.

WATER.

This substance, because it is used in larger quantities than any other,
must be especially pure. All the water used, either for preparation or
analysis, was twice distilled, once with alkaline permanganate and once
alone. For the best work a platinum condenser was used. All that was
needed for analysis was tested in the nephelometer immediately before use.

THE DRYING AND WEIGHING OF THE POTASSIC CHLORIDE.

The final preparation of the salt for analysis is given a special chapter
to itself, because the proper execution of this feature is of very great
importance, not less than the purification of the material. It is clearly
useless to separate 0.001 per cent of a metallic impurity if 0.01 per cent
of water is allowed to remain in the salt.

Obviously, superficial drying can not remove the imprisoned moisture
in the crystals, hence they must be fused.1 Except in a few preliminary
experiments, where a common platinum crucible was used, this fusion
was conducted in a platinum boat or open bottle contained in a porcelain
tube through which a current of nitrogen was passing. The experience
in a number of similar cases, that the simultaneous presence of oxygen
and traces of hydrochloric acid inclosed in the crystals is likely to cause
perceptible corrosion of the platinum boat, was confirmed in this case ;
hence an inert gas \vas needed. The nitrogen was prepared by the wrell-
known method of Wanklyn, by passing air charged with ammonia
over red-hot copper. The excess of ammonia was carefully eliminated
by washing the gas with much dilute acid. Because fused sodic chloride
was found to be essentially free from dissolved nitrogen, we did not think
it necessary to fuse this very similar potassium salt in a vacuum.

Potassic chloride, although not so hygroscopic as some other salts,
used in similar researches, is nevertheless far too hygroscopic to weigh
safely when directly exposed to the air of the balance room. Its ten-
dency to attract water was seen in perceptible crackling when the tube

aRichards, Zeit. phys. Chem., 46, 189 (1903).

THE DRYING AND WEIGHING OF THE POTASSIC CHLORIDE. 15

containing fused salt was opened in the moist atmosphere of the beaker to
be used for the solution of the salt.

At first the bottling apparatus so often used in the Chemical Laboratory
of Harvard College was employed to protect the fused chloride from mois-
ture during the determination of its weight, the chloride being contained in
a platinum boat protected by a glass-stoppered weighing bottle. Because,
however, the weighing room was not wholly constant in temperature or
in moisture, time and trouble were needed to obtain exact weighings, even
although the apparatus was always weighed by substitution against a
similar empty boat and bottle. The use of quartz weighing bottles proved
to be no advantage, indeed, if anything, a disadvantage. In either case,
if time enough were taken, satisfactory weighings could be made ; but
as time was especially precious, another device was used which wholly
overcame the trouble.

This effective and satisfactory device consisted in the use of small
weighing bottles of platinum, shaped like the long Lawrence Smith
crucible, which were closed by small platinum capsules, fitting into the
bottles like ground stoppers. One of these bottles, containing the potassic
chloride, was supported by a loop of platinum wire in an inclined porce-
lain tube, and above it in the tube was placed its platinum stopper. Glass
stoppers fitted into each end of the porcelain tube, which was encircled
by a suitable furnace. After the potassic chloride had been dried for
a long time at a high temperature, just barely fused in nitrogen, and
cooled in a current of pure dry air, the stopper was shaken into place
and the platinum bottle quickly removed and placed in a desiccator.
This arrangement is essentially similar to the bottling arrangement of
Richards and Parker, except for the additional advantages that glass
is wholly eliminated and that the inclosed air-volume in the tube is much
smaller. Probably the burnished platinum joint is not as tight as the
ground-glass joint; but it is amply tight enough to prevent any appre-
ciable diffusion of moisture during the very brief exposure of the tube.
This was evident from the constancy of the weighings, which were made
by substitution against a similar empty platinum bottle kept in a similar
desiccator. The weighings were very quickly performed and were trust-
worthy. Before being used, the platinum bottles were repeatedly ignited
with pure sublimed ammonic chloride in order to remove iron ; they
remained satisfactorily constant in weight during subsequent experiments,
the one used for the fusion losing only 0.3 mg. in fourteen experiments,
or on the average 0.02 mg. in a single experiment. In one experiment,
where oxygen was present during the fusion, as much as 0.3 mg. was
taken from the boat, the platinum being plainly visible on solution. This
was of course not used for analysis.

THE ANALYSIS OF POTASSIC CHLORIDE.

All weighings, whether of this salt or of silver or argentic chloride,
were made by substitution in the manner already described in other
papers.1 The balance was a short-armed instrument made by Troemner,
of the type adopted in other similar investigations ; it was used only for
this work and that to be described afterward. The weights were of
platinized brass, carefully standardized according to the usual Harvard
method.2

All weighings were reduced to the vacuum standard. Thus from every
apparent gram of silver, 0.00003 gram was subtracted, and to every
apparent gram of argentic chloride, 0.000071 was added, if the tempera-
ture was 20° and the pressure normal. Assuming the density of potassic
chloride to be 1.995, the similar correction for this salt .was -f- 0.00045G.
Changes in temperature and pressure occasionally caused slight but
usually inessential changes in these corrections.3

The wholly colorless, transparent, fused salt was dissolved in the purest
water in a covered beaker of Jena glass under a tightly fitting bell- jar.
The solution was always perfectly clear, except in the rejected case already
cited and one other similar one. After this solution had been very thor-
oughly washed away and used for analysis, the platinum weighing bottle
was heated for a short time in another crucible, and weighed in prepara-
tion for another analysis. The constancy of these weighings has already
been discussed.

The details of preparation having been described, the analytical methods
themselves must be indicated. The problem was, as in the case of sodium,
to determine the amount of chlorine present, the weight of potassium
being found by difference. Both of the previously used methods for
finding the amount of chlorine were used, and the details are so much
like those discussed by Richards and Wells4 that much may be assumed
as understood. For a full understanding of the present work, that upon
sodium should be read in connection with it.

The two methods for the determination of chlorine, namely, the weigh-
ing of the precipitated argentic chloride on the one hand, and the dis-
covery of the equivalent amount of silver on the other, are discussed
below in order.

1For example see Richards and Rogers, Zeit. anorg. Chem., 10, 19 (1895) ; also
Richards and Wells, Journ. Am. Chem. Soc., 27, 465 (1905).

2Richards, Journ. Am. Chem. Soc., 22, 144 (1900). These weights had an average
density of 8.3.

3Richards and Wells, Journ. Am. Chem. Soc., 27. 465 (1905).

4Loc. cit.

THE PRECIPITATION AND WEIGHING OF ARGENTIC CHLORIDE. 17

THE PRECIPITATION AND WEIGHING OF ARGENTIC CHLORIDE.

For purpose of analysis, the solution of potassic chloride was carefully
and completely transferred to a large Jena glass Erlenmeyer flask with a
finely ground glass stopper, and diluted to the volume of a liter or more.
To this in the dark-room under the red light was added exactly the calcu-
lated weight of argentic nitrate, never more concentrated than one-fiftieth
normal ; and the mixture was shaken for a short time in order to aggre-
gate the great mass of the precipitate. Because of the absence of an
excess of argentic nitrate, the danger of the occlusion of this salt was
slight. On the following day the supernatant liquid was usually quite
clear, and to it now was added the excess of perhaps 0.05 gram of argentic
nitrate needed for complete precipitation. This method of treatment
was found to be more satisfactory than the immediate addition of all the
argentic nitrate at once, as in this case long and tiresome shaking and
washing of the precipitate was needed to eliminate wholly the occluded
argentic nitrate. Thus was one of the chief difficulties met by previous
experimenters almost entirely overcome. It was shown in the paper on
sodium that the simplest test for the presence of argentic nitrate is the
appearance of the argentic chloride after fusion, even a small trace of
nitrate causing a perceptible gray-violet cloudiness. In all cases when the
above precaution was used the fused mass was perfectly clear and trans-
parent. This is, however, an anticipation of the later part of the process ;
the collection, ablution, and desiccation of the precipitate should first be
described.

In preliminary experiments, the Gooch perforated platinum crucible
with asbestos mat was used with all the precautions previously adopted
in this laboratory. In this way it is certain that good results may be
obtained ; but the necessary second filtration of the filtrate in order to
collect the traces of disintegrated asbestos is a tiresome process. On this
account preliminary experiments were made with Gooch crucibles pro-
vided with a smooth and burnished mat of platinum sponge, as prepared
by Heraeus.1 Such crucibles are named by him after Neubauer, but
might more properly be called Gooch-Munroe crucibles. In these tests
it was found that such crucibles as a matter of fact answer very well
for the purpose in hand if certain special precautions are taken. It
is not permissible to ignite them at a high temperature, unless the pre-
cipitate also is to be ignited at a high temperature. A crucible brought
to constant weight at 150° was found to lose 0.15 mg. on ignition, prob-

1Zeit. Angew. Chem., 14, 923 (1901). A number of experimenters have recom-
mended and used a mat of platinum sponge in a Gooch crucible, but Munroe was
probably the first. J. Analyt. Chem., 2, 241 (1888). Also Chem. News., 58, 101.
1885.

18 THE ANALYSIS OF POTASSIC CHLORIDE.

ably because of loss of previously adsorbed water. Moreover, when a
dilute solution of argentic nitrate, potassic nitrate, and nitric acid, such as
remains after an analysis, was passed through the filter, and this was five
times washed with water, and dried at 150°, yet 0.15 more was gained,
probably due to adsorbed salts. Very- thorough washing after this
brought the crucible back to its original weight after drying at 150°.

These experiments showed that the washing of the platinum sponge
must be very thorough, and that a definite temperature must always be
used for drying. Other experiments showed that constant weight could
always be obtained, if these conditions were fulfilled.

It is well known that argentic chloride sometimes clings to a platinum
surface, and always shrinks much on drying. On this account the
somewhat delicate upper surface of the sponge was protected by a closely
fitting disk or diaphragm of platinum punched with many fine holes.
This diaphragm, merely laid upon the top of the sponge at the base of the
crucible, was easily loosened with the precipitate, and formed an effectual
protection for the sponge. After this was done, no platinum was ever
lost from the upper surface ; and none was ever carried away mechanically
from below in the wash-water.

In spite of the presence of the disk, small particles of argentic chloride
always clung here and there to the sponge. As a possible means of
removing these when preparing for a new analysis, potassic cyanide was
tested. It was found, however, that not only was this salt adsorbed by
the sponge, but that in the presence of air platinum was dissolved even
to the extent of a milligram --a fact not new, but none the less pertinent.
Thiosulphate was not tried, because sulphur was an impurity little wished.
Finally, while ammonia for a short time was inefficient, this liquid, when
concentrated and applied for twelve hours or more, dissolved every trace
of the silver salt without harming the crucible.

The technique of the Gooch-Munroe crucible .having thus been mas-
tered, this utensil was used with satisfaction as a means of collecting
and weighing the precipitate in hand. The latter was first washed often
by decantation, as is recounted below, and finally on the filtering crucible,
which was dried to constant weight by a temperature gradually rising to
150°. After careful weighing, the main mass was separated from the
clinging platinum disk, and was carefully fused in porcelain, as has often
been described. The accurately determined loss on fusion, amounting
sometimes to a milligram, was calculated from the part to the whole,
and applied as a correction to the total weight of the precipitate.

This weight, even as thus corrected, did not, however, exactly represent
the total in weight of the chlorine, as some of the argentic chloride was
dissolved bv the wash-water. The ablution had been conducted in three

THE PRECIPITATION AND WEIGHING OF ARGENTIC CHLORIDE. 19

stages, just as in the often-cited investigation of sodium. The first stage
included the mother liquor and first five wash-waters, containing inten-
tionally as much argentic nitrate as the mother liquor, and added to remove
the greater part of the potassic nitrate. This first quantity of liquid was
entirely free from chlorine in every accepted analysis, as was shown by
careful testing in the nephelometer, and so was at once cast aside.

The second stage of washing was conducted with very dilute nitric
acid, and yielded 0.5 or 0.6 liter of a very dilute solution of mixed nitric
acid and argentic chloride and nitrate. The traces of chloride which it
contained were carefully estimated in the nephelometer after addition
of excess of argentic nitrate.

The third stage of the washing, also conducted with pure water acidified
with nitric acid to prevent colloidal irregularities, yielded a liter or more
of liquid containing much argentic chloride. The analysis of this liquid
constituted a very important part of the work, and gave much trouble.

In order to be as certain as possible of the weight of chlorine it con-
tained, this liquid was tested in five cases according to two very different
methods. The first of these consisted in evaporation of the solution
under diminished pressure and actual weighing of the precipitate, and
the second consisted in a new modification of the nephelometer test.
Although the latter evidently gave the more satisfactory results, the
former is worthy of brief discussion, because its outcome certainly repre-
sents the maximum value.

In order to carry out these two parallel quantitative tests, the collected
wash-water was divided into two parts ; and the larger part, about 90
per cent of the whole, was evaporated under low pressure at 50° in a
specially made Jena glass 2-liter flask with a glass stopper. In the course
of this evaporation the argentic chloride began to separate' out when the
volume had been reduced to about 0.2 liter, if 2 mg. of the salt were
present — a fact agreeing well with the recent observations concerning this
solubility.1 The dried residue was dissolved in a very little ammonia
freshly distilled in platinum, and washed into a small precipitating flask,
where it was reprecipitated with excess of argentic nitrate and nitric acid.
This trace of precipitate was collected and weighed on a fresh Gooch-
Munroe crucible in the same way as the larger mass. The results are
given below in comparison with those obtained from the same solutions
by the nephelometer.

An important modification was introduced into the nephelometric deter-
mination. One of us has repeatedly pointed out that in order to obtain
satisfactory results with this instrument the solution to be tested and
that used as the standard of comparison must be treated in exactly the

1See for example Bottger, Zeitschr. phys. Chem., 56, 93 (1906).

20

THE ANALYSIS OF POTASSIC CHLORIDE.

same way.1 Hence the use of a ground-glass plate2 as a standard of
comparison is a very questionable proceeding. It appears that the precipi-
tation of argentic chloride from solutions of this salt differ perceptibly
in mechanism from the precipitation of the same substance from other
chloride solutions, even when argentic nitrate in great excess is used
in each case.3 Accordingly, in order to secure a perfectly satisfactory
comparison, both precipitates must be dissolved in ammonia and repre-
cipitated. The precipitate then appears in each case in precisely the same
condition, and yields trustworthy results.

The present tests were carried out in the following manner: Two
test-tubes of precisely the same volume (0.025 liter) were provided. Into
one of these was placed 0.015 liter of the wash-water, and into the other,
serving as the standard of comparison, a like volume of water containing
about as much carefully measured chlorine (in the form of potassic
chloride) as was present in the first. Into each was now run 5 ml. of a
three-hundredth normal solution of argentic nitrate. The solutions were
stirred with a platinum stirrer, previously cleansed with ammonia and the
purest water. After five minutes the precipitates were well formed ; they
were then both dissolved with the help of a milliliter of freshly distilled
ammonia, and reprecipitated with a slight excess of nitric acid, being filled
to similar marks near the top of the tubes. The two cloudy solutions,
thoroughly stirred, were allowed to stand and compared optically in the
nephelometer in the usual way.

The following table compares the results obtained by the two methods.
The varying solubility of the halicle in the wash-water was mainly due to
difference in temperature at the time of the ablution.

The Comparison of Weights of Argentic Chloride found Ncphelomctrically and

by Evaporation.

Weights of argentic chloride.

Experiment No.

\ olume.

By evaporation.

By nephelometer.

Difference.

Liters.

Mg.

Me.

Mg.

10

1.45

3.96

3.12

- 0. 84

11

1.88

3.82

3.22

— 0. (50

12

1.18

2.59

1.97

- 0. 62

13

0.90

1.70

1.35

- 0. 35

14

1.00

1.74

1.45

— 0. 29

'Am. Chem. Journ., 31, 242 (1904); 35, 509 (1906).
2Wells, Am. Chem. Journ., 35, 99 (1906).

sRicharcls and Wells, Carnegie Inst. Pub. 28 (1905) ; Journ. Am. Chem. Soc.,
27, 485 (1905).

THE PRECIPITATION AND WEIGHING OF ARGENTIC CHLORIDE.

21

Thus the nephelometer always indicated less argentic chloride than
the gravimetric process. At first the nephelometer was suspected, but
the steadily diminishing difference between the two series of results indi-
cated that they were approaching the point where they would indicate
the same values. As the nephelometric treatment was invariable, but
on the other hand the evaporating-flask might easily be attacked, it
seemed probable that the decreasing difference exhibited by the figures
was due to something added to the weight of the precipitate from the
latter source. Apparently, as is reasonable, the flask was less and less
attacked as it continued in use.

For this reason the gravimetric results were wholly rejected, and the
nephelometric ones alone used in the calculation of the final results.

THE RATIO OF POTASSIC TO ARGENTIC CHLORIDE.

The final results of the series of analyses discussed above are recorded
in the following table. A number of preliminary experiments are
omitted from this table, since it is clear that no doubtful or imperfectly
executed experiment should find a place in such a table of final data.
The list is nearly consecutive, however, as but few experiments met with
misfortune after the processes had been mastered. The original number-
ing of the experiments is retained ; they were recorded in the notebook in
chronological order. All the argentic chloride referred to in this table
was clear and colorless ; a fact which is one of the best proofs that it
was free from argentic nitrate or from organic dust.

Final Series of Determinations of the Ratio KCl: AgCl.

No. of
Analysis.

Preparation
of KCl.

Weight of
fused KCl
in vacuum.

Weight of
fused AgCl
in vacuum.

AgCl : KCl
= 100.000 : A'.

Atomic
Weight K if
Cl = 35.473.

Grams.

Grams.

13

a

4. 36825

8. 398(i

52.012

39. 114

14

a

5. 56737

10. 7038

52. 013

39.115

15

a

6. 41424

12. 3323

52. 012

39. 114

20

b

3.27215

6.2913

52.011

39. 112

31

b

4. 83028

9. 2870

52.011

39.112

A. V6rjl££ 6

52.0118

39. 1134

This result will be discussed after the next series has been presented.
It has a probable error, calculated by the method of least squares, of only
about 0.0004 ; accordingly further repetition was deemed unnecessary.

THE ANALYSIS OF POTASSIC CHLORIDE.

THE RATIO OF POTASSIC CHLORIDE TO SILVER.

In this series of experiments, weights of the purest silver equivalent to
entirely new portions of fused potassic chloride, as calculated from the pre-
ceding table, were dissolved in nitric acid, and the two equivalent solutions
were mixed. Great care was used. The presence of an excess of silver
or of chlorine was then determined with the nephelometer, exactly accord-
ing to the details of manipulation adopted by Richards and Wells, whose
accounts should be consulted for particulars. The table below contains
the final results ; the preliminary practice experiments are omitted for the
same reason as before.

Final Scries of Determinations of the Ratio Ag: KCl.

No. of
Analysis.

Preparation
of KCl.

Weight KCl
in vacuum.

Weight Ag
in vacuum.

Ratio Ag : KCl

= 100.000 : X.

Weight of K if
Cl = 35.473.

19

a

Grams.

3. 88074

Grams.

5.61536

69. 109

39.117

21

a

7. 44388

10.77156

69. 107

39.114

24

C

5. 00681

7. 24514

69.106

39. 113

27

e

5. 04833

7. 30515

69.107

39.114

30

e

8. 19225

11.85412

69. 109

39.117

32

d

4. 99795

7. 23230

69.106

39.113

33

f

5.16262

7. 47042

69.107

39.114

Average

69 1073

39 1145

In this case the "probable" error is even somewhat less than before,
being under 0.0004.

DISCUSSION OF FINAL RESULTS.

Thus two results have been obtained, giving for the atomic weight of
potassium the value 39.113 by reference to argentic chloride, and the
value 39.114 by reference to pure metallic silver. The close agreement
of these results is an important evidence of their verity, and a striking
confirmation of the new atomic weight of chlorine found by Richards and
Wells. The atomic weight of chlorine is very simply calculated from the
results of the two series above as follows: Cl = = (69.1073/52.0118-
1.00000) 107.93 -- 35.47.5 --very near the most likely value, 35,473.

Such difference as exists is probably due to the slight remaining trace
of occlusion of foreign salts by the argentic chloride. In the present case
this source of error was eliminated more successfully than ever before,
hence the agreement between the two values of the atomic weight of
potassium was closer than usual.

DISCUSSION OF FINAL RESULTS. 23

It may be noted that the difference between the new value and the
old value of Stas is somewhat less than in the case of sodium. Stas's
results with potassic chloride led him to the value 39.146, about 0.032
hie-her than the new result, while with sodic chloride the difference was

o

0.042. A large part of this difference in each case is due to Stas's error
in the atomic weight of chlorine ; corrected for this error, the differences
become 0.016 and 0.026, respectively. The differences are to be referred
to the same causes in this case as in the others - - namely, to Stas's incom-
plete knowledge concerning solutions of argentic chloride, to his practice
of dropping solid salt into the precipitating solution, and to the presence
in his preparations of traces of impurity taken from containing vessels.
In the case of potassium one or more of these errors must have been
less than in the case of sodium.

The results of Richards and Archibald, although very few in number
and not intended to figure in a discussion of this kind, were somewhat
better -- probably because solid salt was never used directly in precipita-
tion. Assuming the present research to yield the true value, their errors
in the two series were, respectively, 0.009 and 0.014. The result of
Archibald alone for the ratio of silver to potassium chloride was about
the same amount (0.008) different from the present value; but his result
for the other ratio was less satisfactory, having an error of 0.021, even
greater than Stas's. These differences are not surprising, because less
was known at that time than at present concerning the behavior of
argentic chloride in solution.

Although the present paper presents strong evidence that the atomic
weight of potassium is really as low as 39.114, more remains to be done.
It would have been desirable to have used also some other wholly different
method of preparing potassic chloride, and, moreover, to have evaporated
large samples of the salt in nitrogen in order to discover a possible non-
volatile residue. It is very doubtful if these additional experiments would
have altered the present result, especially considering the precautions
taken in the work and the result of the following research ; but never-
theless it is planned to pursue these matters further.

Even when every conceivable precaution is taken, a single salt is not
an adequate basis for the certain decision of an atomic weight. For
this reason a parallel investigation on potassic bromide was simultaneously
in progress at the Chemical Laboratory of Harvard College. The next
communication, describing this other research, must be considered in
connection with the work which has just been described. As will be seen,
excellent confirmation of the present work is afforded by the work with
the bromide.

24 THE ANALYSIS OF POTASSIC CHLORIDE.

SUMMARY.

This investigation concerning the quantitative composition of potassic
chloride resembled in many respects the recent investigation of Richards
and Wells on sodium.

In several details, however, improvements were introduced which
effected a considerable saving of time and a perceptible gain in accuracy.

The precautions necessary for the accurate use of the Gooch-Munroe
perforated crucible were ascertained ; its employment was found to be
advantageous.

Platinum weighing bottles with conical ground-platinum stoppers were
used instead of boats and glass tubes for weighing the potassium salt.

Occlusion of argentic nitrate by the precipitated chloride was dimin-
ished by allowing the latter to stand for a long time in a solution contain-
ing neither excess of silver nor excess of soluble chloride, and by adding
more argentic nitrate only after the precipitate had assumed a fairly per-
manent condition of aggregation.

The nephelometric estimation of small amounts of suspended argentic
chloride was increased in accuracy by redissolving in ammonia both of the
opalescent precipitates to be compared and reprecipitating, in order to
equalize the conditions.

As final results, the outcome of twelve experiments, 100.000 parts of
silver were found to correspond to 52.0118 parts of potassic chloride, and
100.000 parts of argentic chloride were found to correspond to 69.1073
of this salt.

The corresponding values for the atomic weight of potassium (if silver
is assumed to be 107.930 and chlorine 35.473) are 39.1134 and 39.1145, in
unusually close agreement.

II

A REVISION OF THE ATOMIC WEIGHT OF POTASSIUM

THE ANALYSIS OF POTASSIC BROMIDE

BY THEODORE WILLIAM RICHARDS AND EDWARD MUELLER

CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF HARVARD COLLEGE

A REVISION OF THE ATOMIC WEIGHT OF POTASSIUM.

THE ANALYSIS OF POTASSIC BROMIDE.

INTRODUCTION.

The foregoing quantitative study of potassic chloride by Dr. Arthur
Staehler and one of the present authors affords strong evidence that
the atomic weight of potassium is about 39.114, slightly lower than the
value based upon the work of Stas. The authors fully appreciated, how-
ever, that the investigation of a single compound is not enough to
establish a chemical constant so important as this, and, accordingly, the
present investigation was prosecuted simultaneously. It was expected
that the two investigations might either support one another or else, by
affording incompatible results, lead to the discovery of a constant error in
one or the other, and thus pave the way for further advance in knowledge.
As will be seen, the work on the chloride was satisfactorily confirmed by
the work on the bromide.

In the present case the careful study of potassic bromide was particu-
larly necessary, because there already exist two concordant series of
experiments upon this substance, performed by the old masters Marignac
and Stas, pointing to a value in the neighborhood of 39.14, instead of the
before-mentioned new value 39.11. In this case the higher value for
potassium is not diminished, as in the case of the chloride, by an additive
correction in the atomic weight of the halogen, because Baxter has
shown Stas's estimate for bromine to have been nearly correct.1 Hence
the discrepancy remains one too serious to be tolerated.

The careful study of this work of Stas and Marignac affords convincing
evidence that the potassic bromide used by them for analysis was not
sufficiently pure for the purpose. Stas admitted that some of his prepara-
tions were not even wholly soluble in water, and his method of procedure
was such that some of them probably contained platinum and hydroxide.
Because speculations of this kind concerning work so long past are of
but little value, it was clearly necessary to repeat this work with modern
care ; and the following pages recount the details of the repetition.

1A brief review of Stas's and Marignac's work may be found in Clarke's "Recal-
culations" (1897), p. 47. Baxter's work is to be found in Proc. Am. Acad.. 42, 201,
1906; Journ. Am. Chem. Soc., 28, 1322 (1906). See also Richards, Trans. Am.
Phil. Soc., 43, 116 (1904).

2?

28 THE ANALYSIS OF POTASSIC BROMIDE.

In common with others of the same type, the investigation easily resolves
itself into several sections and will be discussed tinder the following
heads : The preparation of materials ; the drying and weighing of potas-
sic bromide ; the determination of the ratio of argentic to potassic bro-
mide; the determination of the ratio of silver to potassic bromide; and
the discussion of the results. Of the laboratory tasks the preparation
of the materials was by far the most laborious and puzzling, while the
analytical work was comparatively simple.

THE PREPARATION OF MATERIALS.
THE SOURCE OF THE POTASSIUM.

The task of preparing pure potassic bromide, simple as it may appear,
is by no means an easy one. In view of our experience, it seems probable
that neither Marignac nor Stas ever prepared samples pure enough to
correspond with the other precautions which they took. As in the case
of sodic bromide,1 the salt itself when once made can not be effectively
purified. In this respect it differs widely from the chloride. We found
after due trial that the potassium and bromine must be purified sepa-
rately in such forms as to introduce no foreign matter. Accordingly,
the two substances, already adequately purified, were caused afterwards
to yield potassic bromide ; and this salt, when recrystallized, gave the final
substance for analysis. The purification of the potassium materials will
be discussed first.

It was necessary to provide potassium material which should be free
from the suspicion of introducing any impurity with the metal desired,
either basic or acid. Potassic nitrate, which had served so well in the case
of the chloride, was not suitable for the present purpose, because the
destruction of the nitric acid involves too great a sacrifice of laboriously
prepared hydrobromic acid. After due consideration, potassic oxalate
was tentatively chosen. It was necessary to prove that this salt can be
easily and certainly purified by recrystallization, and especially to show
that the other alkali metals can thus be separated. The following tests
accomplished this proof.

A saturated solution of potassic oxalate containing a purposely added
admixture of 10 per cent of sodic oxalate was crystallized and the crystals
drained centrifugally. By the flame test, a pronounced difference was
noticeable between the amount of sodium present in the mother liquor
and that in the crystals. A second recrystallization and whirling was
still more satisfactory ; the crystals gave no sodium test to the eye, while

Richards and Wells, Proc. Am. Acad., 41, 435 (1906) ; Zeit. phys. Chem., 56.
348 (1906).

THE PREPARATION OF MATERIALS. 29

the mother liquors gave a distinct yellow. This indicates that sodium
has a decided tendency into the mother liquors, and hence can be sepa-
rated by fractional crystallization.

Again, 1 per cent of lithium salt was added to potassic oxalate with
similar outcome. Rubidium and caesium were tested likewise. A sat-
urated solution of potassic oxalate and a salt of each of these metals was
made of such strength that the rubidium or caesium could easily be found
by means of the spectroscope. Each solution was evaporated until satu-
rated, and cooled ; and the crystals were whirled. The mother liquor gave
a test for the impurity, the crystals gave none.

No other oxalates were seriously to be feared, because the insoluble ones
would appear at once and demand filtration, and soluble double oxalates
could hardly be isomorphous, and would therefore tend into the mother
liquors. Traces of calcic oxalate in fact appeared in the former fashion,
and traces of iron in the latter.

In view of these satisfactory results, it was evidently only necessary
to crystallize the potassic oxalate often enough to make the separation
complete ; and since potassic oxalate has a fairly convenient change in
solubility with the temperature, the yield after a considerable number of
crystallizations is good, especially if pains are taken to carry out the pro-
cess systematically.

Two very pure samples of oxalate were made, from two different sources.
As one source of material, a commercial sample prepared by Merck was
used. It had been labeled "Potassium oxalate, neutral, highest purity,"
and contained as a matter of fact only the traces of calcium and iron
already referred to, but no lead or other discoverable impurity. A large
quantity of a solution of this salt was filtered hot into a platinum dish,
where it was five times systematically recrystallized, the crystals being
centrifugally whirled each time.

Careful scrutiny in the spectroscope revealed in the product no other
spectroscopic lines than those of potassium, and careful qualitative tests
proved the absence of lead and iron. The salt dissolved without a trace
of residue in water, and was quite pure enough to serve as a starting
point for further operations, to be described under the heading "Potassic
bromide."

A second sample of potassic material was obtained from Merck in the
form of the hydroxide, being marked "Potassium Hydrate, Chemically
Pure Reagent ; Conforms to the standards of Dr. Krauch." It was in
fact very pure, giving a perfectly clear solution both before and after
neutralization. It was neutralized with a specimen of oxalic acid which
had been carefully -purified by Mr. G. E. Behr, jr., for other work. This
acid contained no halogens, and left no residue on volatilization in plati-

30 THE ANALYSIS OF POTASSIC BROMIDE.

num. Slightly more than the equivalent quantity of acid was added, and
the oxalate obtained was therefore slightly acid. As will be shown, the
acidity was later the cause of much inconvenience, and had better have
been avoided. The oxalate was four times systematically recrystallized in
platinum with centrifugal draining each time. The final product, like the
previous one, gave no tests for impurities except a slight excess of acid;
it was used as the source of potassium for one preparation of potassium
bromide.

BROMINE.

Although the methods previously used at Harvard had afforded a
satisfactory yield of very pure bromine,1 it was desired to add to the
knowledge on the subject by testing another method. Through the gener-
osity of the Mallinckrodt Chemical Works of St. Louis, Missouri, and
also of the Dow Chemical Works of Midland, Michigan, a large quantity
of potassic bromate was placed at our disposal. Repeated crystallization,
very kindly carried out for us on a large scale by these firms, had been
the method used in its purification.

This material, which served as our starting-point, was already very
pure, a fact which was indicated by three syntheses of argentic bromide
from purest silver and potassic bromide obtained by decomposing the
Mallinckrodt bromate. These syntheses, carried out with all necessary
precautions, gave, respectively, 57.441, 57.440, 57.441 as the percentage of
silver in argentic bromide. According to Baxter the true amount is
57. 445. 2 The slight difference indicates a trace of iodine rather than
chlorine ; but this impurity can be easily expelled, while chlorine is far
more troublesome.

It having been demonstrated that recrystallization is an effective means
of purifying the salt from chlorate and chloride, a large quantity of
potassic bromate was three times recrystallized in porcelain, the crystals
being centrifugally whirled each time and all mother liquors discarded.
This gave an exceedingly pure bromate, much purer than Stas could have
obtained,3 because the centrifugal draining makes so great a difference
in the rate of purification.*

A large quantity of the salt was decomposed into the bromide in a
platinum dish by the heat of an alcohol lamp. The potassic bromate in
decomposing attacked the platinum, and the mass of bromide was slightly

1See Proc. Am. Phil. Soc., 43, 119 (1904), for references and Stas, Mem. Acad.
P.elgique, 43, II, 38 (1882).

2Baxter, Journ. Am. Chcm. Soc., 28, 1332 (1906). See also Richards, Trans.
Am. Phil. Soc., 43, 119 (1904).

3Untersuchungen, 160.

4Richards, Journ. Am. Chem. Soc., 27, 104 (1905).

THE PREPARATION OF MATERIALS.

brownish in color, but this caused no inconvenience in the present case,
as the bromine was to be subsequently distilled. In order to liberate the
halogen, the bromide was treated in strong solution with less than the
calculated quantity of bromate and an excess of pure sulphuric acid. The
acid was added drop by drop to the mixed solution of salts in an apparatus
made entirely of Jena glass, which was kept surrounded by ice-water.
The bromine which volatilized on account of the heat of the reaction was
caught in a condenser under water; to this was added the remaining
bromine, which had been separated from the supernatant solution by
means of a separating funnel.

The bromine thus obtained was twice distilled by steam heat into water
in a Jena glass condenser, packed in ice. It was kept in a well-seasoned
bottle with double ground-glass stoppers.

The product was certainly free from non-volatile materials, and could
have contained only iodine and perhaps a trace of sulphuric acid as
impurity. It will be seen that these were eliminated in the subsequent
work. This was the only sample of bromine prepared ; it was used in all
the preparations of bromide, as the quantitative results indicated that
it was very pure.

POTASSIC BROMIDE.

The problem of obtaining potassic bromide from the oxalate and bro-
mine had been carefully considered before these substances had been
prepared. Only two methods recommended themselves for the present
purpose, although the bromide can be obtained from these two materials
by a variety of reactions, which would ordinarily be acceptable. Here,
however, simplicity, completeness, and theoretical correctness must all be
fulfilled ; that the methods adopted really possessed these advantages in
their favor must be admitted after careful perusal of the following descrip-
tion of the details.

In some preliminary experiments it was shown that by adding an excess
of bromine to the oxalate and evaporating off the excess, a very pure
bromide quite free from oxalate is obtained. Accordingly, a quantity of
the first sample of five times crystallized potassic oxalate was treated with
an excess of bromine, the bromine being added in small quantities from a
dropping-funnel. The solution was held in a quartz dish, which was
carefully hooded to prevent entrance of dust. When the action had
ceased, the solution was heated on a steam-bath until any traces of iodine
and all excess of bromine had been expelled. The solution was found
to be entirely free from oxalic acid upon testing with calcic nitrate in a
solution faintly acid with nitric acid. Although it was wholly colorless
while in the quartz dish, after being concentrated in platinum it developed
a very faint yellow tinge. Upon recrystallizing three times this color was

32 THE ANALYSIS OF POTASSIC BROMIDE.

finally concentrated in the extreme mother liquor, and was found to be
due to a trace of platinum ; no iron was present. Even the first crystals
were perfectly white, the second and third mother liquors also showed
no trace of color, and only the first of the mother liquors gave a test
for platinum.

The material thus prepared, after being three times recrystallized in
platinum with centrifugal draining, was called sample I, and served for
analyses 1 to 11.

The second sample of potassic bromide was prepared from the second
sample of four times recrystallized potassic oxalate. The method was the
same as with the previous sample. An excess of bromine was added to
the oxalate in quartz ; when action ceased, the bromine was driven off by
heating on a steam-bath. However, on testing for oxalate, a perceptible
quantity was found still undecomposed. This was unexpected, in view of
the fact that the first preparation of bromide had given no trouble in this
respect. The difficulty was traced to the presence of a small percentage
of acid salt in the oxalate ; the presence of acid has an inhibitory effect on
the reaction of bromine on the oxalate ion. Obviously, the simplest
remedy would have been to add pure potassic hydroxide ; but this was not
added because at that moment there was none at hand. Moreover,
because the reaction was being performed in quartz, it was thought best
not to use a caustic alkali for fear that the dish might be slightly attacked.
The addition of a small amount of very pure recrystallized bromate to
this slightly acid solution served better, because its use was unattended
with danger. Its employment was entirely effective ; the ionized hydrogen
was removed, and after adding more bromine and heating, a solution of
bromide was obtained which contained no trace of oxalate.

This specimen was recrystallized entirely in quartz, in order to prevent
a recurrence of the trouble experienced with sample I, as well as to see
if the variation of conditions affected the atomic weight. Two crystalli-
zations were made with centrifugal draining, and every precaution ; and
there was obtained finally a very pure, colorless product with which were
made two analyses, Nos. 12 and 13. That a uniformly pure material had
been made is shown by the close agreement of these two analyses with
the mean of all.

As a further precaution against accidental error, it was thought best
to develop a method essentially different from that just discussed, and
to prepare therewith a third specimen of bromide. In brief, this method
consisted in the separate preparation of pure ammonic bromide and
potassic hydroxide, and the production of potassic bromide by the evapor-
ation of the mixed solutions of these substances. Thus can be obtained
a salt contaminated only with excess of volatile ammonic bromide.

THE PREPARATION OF MATERIALS. 33

The preparation of pure ammonic bromide was very easy. Ammonia
essentially free from carbon compounds was redistilled into water in a
platinum dish. Into this was dropped the purest bromine, which fell
through the liquid into a small porcelain crucible resting on the bottom
of the dish. In this way the bromine itself could not come into contact
with platinum and attack it. The reaction took place very rapidly, yield-
ing ammonic bromide which could have contained no non-volatile ingre-
dients, because all its constituents had just been distilled.

The preparation of pure potassic hydroxide was less easy to devise,
because more original ; but the execution was almost as easy. The
problem was solved by the use of an electrolytic process which will be
described in greater detail elsewhere. In brief, a saturated solution of
potassic oxalate (sample I) was electrolyzed between a pure mercury
cathode and a platinum anode in a porcelain dish cooled with ice, the
current from four storage cells being used. When the amalgam became
solid, the current was stopped, the solution decanted, and the amalgam
washed, being triturated with an agate pestle under water, till no test
for oxalate could be obtained. The pure amalgam was transferred to a
platinum dish, covered with water, and made the anode of a dense current
until only a little of the potassium amalgam remained undecomposed.
Thus pure potassic hydroxide was formed. This solution was poured into
the ammonic bromide solution, the mixture was evaporated and crystal-
lized, and the salt thus obtained used for analyses 14, 15, and 16.

The bromide thus obtained must have been very pure. During the
electrolysis, the high concentration of the potassic oxalate1 would tend
to allow the deposition of potassium only.2 At any rate, metals less easy
to deionize could hardly have been set free. During the later decomposi-
tion of water by the amalgam, all metals with less tendency to ionize
would have remained in the mercury, because some of the amalgam was
left undecomposed. Any trace of iodine which the bromine may have
contained must have been driven off by the fusion just before analysis,
this fusion being prolonged in order to expel ammonic bromide. The
results of the analyses of this sample indicate its essential identity with
the two preceding preparations. Since there was no known source of
impurity in any one of the preparations, this was not surprising.

Although our consistent employment of insoluble vessels rendered the
presence of colloidal silica unlikely, an effort was made to test for this
impurity of sample II by volatilizing 3 grams of it in a stream of
nitrogen. Unfortunately, the high temperature of the electric furnace
caused the platinum boat containing the salt to weld to the foil which

Bunge, Berichte, 9, 78 (1876).
2Berthelot, Ann. Chim. Phys. [5], 18, 433 (1879).

34: THE ANALYSIS OF POTA55IC BROMIDE.

surrounded it in the tube, so that no certain quantitative evidence could
be obtained. Two minute black spots were visible in the boat; nothing
else could be seen upon very careful scrutiny. If there were any silica
present, the amount must have been too small to have had any appreciable
influence on the final atomic weights. Time and material were lacking,
hence the experiment was not repeated.

SILVER.

The essential details of the preparation of pure silver, as developed by
Richards and Wells, were followed in detail, and the reader is referred
to their paper, as well as to the preceding paper of Richards and Staehler.
for the particulars. Two samples of silver were made ; they were found
to be identical in quantitative behavior.

The source of the material used for the first preparation was the pure
precipitated argentic chloride remaining from the work of Richards and
\Yells on sodium. It was reduced to metallic silver with invert sugar in
a strong solution of sodic hydroxide. The silver thus obtained was
washed free from soluble matter and dissolved in nitric acid ; the argentic
nitrate after crystallization and whirling was reduced with ammonic
formate, yielding a beautiful crystalline mass of metal.

This pure silver, after very thorough washing, was fused on lime by
means of a blast lamp whose tip had been carefully cleaned. The buttons
which were thus obtained were cooled in the reducing-flame, washed free
from lime, scrubbed with cleaned sea-sand, etched with strong nitric acid,
and washed with the purest water, to serve as anodes in the electrolytic
purification which was the next step. The electrolysis was carried on
as already described in previous similar Harvard researches, and every
step of the subsequent work may be found in detail in the preceding papers
on sodic and potassic chloride.1 The metal was finally fused as usual on
a boat of the purest lime in an atmosphere of hydrogen under a tension of
50 mm. The larger pieces, after etching with nitric acid, were cut into
smaller ones of convenient size with a clean cold-chisel. They were again
etched in order to dissolve all traces of iron,2 scrubbed with clean sand,
washed with a jet of water, and etched yet again with acid, paying par-
ticular attention to the cut edges. Finally the pieces were dried over an
alcohol lamp and kept in a desiccator over potassic hydroxide for use.

This silver was used both in most of the titrations against potassic
bromide and as the source of argentic nitrate used in the precipitation of
argentic bromide to be weighed.

aSee Proc. Am. Acad.. 38. 450 (1903) : and Richards and Wells, Journ. Am.
Chem. Soc., 27, 473 (1905) ; Richards and Staehler, preceding paper.

2Richards and Archibald, Proc. Amer. Acad., 38, 450 (1903) ; Baxter, Journ. Am.
Chem. Soc., 28, 1329 (1906).

THE PREPARATION' OF MATERIALS. 35

A second sample of silver, prepared from somewhat less pure initial
material, was put through a more elaborate and exhaustive round of
purification which need not be detailed. The preparation included five
crystallizations as nitrate and two successive precipitations as metal by
formate. This silver was fused into buttons of a size suitable for imme-
diate use without cutting-. They were cleaned by etching and washing,
dried and preserved free from impurity as usual, and were used in analyses

7 Snd 9" NITRIC ACID.

For preliminary work, ordinary ''chemically pure'' nitric acid was
distilled once, using a platinum condenser and 'Jena glass receiver, the
first third of the acid being discarded. For the final analyses this redis-
tilled product, already very pure, was once again distilled, as before, the
first third being rejected. This distillate gave no test with the nephel-
ometer, after dilution.

\\ J\ J. ILIv.

As usual in such work, all the water used had been carefully purified by
double distillation, once from a fairly strong solution of alkaline potassic
permanganate, and then alone. The distillate was caught in Jena glass
receiving flasks which were provided with special adapters to prevent
the access of dust. The first and last fractions were always rejected, only
the middle portion being used. The connection of the Jena glass boiling-
flask with the pure tin condenser was made without rubber or cork.1
Such water gave even- evidence of sufficient purity, as has been frequently
pointed out. TH£ LABORATORY.

A very essential precaution in every stage of this work was to effectually
exclude hydrochloric acid. Accordingly, all the preparations of the bro-
mide were carried out in a private laboratory, used especially and only
for this work, and wholly free from the interference caused by large
classes or other researches. Great care was taken not to allow any
hydrochloric acid in the room ; indeed, not a drop was used there during
the entire research. A separate ventilating fan prevented fumes from
other laboratories from interfering with the work.

UTENSILS.

As usual, great care was taken to avoid the use of any vessels which
under the given conditions might be attacked and thus pollute the sub-
stance in hand. "\Yhen glass and porcelain were not harmful, as in the
early stages of the preparation of silver, such receptacles were employed ;
but in the purification of the substance for analysis, platinum and quartz
were always used.

iRichards, Proc. Am. AcacL 30, 330 (1S94).

36 THE ANALYSIS OF POTASSIC BROMIDE.

THE DRYING AND WEIGHING OF POTASSIC BROMIDE.

For reasons already mentioned in the paper on potassic chloride, the
salt under investigation must be fused before weighing. Unfortunately,
potassic bromide when fused in air in a platinum vessel attacks the plati-
num to a considerable extent. Although it is shown later that this action
does not seriously affect the weight of silver needed to precipitate the
bromine in the salt, it was nevertheless desirable to avoid all such irregu-
larities. No such action is observed when the fusion takes place in an
atmosphere of pure dry nitrogen. Accordingly, in all work to be re-
counted, the potassic bromide was protected in this way during its fusion
in a well-seasoned platinum boat, and was finally, by means of the Harvard
"bottling apparatus," shut up in a tight weighing bottle in an atmosphere
of pure dry air. The details have already been adequately given in several
places, especially in the account of the recent work on the atomic weight
of caesium.1 The only difference in the present procedure was demanded
by the high fusing-point of potassic bromide (750°) ; on account of this,
a porcelain ignition-tube was substituted for one of glass, as in the case
of calcic chloride.2

Even with all possible care, however, the platinum boat was distinctly
attacked, almost always losing in weight during the fusion. In two of the
cases — analyses 1 and 14 — where the potassic bromide was fused for
some time in the boat, the corrosive action was very considerable, causing
losses of weight of 0.0007 and 0.0014 gram, respectively. One would be dis-
posed to reject these determinations entirely, except for the fact that their

KBr
average result, — : - = 1.10317, is almost exactly identical with that of

the average of results 5 and 6, in which the boat was scarcely attacked at

KBr
all, namely, —r- - = 1.10318. Instead of being rejected, these results

6

are therefore retained and used as evidence that even a considerable
amount of dissolved platinum has no perceptible effect on the weight of
silver precipitated by the salt. In a majority of the other cases, the change
in weight of the boat during the fusion was less than 0.0001 gram. The
average loss in analyses 10, 11, 12, and 16, in which the argentic bromide
was weighed, was less than 0.00009 gram ; hence even if platinum had
been present in the potassic bromide in a state of fine division it could not
have exerted any essential effect upon the result of this series of analyses.

'Richards and Archibald, Proc. Amer. Acad., 38, 451 (1903) ; Zcit. anorg. Chem.,
34, 362 (1903).

2Richards, Journ. Amer. Chem. Soc., 24, 374 (1902).

THE DRYING AND WEIGHING OF POTASSIC BROMIDE. 37

As a matter of fact, the potassic bromide always gave a perfectly clear
aqueous solution except once, namely, in analysis 14. In this case several
small flakes of platinum were noticed in the salt both before and after the
salt was dissolved in water.

The solutions were always perfectly neutral to phenolphthalem, a fact
which is adequate proof of the retention of all the bromine during fusion,
and also of the absence of oxalate from the potassic bromide crystals.

The third sample of potassic bromide had contained ammonic bromide,
which was expelled during the fusion. During the expulsion of this salt,
the boat was placed near the exit end of the porcelain tube, and there
gradually heated to redness in a current of nitrogen. During the entire
operation the middle portion of the porcelain tube was kept red-hot in
order to prevent backward diffusion of the ammonic bromide, which by
condensing might later cause a contamination of the potassic bromide.
All the ammonic bromide having been sublimed, the boat was pushed into
the middle of the tube, and the potassic bromide fused as described above.
There being a possibility that even at the fusing-point of potassic bromide
some ammonic bromide might still be retained, another portion of the same
third sample of bromide was fused under approximately the same condi-
tions as in the analyses. The solution of the bromide thus obtained was
submitted to the Nessler test, being compared with a tube of pure water
containing the same amount of reagent. No color was visible in either
tube, hence no appreciable amount of ammonic bromide remained to con-
taminate the potassium salt.

The salt thus prepared for analysis was weighed in its glass-stoppered
weighing-bottle by substitution, using as the substituting tare a precisely
similar weighing-bottle. Successive weighings of the same specimen were
always practically identical. The balance and weights were similar to
those used with sodic and potassic chlorides, and all the precautions were
the same. At each weighing the heights of the barometer, the temperature
of the balance-room, and the humidity as indicated by a hygrometer were
recorded. The density of the displaced air could thus be determined for
each case. The variations from the average were, however, not enough to
affect essentially any of the results, as weighings were not made under
any abnormal atmospheric conditions. The weights were of course stand-
ardized by the usual Harvard method.

The specific gravity of potassic bromide must be known to within five
one-hundredths of a unit in order to correct the weighings to the vacuum
standard with sufficient accuracy. Its determination1 has been undertaken
with a variety of methods, with a variety of results, ranging from 2.20 to

1For the literature see Landolt u. Bornstein (Meyerhoffer). Physikalisch-Chem-
ische Tabellen (1905), and Clarke, Constants of Nature (part 1), Macmillan (1888).

38 THE PREPARATION OF MATERIALS.

2.756. The more recent results all tend towards the highest value, found
by Krichmeyer1 (2.756). This appeared to be the most probable, because
the usual errors cause low results ; but as this determination was made
with only a single crystal, and as the salt weighed in the present research
had been fused, it was thought best to investigate further. Accord-
ingly, a pure sample of potassic bromide was further purified by being
three times recrystallized with centrifugal draining, when it gave no flame
test for sodium.2 This product was fused in platinum and coarsely
pulverized ; its specific gravity was determined in an Ostwald pycnometer,
modified for use with solids, using toluol as the fluid to be displaced. The
toluol had been redistilled and possessed a density 0.8608 at 25°, referred
to water at 4° ; 8.2568 grams of potassic bromide displaced 2.6009 grams
of the toluol at 25°, a result which indicates a density of 2.73. Time was
lacking for a repetition of the experiment, but this preliminary result was
enough to show that the fused salt is essentially in the same state as the
crystallized salt. The difference between the two values is too small to
affect the vacuum correction ; the average value 2.74 was used.

THE PRECIPITATION AND WEIGHING OF ARGENTIC BROMIDE.

In order to determine the weight of argentic bromide obtainable from
the bromine in potassic bromide, the latter salt was precipitated by a very
slight excess of argentic nitrate, both substances being dissolved in large
quantities of the purest water. The method differed but slightly from
that which had been used recently in so many other cases of the same
kind. The argentic nitrate was prepared from a weighed amount of
pure silver, but no attempt was made in this series to determine the exact
amount of silver required, as it was desired not to complicate the process
before weighing the argentic bromide. Care was taken to have an excess,
but only a slight excess. The silver was dissolved in nitric acid with all
the usual precautions, and the precipitation was carried out in orange-red
light in the dark-room devoted to accurate work of this kind. The potas-
sic bromide had been dissolved out of the boat by digestion in a large
platinum dish, and been transferred to a 2-liter glass-stoppered Jena
Erlenmeyer flask with careful rinsing, and every precaution to prevent
any gain or loss. After being well shaken, the flask containing the silver
bromide was allowed to stand until the mother liquor had become clear.
The solution was filtered through a platinum Gooch-Munroe crucible,
and the precipitate was washed by decantation, first with an extremely
dilute acid solution of silver nitrate, and finally with exceedingly dilute

iKrichmeyer, Z. physikal. Chem., 21, 81 (1896).
2See Krichmeyer s experience, loc. cit.

THE PRECIPITATION AND WEIGHING OF ARGENTIC BROMIDE. 39

nitric acid to prevent colloidal solution. With this latter faintly acid
liquid the precipitate was transferred to the crucible. The silver bromide
was dried at least twelve hours in the filtering crucible at 130° in an elec-
tric oven,1 and weighed. It was then fused in a porcelain crucible, in a
furnace where it was effectively protected from flame gases, in order to
determine the trace of moisture always retained. The wash-waters and
the solution obtained on neutralizing the ammoniacal rinsings of the flask
gave no indication of the presence of bromine on careful testing in the
nephelometer.

In using the Gooch-Munroe crucible, all the precautions pointed out
in the preceding paper were carefully heeded. A perforated plate was
always placed on top of the friable sponge to prevent rupture. After each
determination any bromide clinging to it was leached out with potassic
cyanide, which was then washed away with nitric acid, and finally very
thoroughly with water. The solvent action of the cyanide occasionally
loosened portions of the spongy film, but there was no difficulty in repair-
ing the injury. Upon drying in the electric oven, the crucible was ready
for another analysis.

In addition to the loss of weight on fusion, two other very small and
somewhat uncertain corrections were applied to the weight of the argentic
bromide, but because they were of about the same magnitude and of
opposite sign, their effect was practically negligible. These were a cor-
rection for platinum corroded from the boat during the fusion of potassic
bromide, and a correction for argentic bromide dissolved in the water used
for washing. Because these corrections have not been considered in most
work of this kind, a word about them may not be amiss.

In the series under discussion the average loss of the boat in each deter-
mination was less than 0.00009 gram. All of this trace of platinum may
have been present as invisible dust in the solution, and thus may have been
weighed with the argentic bromide, or all of it may have been in a soluble
form and may have remained in solution. Because of this uncertainty,
a compromise was made, and half of the loss of weight of the boat (in the
mean 0.00004 gram) was subtracted from each weight of argentic bro-
mide. As the average total weight of argentic bromide was over 4 grams,
this compromise could not have introduced an error as great as 1 part in
100,000 in either direction.

According to Stas,2 argentic bromide is wholly insoluble in water, but
recent experiments show that in the flocculent form it is unquestionably
soluble to a slight extent.3 As in the case of the chloride, its solubility is

Richards, Am. Chem. Journ., 22, 45 (1899).
2Stas, Oeuvres, I, 89.

3B6ttger, Z. fur physik. Chemie, 46, 602 (1903) ; Kohlrausch und Rose, Zeit.
phys. Chem., 12, 234 (1893) ; also, Richards, Proc. Am. Acad., 30, 385 (1894).

THE ANALYSIS OF POTASSIC BROMIDE.

greatly diminished by the addition of an excess of either precipitant ; but
whether or not any is dissolved by a very dilute acid solution of argentic
nitrate, such as that used in washing, it is practically impossible to discover.
It is not unlikely, however, that about as much was dissolved by this solu-
tion as by the dilute hydrobromic acid used by Baxter in his admirable
work on the atomic weight of bromine ;l and in this solution it is easy to
find the amount of dissolved substance. Baxter found as a matter of fact
in his last seven most exact syntheses an average of 0.00004 gram of
argentic bromide in each of the wash-waters — a figure which was added
to each of our weights of argentic bromide, because the other circumstances
of the analysis were similar in the two cases. On the average, this cor-
rection exactly eliminates the other ; and except for the sake of complete-
ness, they might both have been wholly neglected.

Of course all the weighings were corrected to the vacuum standard, by
adding 0.000041 gram to every apparent gram of argentic bromide and
0.00029 gram to every apparent gram of potassic bromide, as calculated
from the figures 6.473, 2.74, and 8.30, for the densities of argentic bro-
mide,2 potassic bromide, and the brass weights respectively. All the
determinations made are given in the table below.

The Ratio of Argentic Bromide to Potassic Bromide.

Experiment
number.

Weight of KBr
in vacuum.

Corrected Weight
of AgBr in vacuum.

Parts of KBr
for 100.00 parts
of AgBr.

Atomic Weight of
Potassium
Br = 79.953.

Gramt.

Grams.

10

2. 19027

3.45617

63.3728

39. 114

11

4. 19705

6. 62285

63. 3723

39.113

12

2.06723

3. 26206

63.3719

39.112

16

2.58494

4. 07889

63. 3736

39.115

Total average...
Probable error

11.0395

17. 41997

63. 3727
0. 0003

39. 1135
0. 0004

The extreme deviation from the mean corresponds to an error of weigh-
ing the potassic bromide of 0.00004 gram, a reasonable quantity. The
"probable error" indicates that there is but little chance that the atomic
weight of potassium is much below 39.113 or much above 39.114, if con-
stant chemical errors were successfully excluded. Stas found in a single
experiment the number 63.383 instead of 63.37S.3

With unlimited time more determinations might well have been made,
but the agreement of these four results is so good that further repetition
seemed to be not very urgent.

iBaxter, Journ. Amer. Chem. Soc., 28, 1322 (1906).
2Baxter and Hines, Amer. Chem. Journ,, 31, 220 (1904).
3Stas, Untersuchungen (Trans. Aronstein 1867), page 340.

DETERMINATION OF THE SILVER NEEDED FOR PRECIPITATION. 41

THE DETERMINATION OF THE SILVER NEEDED FOR PRECIPITATION.

The silver titration method of Gay-Lussac, as developed by Pelouze,
Mulder, Stas, and more especially by the recent work at Harvard, yields
very consistent and accurate results, if the proper conditions are carefully
observed. The reading of the end-point is rendered easy by the use of
the nephelometer.1 In the case of the somewhat soluble chloride, various
precautions must be strictly heeded ; but with argentic bromide, which is
almost insoluble, the matter is a simpler one, and a very slight excess of
either bromide or silver can be easily determined.

The method used in the present case is easily inferred from previous
Harvard work of the same kind. From the weight of a piece of the purest
silver, the equivalent amount of potassic bromide was calculated ; slightly
more than this amount of substance was then fused as before in a plati-
num-iridium boat placed in the porcelain tube of the bottling apparatus.
From the weight of fused bromide, the equivalent quantity of silver was
calculated; the greater part of the difference between this calculated
weight and that of the original piece of silver was added in the shape of
pure silver wire,2 and any final difference of 0.1 or 0.2 mg. was made up
with a dilute solution of silver nitrate. The silver was dissolved in nitric
acid and the nitrous acid expelled as usual, and the solution was then
diluted to about tenth normal.

To the dilute bromide solution, with continual agitation, was added this
dilute argentic solution; the total volume including the water used in
rinsing usually amounted to about 1.5 liters. After shaking steadily for
15 minutes, and occasionally for a day, the mixture was allowed to settle
during another day. When clear above, about 0.05 liter of the aqueous
solution was withdrawn and tested in the nephelometer, one tube being
treated with an excess of bromide, the other with an excess of argentic
nitrate. If any difference in the opalescence was noticeable after due time
had been allowed for the very faint clouds to attain their maxima, the
slight deficiency was made up in the flask by means of solutions containing
approximately 1 milligram of argentic nitrate or of potassic bromide per
milliliter. These additions were continued till equality in the opalescence
was attained. From the sum of the original weights and subsequent addi-
tions, the total amounts of bromide and silver were obtained ; from these,
reduced to vacuum, the ratio was calculated. All the analyses which were
performed are given in the table, excepting No. 2, which was rejected for
just cause before it was finished. Vacuum corrections of -f- 0.00029 for
every gram of potassic bromide and — 0.00003 for every apparent gram

iRichards and Wells, Am. Chem. Journ., 31, 235 (1904).
2Richards and Parker, Proc. Am. Acad., 32, 60, 1896.

42

THE ANALYSIS OF POTASSIC BROMIDE.

of silver were applied. Most of these experiments were made before
those given in the preceding table, a fact which may account for the
slightly less satisfactory agreement of the individual results. Because the
deviations could not be traced to any definite cause of disturbance, they
must be ascribed to accident.

The Ratio of Potassic Bromide to Silver.

Experiment
number.

Weight of KBr
in vacuum.

Weight of Silver
in vacuum.

Parts of KBr
corresponding to

Atomic Weight of
Potassium

100.00 parts Ag.

if Br = 79.953.

Grams.

Grams.

1

4. 33730

3.93164

110.318

39. 113

3

4. 18763

3. 79587

110. 320

39.115

4

4. 15849

3. 76943

110. 321

39. 116

5

3. 67867

3. 33450

110.321

39.116

6

3. 60484

3. 26776

110.315

39. ] 10

7

4. 78120

4. 33387

110. 322

39. 118

8

5.67997

5. 14860

110. 321

39. 116

9

6. 41587

5. 81571

110. 320

39.115

13

2. 88134

2. 61184

110.318

39.113

14

3. 64383

3. 30309

110.316

39.111

15

3. 12757

2. 83504

110.318

39. 113

Total average

110.3190

J39. 1143

Probable error

0. 0004

0 . 0004

1 The average 39.1143 is calculated from the average 110.3190, not from the average of the individual values of
the atomic weights in the column above. The difference is of course very slight.

The "probable error" is as small as before because of the greater
number of determinations ; and the mean deviation from the average value
is only one in the last decimal place. It will be observed that these results
point to the limits 39.113 and 39.115 as the extreme values between which
the atomic weight of potassium must fall, in essential agreement with the
previous results.

Marignac's seven experiments on this ratio give values ranging from
110.303 to 110.369, while Stas's fourteen results ranged from 110.332 to
110.361.

Obviously the results furnish a means of calculating the atomic weight
of bromine, when taken in connection with the foregoing series, entirely
independent of any other work. Thus Br = (110.319/63.3727 - - 1.00000)
107.93 = 79.954, a value almost identical with Baxter's value, 79.953. This
is excellent proof that the bromine used in the present research was pure,
and that the occlusion of electrolytes by argentic bromide was small.

THE ATOMIC WEIGHT OF POTASSIUM. 43

THE ATOMIC WEIGHT OF POTASSIUM.

The preceding paper and the present one together yield four ratios
determined with modern precision, which together fix the atomic weight
of potassium as definitely as could be expected. The respective values are
as follows:

From the ratio of argentic to potassic chloride K = 39. 11 34

From the ratio of metallic silver to potassic chloride .... K = 39. 1145

From the ratio of argentic to potassic bromide K = 39. 11 35

From the ratio of metallic silver to potassic bromide .... K = 39. 11 43

Average atomic weight of potassium, if Ag= 107.930 . . . K = 39. 11 39

These figures are interesting and significant. The maximum departure
from the mean is only 1 part in 70,000, and such differences as exist in the
figures are explicable. It is likely that the slightly lower value given by
the first member of each pair of series was due to a trace of occlusion of
potassic nitrate by each of the precipitates — a circumstance which can not
be absolutely prevented. Therefore the higher values, averaging 39.1144,
are more probable. The differences are, however, wholly negligible at
present.

In this connection it is interesting to note that Clarke in 1897, from a
miscellaneous collection of partly uncertain results obtained by others,
decided upon the almost identical value, 39.112,1 although at the same time
from similar results he obtained values much too low for chlorine and
iodine, somewhat too low for bromine, and much too high for sodium.

The very close mutual agreement of the new results obtained from two
different compounds is a satisfactory verification of the relative values
yielded by the recent work on the atomic weights of chlorine and bromine.2
The value of chlorine found from the work on potassium chloride was
35.475 ; and that of bromine from the present work is 79.954. The ratio
of chlorine to bromine is thus found to be 0.44369, whereas the ratio com-
puted from the work of Richards and Wells and of Baxter is 0.44367.

Against such an accumulation of concordant data as that just presented,
the older figures can have no important weight. Whatever may have been
the cause of the irregularity and internal inconsistency of Stas's results
with potassic chloride and bromide, there seems to be little reason to
doubt that the outcome of the present investigation, 39.114, really repre-
sents the atomic weight of potassium.

It is needless to point out that this change in the atomic weight of potas-
sium will affect many other atomic weights.

Recalculations, p. 57.

2Richards and Wells, loc. cit; Richards, Trans. Amer. Phil. Soc., 43, 116 (1901) ;
Baxter, loc. cit.

44 THE ANALYSIS OF POTASSIC BROMIDE.

SUMMARY.

This investigation upon the atomic weight of potassium presents, among
other considerations, the following additions to the knowledge of the
subject:

(1) The problem of preparing pure potassic bromide was solved in
two ways.

(2) An unusually satisfactory method for preparing pure potassium
hydroxide was developed. This method is applicable to other alkalies, and
will be described elsewhere in greater detail.

(3) The ratio of silver to potassic bromide was redetermined, and
found to be 100.000: 110.319. The atomic weight of potassium was thus
found to be 39.1143, if silver is 107.930 and bromine 79.953.

(4) The ratio of argentic bromide to potassic bromide was found to be
100.000: 63.373. This determination yielded an essentially equal value,
K = 39.1135.

(5) These values confirm in a striking manner the simultaneously
executed work upon potassic chloride, and unite with them in showing
that the atomic weight of potassium is 39.114.

(6) By thus agreeing, these four values support the new value for the
atomic weight of chlorine in relation to silver and bromine.

Ill

THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE
AND THE ATOMIC WEIGHTS OF NITROGEN

AND SILVER

BY THEODORE WILLIAM RICHARDS AND GEORGE SHANNON FORBES

CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF HARVARD COLLEGE

THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, AND THE
ATOMIC WEIGHTS OF NITROGEN AND SILVER,

INTRODUCTION.

The composition of argentic nitrate is one of the questionable premises
in the lively argument which has recently taken place concerning the
atomic weights of nitrogen and silver.1 Although Stas's syntheses of this
salt were carried out on a large scale, and far more carefully than those of
anyone before him, several points concerning the details of the work were
not investigated with the care which modern physicochemical knowledge
demands. Accordingly, a repetition of this work of Stas's seemed to be
worth the trouble involved ; and the following pages contain a brief
account of nine months' thought and labor upon it.

The method appears at first sight to be extremely simple, consisting
merely in the weighing of pure silver, the dissolving of this silver in nitric
acid, and the weighing of the resulting nitrate. The preparation of pure
silver having been already solved, the great difficulty consisted in the
procuring of satisfactory evidence that the nitrate was free from impurity,
and in making sure that none of the silver was lost during the process.
The main emphasis of the subsequent discussion will therefore be laid on
these points, the other details being often indicated with but few words,
because they so closely resemble the details of previous investigations
carried out in the Chemical Laboratory of Harvard College.

The research naturally divided itself into four sections, namely, first,
the preparation of pure materials ; second, the quantitative synthesis ;
third, the determination of the purity of the product ; and fourth, the final
result and its relations to the table of atomic weights. These will be con-
sidered in order.

PREPARATION OF PURE MATERIALS.

All the substances used in the research were purified with very great
care. Nitric acid and silver were the two most important.

Nitric acid was supplied by two firms. Each sample was warranted
by the manufacturers to be of a very high grade of purity. Each was
redistilled twice just before adding to the silver, using only the middle

especially Guye, Nouvelle Rech. s. I. Poids Atom, de 1'azote, Soc. Ch., Paris,
1905. The reader is referred for a convenient resume to the Report of the Interna-
tional Committee on Atomic Weights, Journ. Am. Chem. Soc., 28, 1 (1906), and
many other places; also to Clarke, Journ. Am. Chem. Soc., 28, 293 (1906); Gray,
Trans. Chem. Soc. (London), 89, 1173, (1906).

47

48 THE QUANTITA\7E SYNTHESIS OF ARGENTIC NITRATE, ETC.

portion of each distillate. None of the samples left a trace of nonvolatile
residue on evaporation. No difference could be detected in the results
because of the difference of source of the acid, nor was the constancy
destroyed when a single distillation only was made. (Experiment 11.)
Hence little anxiety was felt concerning the purity of this material.

Silver. - - The researches of Richards and Wells have shown how to
prepare silver of unquestioned purity. Preliminary determinations 1, 2,
3, 4, 5, 7, and 8 were made on a sample (A) left over from the above-
mentioned research, crystallized fifteen times with nitric acid, precipitated
with formate, and fused on purest charcoal, but not electrolyzed or fused
in hydrogen. No. 9 was made with a sample prepared by Professor
Baxter for his final work on bromine. It had been through the chloride
and formate treatment, electrolyzed, fused in hydrogen, cut with a saw,
etched with nitric acid, boiled with water, dried at dull redness in vacuo,
and given us ready to weigh. We are much indebted to him for furnishing
this check on our silver. The final determinations were made with silver
derived from several preparations. The first of these (C) was recrystal-
lized sixteen times as nitrate from water and redistilled nitric acid, and
then precipitated twice in succession with formate. A part of the final
formate product was fused on the best lime in carefully purified hydrogen
made from aluminum and sodic hydroxide. Sample D had been precipi-
tated as chloride, reduced with best alkaline sugar, washed free from
chloride, dissolved in nitric acid, filtered and crystallized as nitrate in
platinum six times from nitric acid distilled in a platinum condenser.
Centrifugal treatment eliminated the mother liquor. The last crop of
crystals was precipitated with formate in a silver dish, washed free from
ammonia, and fused in a cup of pure lime. Sample F was obtained by
the electrolysis of a button of very pure silver from Colorado, which had
been fifteen times recrystallized as nitrate, precipitated with formate, and
fused on lime. Samples C, D, and F were all cleaned by etching and then
purified by electrolysis through a nitrate solution made from some of the
same silver and the purest nitric acid. They were then fused separately,
on a well-seasoned lime boat, in a new porcelain tube, in an atmosphere
of pure electrolytic hydrogen. The Hempel stoppers fitted so well that
the pressure could easily be reduced to a fraction of a millimeter by a
Geryk oil-pump when desired. The heating was accomplished by a large
Heraeus electric furnace which fused the silver without overheating any
part of the tube. In all cases the initial fusion was completed in hydrogen
at atmospheric pressure, but in half the fusions the tube was evacuated
before the temperature was lowered. No spurting or boiling could be
observed through the glass window when the pressure was reduced, and

PREPARATION OF PURE MATERIALS. 49

the silver thus prepared gave the same combining weight as that cooled
under a full atmosphere's pressure of hydrogen. Thus the conclusion of
Richards and Wells and of Baxter that silver can not dissolve a weighable
amount of hydrogen was confirmed.

The buttons thus obtained were etched to remove lime, and if too large
to go into the flasks used for the synthesis, were cut with a cold chisel or
a jeweler's saw, observing all the precautions recommended by Richards
and Wells. The fragments freed from superficial iron were washed and
dried, sometimes in the electric oven in air at 150° for an hour, sometimes
in a vacuum at dull redness, and sometimes in a reduced pressure of hydro-
gen. Judging from the constant combining weight, all these methods
were equally good.

Water. — The water was distilled first with alkaline permanganate
through a glass condenser, and then, after the addition of a small drop
of dilute sulphuric acid, through a carefully cleaned condenser of block
tin. Needless to say, dust was excluded as much as possible, and distilla-
tions were conducted immediately before the water was needed, in order
to avoid absorption of gases or solution of solid matter.

Air. — It will be remembered that Stas allowed his solutions of argentic
nitrate to evaporate very slowly, the vapors diffusing out through the neck
of his flask and condensing in a suitable receptacle. Under these condi-
tions the evaporation required seventy-two hours of continuous heating —
an unnecessarily prolix process. To hasten the escape of aqueous vapor,
it was resolved to maintain a gentle current of pure dry air throughout the
process. The air for this purpose was delivered from a water pump,.
which must have removed some of the original impurities. It was passed
first through a tall Emmerling tower filled with beads moistened with
concentrated sulphuric acid, to which a trace of potassic bichromate had
been added, and then through two more towers filled with a concentrated
solution of pure potash, being thus freed from ammonia and from acid
gases. Next it was passed through a tall drying tower of stick potash, to
a hard glass tube containing platinized asbestos heated to dull redness by
a Bunsen burner. The hot platinum was intended to destroy organic
matter. Hence it passed into a trap designed to catch asbestos shreds,
and finally, the air was dried by two towers of broken potash. The whole
purifying train was put together without rubber, all the pieces being
blown together except the hard glass tube, which was connected with the
soft glass on either side with joints so well ground that no lubricant was
necessary. The towers were glass-stoppered. Air thus purified and dried
contains nothing to injure the silver nitrate except possibly a minute
trace of ammonia, which might have come out of the potash solutions.

50 THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

THE SYNTHESIS OF ARGENTIC NITRATE.

The materials having been prepared, the next step was to combine them.
This combination must take place in a vessel wholly free from a sus-
picion of solubility on the one hand, and arranged so as to prevent loss of
material on the other hand. At the same time, it was important to be able
to evaporate and to weigh the argentic nitrate in the same vessel. After
a careful consideration of the somewhat conflicting claims of these require-
ments it was decided to carry out the process in small round-bottomed
flasks of fused quartz of from 0.03 to 0.04 liter capacity ; with necks
11 cm. in length and 6 to 9 mm. in internal diameter. Collars of platinum
wire provided with loops permitted the suspension of these flasks from
the hook of the Troemner balance. Before weighing, they were heated in
the oven to be presently described to 250° for at least half an hour, a
moderate air-current sweeping through them all the time. The cover-
ings were then lifted, and each flask was removed by a hook of platinum
wire sealed into a glass rod, and transferred to a large desiccator pro-
vided with a suitable support. Two flasks similar in weight and surface
were treated in the same way and kept in the same desiccator. After
two hours near the balance the first flask was weighed against a tare, its
weight being determined by substitution of the first by the second flask
and a few small weights. Upon removal from the desiccator both flasks
absorbed moisture from the air, often to the extent of 0.0001 gram, but
they soon reached constancy with respect to one another, which was all
that was needed. No final weighings were made when the hygrometer
in the weighing-room stood above 40, and under these circumstances
flasks could be weighed on successive days with variations not exceeding
0.00003 gram.

A new Troemner balance, sensitive to 0.01 mg. was available for the
research. The brass weights were carefully standardized at the begin-
ning of the year, and again upon starting the final series ; and the rider
also was calibrated, and the necessary correction employed. For cor-
rection to the vacuum standard 0.000132 was added to each apparent
gram of argentic nitrate, and 0.000030 was subtracted from each apparent
gram of silver. This was on the assumption that the two specific gravities
concerned were respectively 4.35 and 10.49, that of the brass weights
being 8.3.

The silver was now weighed by substitution, and carefully pushed
down the neck of the flask, which was held meanwhile in a horizontal
position to avoid breakage. Next the whole was transferred to a large,
clean, empty desiccator, whose sides had been moistened ; and the flask was
laid on a suitable support of glass and platinum tilted 45° from the ver-

THE SYNTHESIS OF ARGENTIC NITRATE.

51

\

tical. A sufficient quantity of purest nitric acid mixed with half its
volume of distilled water was poured in from the platinum crucible into
which it had just been distilled. The cover of the desiccator was replaced,
but its stopper was first removed, and covered with a watchglass and a
clean beaker. The apparatus was now kept all night in a warm place,
between 40° and 50° C. ; thus the silver when dissolved remained in solu-
tion, though the volume was restricted to 2 cm. of solution per gram of
nitrate. In the morning the process was found to be complete. After
washing down the neck with a few drops of freshly distilled water, the
flask was placed in the oven as before. The desiccator was washed with a
little water, the washings being tested for silver in the nephelometer ; if
any had been found it might have been feared that more had escaped, but
as no trace was ever found, it was inferred that the
long inclined neck had caught all the spray.

Attention is now called to the apparatus for evapo-
ration.1 The main feature of this was the combined
delivery tube for the air current and hood for the
protection of the contents of the flask. This hood H
with its ingress and egress tubes, / and E, is shown
in the diagram (figure 1). The dry air used to
sweep out the aqueous and nitrous vapors entered
from the purifying train at G, where connection was
made by a well-ground joint; passing through the
tube /, it escaped into the body of the flask and out
through its neck. A side tube, E, was attached to
H for connection with a water-pump. The flask
rested on a triangle of platinum wire attached to a
glass tripod, and the whole was placed in a 1.5-liter
beaker which served as the oven. A sand-bath en-
abled this to be kept at any desired temperature.
The cover of the beaker was of sheet copper, pierced
with three holes, for the tubes / and E, and for the
thermometer. As no nitrous fumes escaped into
the oven, this cover remained intact. In the prelimi- Fig. I. -Apparatus for Ev?Po-

. , , . - rating Aqueous Solutions.

nary trials the extra tube Q was lacking, the tube Section about half actual size
/ being extended to take its place. The part which
had projected into the body of the flask was treated
with hot dilute nitric acid and investigated for silver

H

by platinum wire, serv-
ing as air-nozzle.

1In passing it might be, noted that one of us used a somewhat similar arrange-
ment for evaporating solutions of sodic sulphate fifteen years ago (Proc. Am.
Acad., 26, 258 (1891).

52 THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

in the nephelometer. Occasionally, some was found, but the corrections
required were not large. Nevertheless, the possibility that the fused
nitrate had attacked the glass was always disturbing, and the inconven-
ience and uncertainty of this correction made its elimination desirable.
Accordingly, in the final determinations, a section of quartz tubing, Q,
3 mm. in diameter and 15 mm. long, was firmly attached to a platinum
wire and lowered till it barely projected into the body of the flask. This
supporting wire hooked into the collar, so that the quartz tube could be
removed while the silver was dissolving. During the evaporation, how-
ever, it was in place, and the end of the glass delivery tube was shortened
and drawn down to fit into it. The glass tube was still tested as before,
but no silver was ever recovered from it. Hence the flask and the quartz
tube must have retained all the silver originally weighed out.

The evaporation of the liquid was easily and quickly conducted in this
apparatus. If the flask was not more than half full and the air passed
through the drying train no faster than two bubbles per second, it was
possible to maintain the temperature of the oven at 125° without risk of
ebullition. The vapors drawn out as before described passed first through
a U-tube filled with moist glass beads and just sealed with water, after-
wards through a tower of caustic alkali to take out nitric acid which might
attack the water-pump. When the liquid became coated with a film of
crystals the temperature was lowered to 110°; bubbles of vapor now
formed gradually under the crystal layer, and the slight spattering caused
by their rupture could be seen on the inside walls of the flask. The oven
was now suddenly cooled, so that the vapors from the hot liquid, condens-
ing on the walls of the flask, washed down the crust which had formed
during evaporation. Upon continuing the heating, a porous crust was
soon formed, under which the formation of steam caused no spattering.
Finally, the temperature was raised gradually while the last trace of
liquid was disappearing. The solid was dried for a quarter of an hour
at 150° in the dark, and then heated to 23001 to insure complete fusion,
not only of the main portion, but also of any portions projected on the
upper walls of the flask. The temperature was soon lowered to 210°,
which sufficed to keep the mass in fusion, and the air current was con-
tinued for an hour. Then, in subdued light, the flask was lifted out
by a loop of platinum wire passed through its collar, placed on a net-
work of platinum wire, and tilted in all directions to cause the nitrate to
solidify in a thin layer on the walls. Once this precaution was neglected
and the flask was cracked by the contraction of the nitrate on cooling.

1This temperature and the others to be recorded later have not been corrected for
the exposed steam. They do not indicate the exact meeting-point of the salt.

THE SYNTHESIS OF ARGENTIC NITRATE.

53

When the nitrate was wholly solid but still hot, the flask with its con-
tents was transferred to its support of platinum wire in a large, tight
desiccator, which was wrapped in a black cloth and left near the bal-
ance over night. Then the weight of the flask was determined as before.
Thus in the entire cycle of operations nothing but platinum, clean glass,
and pure air touched the outside of the flask.

It remains to be proved that no silver was carried out by the air cur-
rent. This was inferred because no trace of silver was ever found in the
U-tube through which all the vapors passed; but more definite evidence
was obtained by redissolving two finished determinations, and evaporat-
ing them again with the customary precautions.

Determination
number.

AgNOs once
evaporated.

AgNOs twice
evaporated.

AgNOa thrice
evaporated.

Total gain.

8
15

10. 68933
14.20114

10. 68928
14. 20122

10. 68920
14. 20127

— 0. 00013
+ 0. 00013

Average gain

or loss

-f- 0 00000

The first residue in the case of No. 15 was slightly darkened by pro-
longed heating ; still no fixed tendency toward either loss or gain is shown
by these figures, and the reliability of the method is demonstrated.

Two preliminary experiments were not finished, and one was rejected
because of known error. Six preliminary determinations carried out in
the way just described gave results varying from 157.483 to 157.475
parts of nitrate from 100.000 parts of silver, in the mean 157.479. This
is very nearly the same as the final series, given later, but because
of sundry irregularities these determinations are individually much less
trustworthy than the latter. One of the preliminary determinations was
made with a piece of silver kindly given us by Professor Baxter for
the sake of comparison, and for which we are much indebted. It gave
a result perhaps higher than the average, but not by an amount greater
than the limit of error of the experimentation at that time. It is greatly
to be regretted that this piece was not examined after every detail of the
process had been perfected, because the comparison would have been in-
teresting, although not at all essential for the completeness of our work.

When it appeared impossible to improve upon the details of the experi-
mentation, a final series of six consecutive determinations was carried
out with all possible care. In each case, except Nos. 14 and 15, the argen-
tic nitrate was maintained for an hour in a fused condition while dry air
swept over it. In No. 14 only one-quarter of an hour was allowed,
whereas in No. 15 the fusion was prolonged for three hours. Sample D

54

THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

of silver was used in syntheses 10, 11, and 12, and sample F in the
other three.

The Synthesis of Argentic Nitrate.

No. of
synthesis.

Weight of
fused silver
(in vacuum).

Weight of
fused argentic
nitrate
(in vacuum).

Weight of argen-
tic nitrate made
from 100.000
parts of silver.

Grams.

Grams.

10

6. 14837

9. 68249

157.481

11

4. 60825

7. 25706

157.480

12

4. 97925

7. 84131

157.480

13

9.07101

14. 28503

157. 480

14

9. 13702

14. 38903

157.481

15

9. 01782

14. 20123

157.480

Av6r&ff6

'157.480

probable error of this average, computed from the results when car-
ried to the next decimal place, is only 0.0001, a wholly negligible quantity.
Hence repetition of the process was unnecessary.

For this ratio just found to be 100.000 :157.480, Stas obtained an aver-
age of 100.000:157.475 from nine determinations, which ranged from
157.463 to 157.488.

Better agreement than that exhibited by the above table could hardly
be desired or expected, as the greatest deviation corresponded to less
than 0.1 mg. in the weight of the argentic nitrate. This series demon-
strated that it is not necessary to use very large amounts of material
in order to attain a very high order of precision, if only the details of
experimentation are fittingly arranged.

Satisfactory as this series of results appears to be, it is by no means
to be accepted without further question as representing the true weight
of argentic nitrate to be obtained from pure silver. Even in this fused
salt, prepared under such favorable conditions, several impurities might
exist; and because in each case the method of treatment was the same,
these impurities might be constant in amount and therefore not perceiv-
able in the results. Accordingly, attention was now directed to the search
for these impurities; and this part of the investigation was found to be
the most arduous and time-consuming part of it. The following section
discusses this matter.

THE PURITY OF THE FUSED ARGENTIC NITRATE.

55

THE PURITY OF THE FUSED ARGENTIC NITRATE.

The first impurity for which search was made was air. While crystals
obtained from solution, although always containing solvent, may be sup-
posed to be free from air, the fused material can not without question
be assumed to be free from this impurity. The only manner of approach-
ing this question seems to be to fuse the salt a second time in a vacuum,
in order to detect a possible loss of weight. This Stas did in a single case,
with a negative result. Nevertheless, a single experiment, even of
Stas's, does not carry with it much weight; therefore the test deserves
repetition.

This was carried out in the present case without great difficulty on
five of the preliminary determinations. The flasks containing the argen-
tic nitrate which had been fused for an hour in air were lowered into
mammoth test-tubes — long tubes of soft glass 4.5 cm. in diameter —
sealed at the lower end and scrupulously clean. A hood was placed over
the mouth of the flask to prevent fragments of glass from entering, and
the tube was drawn down in two large converging blast flames. A glass
tube was fused on and connected with an efficient mechanical hand
pump; several exhaustions, followed by admission of dry air, excluded
all moisture. Finally, the system was reduced to 2 mm. and sealed, and
heated to 210°. After having been maintained in a state of fusion in the
dark for half an hour, the salt was cooled in the usual cautious fashion,
and air was admitted very slowly. The flask was placed in a desiccator
over night and weighed. The product was always slightly discolored,
indicating decomposition, but showed no serious loss in weight.

*

The Effect of Fusion in Vacuum.

Determination
number.

Original
weight.

Weight after
fusion.

Change.

Grams.

Grams.

1

8. 04489

8.04477

— 0.00012

2

8. 75813

8. 75810

— 0. 00003

3

8. 54170

8.54170

db 0. 00000

4

9. 87850

9. 87837

- 0. 00013

9

10. 76381

10. 76374

— 0. 00007

Forty-five grams of salt lost in all 0.00035 gram. This is less than 1
part in 100,000, and may safely be referred to the trace of decomposi-
tion indicated by the pale-brownish coloration. Not only is it reasonable
to suppose that but little air is dissolved (for according to the law of
Henry nearly all of it should have been expelled by this treatment) but
also one is led to infer that very little water remains to be expelled. This

56

THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

matter is not, however, so simply settled ; the final testing of it cost much
labor.

It has been ordinarily assumed that water is entirely expelled from crys-
tallized or evaporated salts upon fusion. This is in all probability the
case with sodic and potassic chlorides, and other salts which fuse at high
temperatures and show little affinity for water. Whether or not this was
the case with argentic nitrate, which at 210° is miscible with water in all
proportions, was a question which required experimental investigation.

Besides the conservation of weight on fusion in a vacuum another argu-
ment may be adduced to show that most of the water has been driven out.
The combining weight of argentic nitrate seems to be nearly independ-
ent of the time of its fusion in a current of dry air, as is shown by the
following table.

Determination.

Time kept in
fusion.

Weight of AgNOs
formed from
100 parts Ag.

14
9 10 11 12 13

15 minutes
1 hour

157.481
157. 480

15

3 hours

157. 480

It should be stated that the last residue was slightly discolored, and
increased 0.00008 gram upon a subsequent evaporation with water and
nitric acid ; hence a real loss of weight during the extra period of fusion is
indicated; but this was probably due to decomposition of the salt rather
than to escape of water ; therefore the table bears out the contention stated
above. In any case this loss amounts to much less than a unit in the last
decimal place.

Ordinarily, in the past, the investigator has been satisfied with such an
outcome and has gone no further. But in this case we were anxious to
leave no stone unturned; and accordingly a drastic method of treatment
was adopted, which permitted no trace of water to escape being weighed.
This was to decompose completely the argentic nitrate by heat, and to pass
the gaseous product of the decomposition through a weighed tube contain-
ing phosphoric oxide.

A hard glass combustion tube was bent and drawn out in the manner
shown in figure 2, the space between C and D being packed with glass
wool. Into the space between B and C was then introduced about 50
grams of argentic nitrate which had been crystallized from nitric acid,
barely fused in porcelain, cooled, and pounded in a mortar. The tube
was heated to 220° in an air-bath for an hour in a stream of dry air passed
in through A. The argentic nitrate should now be in a condition some-
what comparable to that of the quantitative determination. Finally, a

THE PURITY OF THE FUSED ARGENTIC NITRATE. 57

weighed pentoxide tube was attached at D, and the temperature raised to
500°. Yellow oxides of nitrogen came over, also a fine gray dust caused
presumably by the bursting of
bubbles of decomposing salt.
After an hour the pentoxide

tube was removed, swept out Fig 2 _Tube ^ DecomposiL Of Arctic Nitra«e.
with air, and weighed again. aboul one-el«hth actual 81ze-

A trace of the gray dust had been carried in, and suggested a slight
gain in weight, but by no means enough to account for the great
increase of weight, 0.012 gram. This was finally traced to the adsorption
of nitrous fumes by the pentoxide, 2 mg. being lost upon passing dry
air over the tube for an hour, and the rest giving a very strong test for
nitrous acid after being dissolved in water.

In the effort to eliminate this seriously disturbing effect, recourse
was next had to spirals of copper gauze heated in a combustion tube, in
order to abstract the oxygen from the nitric oxide, leaving only nitrogen.
It is well known that metallic copper adsorbs hydrogen ; hence the spirals
were first superficially oxidized in a stream of dry air, and then reduced
by pure dry carbon monoxide. This was generated by heating the purest
oxalic and sulphuric acids of commerce together, the carbon dioxide being
absorbed by a generous train of pure concentrated potash solution, and the
monoxide dried first by a tower of concentrated sulphuric acid, with
beads, and finally by two towers of pounded potash. The resulting cop-
per should be above the suspicion of containing any considerable amount
of hydrogen or moisture. The argentic nitrate was treated as before,
and the decomposition was continued until traces of yellow fumes were
noted coming past the copper. This meant that nitric oxide had been
coming over for some time, since the last part of oxygen is harder to
detach than the first. When swept out with air, the gas in the pentoxide
tube turned yellow, but it was not then supposed that much nitric oxide
could be absorbed by the pentoxide in so short a time. Nevertheless,
nitrous acid was found upon solution, even although the gain in weight
was only a third as great as before.

Efforts were now made to reduce accidental water from rubber, etc.
All joints were made to overlap, so that the rubber surfaces exposed to
nitric oxide might be small, and large asbestos screens kept them from
being overheated by the adjacent furnaces. Fresh sublimed pentoxide
was used in the weighing-tube, which was carefully imitated as to volume
and surface by a similar tube, to be used as a counterpoise in weighing;
both were opened after coming to the temperature of the balance-room,
wiped with a clean, slightly damp cloth, and weighed by substitution. A
substantial plug of glass wool in the decomposition tube strained the gases

58 THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

before passing over the copper ; it had been heated in the dry air to remove
moisture.

To eliminate the possibility of water in the copper gauze and to test the
apparatus, a blank run was made, removing the decomposition tube, and
oxidizing the copper with dry air. Here the pentoxide tube lost 0.0004
gram, a result which indicated no error in that part of the process.

It is not worth while to record the individual experiments by which the
gradual improvements in apparatus and manipulation were tested, tire-
some and time-consuming as these were in execution. By the time seven
experiments had been performed, it was clear that the fused argentic nitrate
could not contain over 0.004 per cent of water. Even of this small amount
a part was undoubtedly due to nitric peroxide ; for the copper gauze was
not enough to effect a complete decomposition of the gases. Passing car-
bon monoxide through the apparatus while the decomposition was in prog-
ress lengthened the effective life of the reduced copper, but introduced
other complications; and the addition of a second tube of hot gauze, a
meter long, was still inadequate.

The next step was to secure a copper surface so large that all the nitro-
gen oxide resulting from the complete decomposition of 50 grams of
argentic nitrate could be removed, and this without increasing the compli-
cations of the apparatus. Powdered copper oxide looked promising as a
source of this metal, but it must be made from pure materials to avoid
the presence of detrimental impurities. Pure electrolytic copper (Merck)
was dissolved in nitric acid, evaporated to dryness in porcelain, and very
gradually ignited, with constant stirring, to powdery oxide. A hard glass
tube 30 cm. long and 3 cm. internal diameter was used to contain it ; the
ends were drawn down conveniently and the oxide was reduced at a very
moderate heat with carbon monoxide, prepared as usual, except that it
was finally dried with pentoxide. Nearly 300 grams of copper oxide were
present ; therefore, the process was very tedious, since the gas had to pass
through its purifying train at a moderate rate of speed. With this large
mass of finely divided copper the glass wool (from C to D in the dia-
gram) was no longer necessary, therefore it was no longer used. No trace
of powdered nitrate ever appeared beyond the copper.

A clean pentoxide tube, with glass stopcocks, was refilled with resub-
limed pentoxide and glass wool ; dry air was passed through for ten min-
utes to assure constancy. A duplicate tube was also prepared for a coun-
terpoise, and extraordinary precautions were taken in protecting and
weighing these tubes. As another precaution it seemed also expedient to
prepare argentic nitrate nearly as pure as that used in the determinations
for fear of hygroscopic nitrates, silicic acid, and ammonium salts, which
would yield water when decomposed. A moderately pure salt was recrys-

THE PURITY OF THE FUSED ARGENTIC NITRATE.

59.

tallized from nitric acid, filtered, precipitated with formate, washed free
from ammonium salts, and fused on lime in the blast. The buttons were
etched and dissolved in freshly distilled nitric acid in Jena glass, and
the salt was crystallized with ample protection against ammonia, dried
centrifugally and kept in a clean, tight desiccator. There was no ammonia
in this product. Before using it was fused in platinum.

In the first determination of this series the argentic nitrate was heated
too hot (520°) and the gases came over so fast that the reduced copper
was only superficially attacked ; even so it was vastly more efficient than the
gauze. Because, however, some nitric peroxide still escaped, this experi-
ment also had to be rejected, a part of the 0.004 per cent impurity which
it indicated being undoubtedly due to nitrous fumes.

The apparatus was now improved by sealing off the end (A) of the tube
after the argentic nitrate had been fused for a long time in a current of dry
air. Moreover, the last trace of rubber exposed to nitrous fumes was
eliminated by providing an accurately ground joint between the decom-
position tube and the copper oxide. The process was conducted so slowly
at 490° that the argentic nitrate could be entirely decomposed to pure
spongy silver without exhausting all the copper. Care was taken to have
the very pure argentic nitrate in a state comparable to that used in the
determinations. The salt after its preliminary treatment in platinum still
contained a few crystals of unfused nitrate ; accordingly in the tube it was
heated evenly at 220° with shaking to dislodge steam bubbles, exactly as
the determinations had been. Under these conditions reliable results
were obtained. In No. 25, a second weighed pentoxide tube placed in
tandem with the first showed no gain in weight; hence the first tube
caught all the water vapor. The remaining nitrogen was driven out by
pure dry air before weighing. That water vapor was actually present
was shown by the "melting" of phosphoric pentoxide in the first tube.

Water Obtained by Decomposition of Argentic Nitrate.

Experiment
number.

Weight of
AgNOs.

Gain in weight
of drying tubes.

Percentage of
water found.

25

26
27

Average...

Gram

53
52
44

Gram

0. 0015
0. 0011
0. 0014

Percent

0. 0028
0. 0020
0. 0033

50

0. 0013

0. 0027

Thus it is clear that less than 0.003 per cent of water was held by the
argentic nitrate after fusion. There was only one other source from
which this milligram or so of water might have come, namely, from the

60 THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

copper used for the reduction of the nitrous fumes. Less than 0.0002
gram of hydrogen retained by 300 grams of copper, or less than one part
in a million, would have been enough to cause this effect, hence it seemed
rash to assume that the water did not come from this source. As much
hydrogen as this might have come from a trace of moisture in the hun-
dred liters of carbon monoxide used for the reduction of the copper.

The question thus raised was capable of being investigated. For this
purpose the mass of copper, reduced just as it would have been for one of
the preceding experiments, was twice oxidized by a very large volume of
pure ignited air dried with the usual potash towers and finally with phos-
phoric pentoxide. This air, having been already passed over red-hot
cupric oxide in a hard glass tube and thoroughly dried by pentoxide, could
hardly bring with it any water which could be taken up by the following
tube of the same material. On one of these occasions, 0.0009 gram of
water was found, and on another 0.0007 gram. Possibly some of this may
have come from the atmosphere during the manipulation of the tubes ; but
a similar amount must be supposed to have been taken during each of the
previous determinations. Hence it seems to be permissible to subtract
the average 0.0008 gram from the average result of experiments 25, 26,
and 27, namely, 0.0013 gram. This leaves only 0.0005 gram as the maxi-
mum amount of water held by 50 grams of carefully fused argentic nitrate,
introducing an error of only 1 part in 100,000. As the average of
the final series led to a value a trifle over 157.480 grams as the weight of
nitrate obtainable from 100.000 grams of silver, the correction to be applied
for water reduces the result only to 157.479. Possibly not even as much
water as this was present in the quantitatively made argentic nitrate,
because the relative surface exposed for drying was not so great in the
long, narrow tube just used as it was in the quartz flask employed for
experiments 10 to 15.

This unimportant change in the synthetic result from 157.480 to 157.479
was the only apparent outcome of these tedious and often exasperating
experiments on the decomposition of the nitrate ; but in reality more was
shown by them. The experience furnished to the experimenters a strik-
ing example of the essential importance of taking as much care in deter-
mining a small correction as in determining the quantity to be corrected.
It moreover confirmed the impression that except in cases like the chloride
of zinc, when water acts chemically upon the substance, little or no water
is retained by most fused salts. If argentic nitrate, which at 200° is misci-
ble with water in all proportions, sets free so nearly all that it possesses
in a current of dry air at this temperature, it is much more likely that
other salts, fusing at a higher temperature and possessing a less attraction
for water, should be free from it after fusion. This is a reassuring con-
viction, well worth the trouble spent upon the point.

THE PURITY OF THE FUSED ARGENTIC NITRATE. 61

Attention was now directed to other foreign substances besides water
and air which might be present in the synthetic argentic nitrate. Further
careful investigation was needed to show that all the silver was combined
as nitrate and that no other impurities were concealed in the salt. The
pure pearly whiteness of the cold fused material precluded the possibility
of there having been a trace of reduction to metallic silver. The salt was
soluble in pure water without residue or turbidity, and maintained its
clearness when diluted to half or quarter normal ; hence oxide and halogen
salts must have been absent. The only other likely impurity seemed to be
ammonium salts, nitrite, and excess of nitric acid. Of these possibilities,
Stas thought only of the likelihood of the presence of free acid ; but each
was carefully considered in the present work.

It is known that copper dissolving in nitric acid forms perceptible quan-
tities of ammonic nitrate. Apparently, no similar reaction has been noted
with silver, and from an electrochemical standpoint it seemed unlikely;
moreover, ammonic nitrate if present would probably be for the most part
decomposed by the hour's fusion of the silver salt. Nevertheless, thor-
oughness demanded evidence on this point. Nessler solution is the most
convenient for this test, but silver must first be precipitated from solution
in the dark. Light generates chlorine which interferes with the test.

In order to prepare for this search for ammonia, the synthetic nitrate
of silver resulting from several experiments was shaken with a slight ex-
cess of standard pure sodic chloride until the supernatant liquid was clear ;
the latter was decanted, made up to 50 cc., and placed in a comparison
cylinder. The sodic chloride gave no test with Nessler solution.

The cylinder full of liquid prepared from the argentic nitrate was finally
tested by comparison with other cylinders containing known traces of am-
monic nitrate mixed with sodic nitrate in concentration equal to that exist-
ing in the actual determination. The sodic nitrate had been several times
recrystallized with centrifugal draining, and was shown to be free from
ammonia. In this way the following results were attained : Synthesis 12
was found to contain 0.05 mg. of ammonic nitrate, synthesis 13, 0.06 mg.,
and synthesis 14, 0.05 mg. ; in all, 36.5 grams of argentic nitrate were
proved to contain no more than 0.00016 gram1 of ammonic nitrate — less
than one two-thousandth of 1 per cent. The source of this ammonia was
not determined ; it might have come from the reduction of nitric acid or
from the large volume of air used in evaporating the argentic nitrate.
The only possible place where it could have entered with the reagents
used in the test was in the sodic chloride solution ; this was indeed tested,
with negative results, but Nessler's reagent is not very sensitive in the
presence of chloride, and a trace of ammonia might have eluded detection.
If the amount found had been larger, more time would have been spent
upon the matter, and a sample of the argentic nitrate often recrystallized

62 THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

in an atmosphere free from ammonia would have been used in a blank
experiment for comparison ; but with such a small trace of impurity this
was not worth while.

In summing up the results of the test for ammonia, the amount should
be calculated for the quantity of argentic nitrate produced from 100.000
grams of silver. Thus it is found that 157.480 grams of fused argentic
nitrate could not have contained over 0.0007 gram of ammonic nitrate.

The next impurity to be studied, nitrous acid, was sought by means of
sulphanilic acid and naphthylamine hydrochloride. These were proved
with a standard nitrite solution of which 1 liter contained 0.1 gram of
nitrogen. To insure parallel conditions in testing by comparison, pure sil-
ver nitrate was prepared with some care. It was thrice recrystallized
from nitric acid with centrifugal treatment, fused in porcelain, then recrys-
tallized three times more from the purest water in platinum, with centrif-
ugal draining as before. The specimens of fused argentic nitrate result-
ing from quantitative determinations were dissolved in known amounts
of water, and comparison solutions of equal concentration prepared. The
crystallized silver nitrate gave no trace of pink color with the reagents.
A fresh solution to which 0.1 cc. of nitrite solution (0.01 mg.) had been
added gave a pronounced color, while determinations 2, 3, and 11 all
gave a much fainter, barely visible color ; hence it was permissible to con-
clude that the loss of oxygen from the nitrate on fusion could not have
exceeded 0.000005 gram, a wholly negligible quantity.

In seeking for the next impurity, free nitric acid, Stas tested aqueous
solutions of his fused silver nitrate with "tournesol" and found them alka-
line rather than acid. This alkali may have come from his glass vessels.
We used methyl orange, a more sensitive indicator, and noted that the
purest crystallized neutral specimens and the fused salt alike gave equally
pronounced acid reactions in solution. The color was not modified by the
addition of half a milligram of nitric acid ; therefore the equality in color
does not disprove the presence of free acid in our fused product. Dilute
sodic hydroxide freed from carbonate by baric hydroxide threw down a
precipitate without changing the pink color. Hence the indicator was dis-
carded and the investigation continued with the nephelometer.

The test for acid with the help of this instrument was conducted by add-
ing a very dilute standard caustic alkaline solution to the solution to be
tested and by observing if this addition caused a permanent cloud.

Preliminary trials were necessary in the first place in order to determine
the conditions best suited for accuracy. It was found that a concentration
of nitrate no stronger than 4 grams in 0.025 liter was best adapted to this
work, as argentic hydroxide is distinctly soluble in double this concentra-
tion of argentic nitrate. On the other hand, argentic hydroxide precipi-

THE PURITY OF THE FUSED ARGENTIC NITRATE. 63

tated by somewhat concentrated alkali does not quickly redissolve in an
equivalent quantity of nitric acid, mixed with the dilute argentic solution.
It was found that even the precipitate caused by 0.1 ml. of a two-hun-
dredth normal solution may produce a fairly permanent cloud in spite of
the presence of an excess of 0.00003 gram of nitric acid, enough to dis-
solve it. Still more dilute solutions behaved much more satisfactorily,
however. A milliliter of a two-thousandth normal caustic solution (equiv-
alent to 0.00003 gram of nitric acid) added with constant stirring to the
pure standard argentic nitrate formed a cloud easily seen in the neph-
elometer. In case 0.00003 gram of nitric acid had been introduced
before adding alkali, no precipitate was formed under the same conditions.
Hence under these circumstances the test attained a degree of sensitive-
ness suited to the case in hand.

In this way it was found that the argentic nitrate remaining from syn-
thesis 11 gave a distinct opalescence with the addition of 0.00002 gram of
sodic hydroxide, and an obvious cloud with 0.00003 gram. Hence it was
concluded that argentic nitrate fused in a stream of pure air for one hour
contains no weighable excess of nitric acid.

The various suspected impurities having thus been duly sought, it is
instructive and interesting to tabulate the results. These are as follows :

Grams

Weight of fused AgNOs from 100,000 grams Ag 157.480

Correction for weight of —

Dissolved air 0.000

Retained water — 0.0016

Retained ammonic nitrate — 0.0007

Nitrite 0.000

Free acid . 0.000

Corrected weight of argentic nitrate obtainable from

100.000 grams of pure silver 157.478

As the subtractive corrections given in the above table are maximum
values, the weight of argentic nitrate in question can hardly be lower than
this value, 157.478. On the other hand, it can hardly be higher than the
uncorrected value, 157.480. Thus two limits are set, very near together,
between which the true value must lie. Obviously, for the present one
can not go very far astray in accepting the mean value, 157.479 ; and this
value will be used in the following discussion.

64 THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC.

THE FINAL RESULT AND ITS RELATIONS TO THE VALUES OF ATOMIC
WEIGHTS OF NITROGEN AND SILVER.

The conclusion reached by the foregoing series of experiments is that
100.000 parts of silver yield very nearly 157.479 parts of argentic nitrate.
Among other experimenters, Penny found 157.442 ; Marignac found
157.424; Stas found 157.474 in one series of seven determinations and
157.486 in another of two, and Hardin found 157.484. From these older
figures, by an interesting coincidence Clarke calculated exactly the value
found in the present research.1 This coincidence is to be attributed not
so much to the efficacy of the method of calculation as to the fact that in
this case the impurities in the silver happened to balance exactly the im-
purities in the argentic nitrate. At first sight it is incomprehensible how
Stas, working with slightly impure silver, could have obtained any results
higher than the true value ; but it must be remembered that his nitrate was
fused in a glass vessel, which must have been attacked by the strongly
acid nitrate at 220°. Constancy in the weight of the vessel would be no
evidence of absence of action, for silver might partly take in the glass the
place of the sodium taken from it. Thus a gain in weight due to sodic
nitrate might be accompanied with no considerable loss of weight of the
vessel. In favor of this hypothesis is the fact that Stas's results steadily
decreased in each series, as he proceeded with the work ; the flask seems to
have been less and less susceptible to attack, as is reasonable. If the last
two determinations of the first series are taken as the most nearly free from
this cause of error, the number 157.466 obtained from them should furnish,
by comparison with our number, some clue to the amount of gaseous im-
purity in Stas's silver. Thus it appears that his silver must have contained
nearly 0.01 per cent of impurity — probably more if it is considered that
the flask was still not wholly resistant — a conclusion not very different
from that reached by Richards and Wells.

Speculations of this kind concerning older work are rather a thankless
task, however ; there is usually too much that is doubtful to allow them to
serve a very valuable purpose. The only object in pursuing the review at
all is in order to assure one's self that no real inconsistency exists in the
data.

Recalculation of the Atomic Weights (1897), p. 64.

THE FINAL RESULT AND SUMMARY. 65

Attention is now directed to a more important matter, namely, the effect
of the new experimental result upon the table of atomic weights. This is
quickly stated: The newly advocated low atomic weight of nitrogen is
incompatible with the ratio 100.000 : 157.479, if silver is taken as 107.930.
If on the other hand, the new atomic weight of nitrogen is the true one,
silver must have a much lower value than this.

The exact figures are given in the following table :

The Atomic Weight of Nitrogen.

*

If Ag = 107.930, AgNO3 = 169.907 and N = 14.037

If Ag = 107.890, AgNO3 = 169.904 and N = 14.014

If Ag= 107.883, AgNO3= 169.893 and N = 14.010

If Ag = 107.880, AgNO3 = 169.888 and N = 14.008

This series of conditional statements contains in a nutshell the result of
the present investigation. In order to decide between the alternatives,
other compounds must be further studied, especially the chlorates and the
ammonium salts. Investigations in both of these directions have already
been begun in the Chemical Laboratory of Harvard College.

SUMMARY.

Argentic nitrate was made from pure silver, and the gain in weight was
carefully noted.

In the course of the work, a new and convenient apparatus for quantita-
tive evaporation was devised. Quartz flasks were used as a part of it.

The argentic nitrate was fused until constant in weight; it was care-
fully tested for dissolved air, retained water and ammonia, and nitric and
nitrous acids. Only the second and third of these impurities could be
detected by tests proved to be adequate, and these only in mere traces,
between 0.001 and 0.002 per cent in all.

The outcome was that 100.000 parts of pure silver produced 157.479
parts of argentic nitrate. If, then, silver is taken as 107.93, nitrogen must
be 14.037 ; or if silver is taken as 107.880, nitrogen must be 14.008, oxygen
being 16.000.

IV

THE MOLECULAR WEIGHT OF ARGENTIC SULPHATE AND
THE ATOMIC WEIGHT OF SULPHUR

BY THEODORE WILLIAM RICHARDS AND GRINNELL JONES

CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF HARVARD COLLEGE

THE MOLECULAR WEIGHT OF ARGENTIC SULPHATE AND THE
ATOMIC WEIGHT OF SULPHUR.

INTRODUCTION.

The atomic weight of sulphur has been investigated by many experi-
menters; but, as will be seen, the results are far from concordant. A
brief review of these investigations forms the most appropriate introduc-
tion to the present one. The values in the following list have been recal-
culated with modern figures for the other atomic weights involved:1

1814. Berzelius (Phil. Trans., 104, 20) ............ 32.0

1826. Berzelius (Pogg. Ann., 8, 15) ................ 32.08

1833. Turner (Phil. Trans., 123, 539) ............ 32.00

Do ........................ 32.06

1836. Thomson (J. prak. Chem., 8, 370) ........... 32.

1844. Erdmann and Marchand (J. prak. Chem. 31, 396) . . . 31.99

1845. Berzelius (Lehrbuch, 5th ed., 3, 1187) ......... 32.04

Do ........................ 32.16

1848. Svanberg and Struve (J. prak. Chem., 44, 320) .... 32.15

1851. Struve (Liebig. Ann., 80, 203) ............ 31.94

1859. Dumas (Ann. Chim. et Phys. [3], 55, 148) ....... 32.00

1860. Stas2 (Bull. Acad. Belg. [2], 10, 153, 322) ........ 32.06

1878. Cooke (Proc. Amer. Acad., 13, 50) .......... 32.137

Do ........................ 31.980

1891. Richards (Proc. Amer. Acad., 26, 268) ......... 32.043

1898. Leduc (Ann. Chim. et Phys. [7], 15, 94) ........ 32.056

1899. D. Berthelot (Jour, de Phys. [3], 8, 271) ........ 32.050

1904. Jaquerod and Pintza (C. R., 139, 129) ......... 32.01

1905. Guye (C. R., 140, 1242) ............... 32.065

1905. Jaquerod and Scheuer (C. R., 140, 1384) ........ 32.036

The work of the predecessors of Stas is now of historical interest only
and does not merit detailed criticism ; in recent times the atomic weight
of sulphur has rested chiefly upon the experiments of the famous Belgian.

Stas converted silver into its sulphide by heating it in a current of sul-
phur vapor or hydric sulphide; but because he did not know the precau-
tions necessary for preparing perfectly pure silver,3 it is evident that all
of his work in which silver was weighed needs revision. Stas also reduced
argentic sulphate to silver in a current of hydrogen, but in this case he

= 107.93; Cl = 35.473; Pb = 206.9 ; Hg = 200.0; Na = 23.008; C = 12.002.
^ 2Van der Plaats, Ann. Chim. et Phys. [6], 7, 504 and 526 (1886), pointed out that
Stas made an error in calculating his own data. Stas obtained 32.074, but 32.06 is
correct.

3Dumas, Ann. Chim. et Phys. [5], 14, 289 (1878) ; Richards and Wells, Publica-
tion Carnegie Inst. No. 28, p. 66 (1905).

69

70 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

did not take the very important precaution of fusing the sulphate before
weighing ;a and moreover, the possibility that the reduced silver might con-
tain argentic sulphide or sulphate was by no means excluded.

A brief review of subsequent work may not be out of place, although it
is of a different order of precision. Cooke reduced argentic sulphide in a
current of hydrogen, and concluded that the atomic weight of sulphur
must lie between the limits 32.14 and 31.98. In summing up his results
he stated that "this is equivalent to confirming the accepted value of
this constant, so far as any experiments on a scale less extensive than those
of Stas can be of value to this end.":

The determination of the atomic weight of sulphur by Richards was
made incidentally in his work on the atomic weight of copper. The ratio
Na2CO3 : Na2SO4 gave as a result S = 32.075. If sodium is taken as
23.008 instead of 23.053, the result becomes S = 32.043. No very great
confidence was placed in these results at the time, as is shown by the fol-
lowing sentence : "The results are hardly capable of deciding the present
uncertainty in the atomic weight of sulphur."3 It is probable that the
result is too low, as no proof could be obtained of the thorough desicca-
tion of the sodic carbonate.

Very recently numerous atomic-weight determinations have appeared,
depending on a purely physical method, based on the assumption that
Avogadro's hypothesis and the simple gas law PV = RT is strictly true
for gases when infinitely expanded. Since it is impossible to determine the
ratios of the densities of gases with sufficient accuracy at very low pres-
sures, it is necessary to determine this ratio under normal conditions and
apply a different correction in each case, depending on the deviation of
the gas from the simple gas law. It is probable that the ratio of the den-
sities can be, and in most cases has been, determined with sufficient per-
centage accuracy. On the other hand, the correction is hypothetical
and much less certain ; and accordingly, the method has but little value
when the correction is large.

According to Leduc4 the ratio between the densities of sulphur dioxide
and oxygen is 2.04835. If no correction is applied for the imperfection of
the gases, this value leads to 33.55 for the atomic weight of sulphur. The
correction in this case is unusually large and therefore must be known
with a high percentage accuracy if the result is to have any value as
a determination of atomic weights. The following are the general meth-
ods of calculating the correction, but none seems to be of sufficient value
for the present case.

iRichards, Proc. Amer. Phil. Soc., 42, 28 (1903).
2Cooke. Proc. Amer. Acad., 13. 52 (1878).
3Richards, Proc. Amer. Acad., 26, 269 (1891).
4Leduc, C. R., 117, 219 (1894).

INTRODUCTION. 71

The first, called the "method of corresponding states" was developed
for this purpose by Leduc.1 It depends on the assumption of van der
Waals that two gases are in corresponding states and deviate equally from
the hypothetical perfect gas if their temperatures and pressures are equal
multiples or sub-multiples of their critical temperatures and pressures,
respectively. In calculating the correcting factor, the compressibility and
critical constants are used. Leduc2 by this method obtained 64.056
as the molecular weight of sulphur dioxide, and 34.071 as the molecular
\veight of hydrogen sulphide, and hence from both S == 32.056.

The second method is the "method of critical constants" as developed
by Guye.3 The correction is calculated by the use of van der Waals's equa-
tion, the quantities a and b being calculated from the critical constants of
the gases under consideration. In a complicated manner Guye applied a
correction to the quantities a and b of van der Waals's equation, because
they appear to vary slightly with the temperature and the pressure,4 and
in this way was obtained the value 64.065 as the molecular weight of sul-
phur dioxide; hence S = 32.065.

The third method is called the "method of limiting density." It was
originated by Daniel Berthelot in 18985 and has been used by Rayleigh6
and Jaquerod.7 It depends on the experimental determination of the com-
pressibilities of gases at pressures in the neighborhood of one atmosphere.
By an extrapolation, the ''limiting ratio" of the densities at very low pres-
sures can be calculated. This method has reached its greatest perfection
in the hands of Lord Rayleigh, who has shown that for the permanent
gases the deviation from Boyle's law only varies slightly with the pressure
and therefore the extrapolation is fairly safe. This physical method has
been applied with the greatest success to the cases of hydrogen, carbon,
and, especially, nitrogen, because in these cases the correction is compar-
atively small. Here also, however, the application to sulphur is far less
satisfactory. It is to be regretted that Lord Rayleigh's work did not
include compounds of this element.

Jaquerod and Pintza determined the density of sulphur dioxide at 760
mm., 570 mm. and 380 mm. pressure, and from these results calculated the
compressibility. The results were extrapolated to zero pressure on the

'Leduc, Ann. de Chim. et Phys. [7], 15, 5 (1898).

2Leduc, Ann. Chim. et Phys. [?]. 15, 94 (1898).

3Guye, C. R.. 138, 1215 (1904) ; Bull. Soc. Chim. [3], 5 Aout (1905) ; Jour. Chim.
Phys., 3, 321 (1905).

4Guye, Bull. Soc. Chim. [3], 5 Aout, p. xn (1905).

5Berthelot, C. R., 126, 954, 1030, 1415, 1501 (1S98) ; Jour, de Phys. [3], 8, 263
(1899).

«Rayleigh, Phil. Trans. A., 204, 352 (1905) ; A., 196, 205 (1901), and A., 198, 417
(1902).

7Jaquerod and Pintza, C. R., 139, 129 (1904) ; Jaquerod and Scheuer, C. R.,
140, 1384 (1905).

72 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

assumption that the deviations from Boyle's law diminish as the pressure is
lowered. They obtained 32.01 as the atomic weight of sulphur. After-
wards Jaquerod and Scheuer determined the compressibility of sulphur
dioxide through the ranges 400 to 800 mm. and 200 to 400 mm. by a
method similar to Lord Rayleigh's. They found, as was to be expected,
that the deviation from Boyle's law was smaller at the lower pressure.
Nevertheless, in calculating the molecular weight of sulphur dioxide, they
assumed that the deviation from Boyle's law per centimeter of pressure
between 0 and 760 mm. was the same as between 400 and 800 mm. They
obtained 64.036 for sulphur dioxide and hence S = 32.036. If their meas-
urement of the compressibility through the range 200 to 400 mm. is used,
the result becomes 32.052, and it seems probable that yet a higher value
is the true outcome of their experiments. These different conclusions em-
phasize the uncertainty of the method.

Probably the adsorption of sulphurous oxide and hydric sulphide on the
glass of the containing globes introduces an error in both the density and
compressibility determinations, and hence in facts to which the assumption
above mentioned are applied. This possibility of adsorption seems to have
been not sufficiently heeded by any of the experimenters on sulphur com-
pounds. It adds yet another uncertainty to the results.

The upshot of these considerations is the conclusion that none of the
work heretofore done upon the atomic weight of sulphur decides its value.
Hence further investigation is imperative. The problem, for complete
solution, must be approached from many sides, and the work must involve
many compounds of sulphur. The following contribution describes one
section of a comprehensive program by which it is hoped the question may
be answered.

PRELIMINARY EXPERIMENTS.

The sulphate of silver was selected as the most suitable starting-point
for the present investigation, because it seemed probable that this salt
could be prepared in a pure condition and accurately analyzed. A further
reason for studying argentic sulphate lies in the possibility of later com-
bining the results with some ratio involving the sulphide and thus fur-
nishing evidence on the even more important question as to the atomic
weight of silver. Balanced against the advantages is the disadvantage
that only about 10 per cent of its weight is sulphur, causing the experi-
mental errors to be greatly multiplied in the calculations ; but the advan-
tages more than outweigh the disadvantage.

A tentative preliminary plan of operations was to fuse argentic sulphate
in a platinum crucible, dissolve it, precipitate as chloride, and collect and
weigh the precipitate. This plan was thwarted by numerous obstacles.

PRELIMINARY EXPERIMENTS. 73

It was found that the sulphate decomposes slightly when fused, a difficulty
which was later overcome. Again the quantitative solution of the silver
sulphate was a very tedious process. Eight grams fused in a platinum
•crucible and placed in 1.5 liters of water required six weeks for its com-
plete solution, even with frequent agitation. The action on the glass
during this long time, the danger of the access of gaseous impurity, and
the loss of valuable time all made this difficulty a serious one.

Argentic sulphate can be readily dissolved by placing twice its weight
of concentrated sulphuric acid in the crucible and heating at about 300° ;
on cooling, the acid sulphate of silver crystallizes out.1 Upon adding water
to this acid sulphate the normal sulphate is formed as a fine powder, which
can readily be washed into the precipitating flask and dissolved. The
large excess of sulphuric acid thus introduced was far from desirable,
however, for it tends to cause considerable occlusion of argentic sulphate
in the precipitated chloride. In order to test this question, a solution, pre-
pared as described above, and containing 7.4 grams of argentic sulphate,
was precipitated with hydrochloric acid. The precipitate was washed by
•decanting eleven times with very dilute hydrochloric acid and then dis-
solved in ammonia and diluted. The argentic chloride was then reprecipi-
tated with hydrochloric acid ; the solution should contain all the sulphate
previously occluded. After settling, this mixture was filtered, and the per-
fectly clear solution was evaporated in a platinum dish over an alcohol
lamp until ammonic chloride crystallized out on cooling, the volume being
then about 0.1 liter. To this were added about 10 grams of baric chloride.
An undoubted precipitate of baric sulphate was produced, which proved
that there is considerable occlusion of sulphate by argentic chloride pre-
cipitated from solutions containing a large excess of sulphuric acid.

For the sake of comparison, approximately the same amount of argentic
chloride prepared from nitrate was dissolved in ammonia, and 3.5 mg.
of sulphate added. The solution was treated exactly like the preceding
operations, and yielded about the same amount of precipitate. This
experiment gives an approximate idea of the extent of the occlusion.

In searching for a means of wholly eliminating the sulphate it was
found that a small proportion of argentic sulphate in argentic chloride
could be completely converted into chloride by fusion in a current of hy-
drochloric acid gas. This observation led to the development of a new
process in which the entire reaction was carried out in this manner, fused
argentic sulphate being wholly converted into chloride without the need
of dissolving in water. Thus the operation was greatly simplified, and the
chances of error diminished.

iSchultz, Pogg. Ann., 133, 143 (1868).

74 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

The method of preparing the materials, the shape and nature of the
apparatus, and many of the precautions and details of manipulation were
adopted only after numerous preliminary experiments. A detailed account
of this tentative work is, however, unnecessary, since the important results
are implied in the description of the method which was finally adopted.

THE PREPARATION OF PURE MATERIALS.
SULPHURIC ACID.

The best commercial "chemically pure'' acid was twice fractionally dis-
tilled, using a quartz condenser and a platinum dish as receiver. Only
the middle fractions were used; 17 grams left no visible or weighable
residue on evaporation.

ARGENTIC SULPHATE.

Pure argentic chloride residues from previous atomic weight investiga-
tions in this laboratory were reduced with invert sugar and sodic hydrate.
The reduced silver was thoroughly washed, and dissolved in nitric acid ;
and the nitrate was diluted and filtered. It was evaporated on the steam-
bath until saturated, and crystallized by adding an equal volume of con-
centrated nitric acid and cooling. The crystals were drained centrifu-
gally as usual.1 The crystallization from pure concentrated nitric acid
and centrifugal draining was repeated five times, using Jena-glass vessels.
It was finally recrystallized once more, using a platinum dish and redis-
tilled nitric acid. This silver nitrate was dissolved in a small amount of
water in a platinum dish, and an excess of the purest sulphuric acid, diluted
with an equal volume of pure water, was poured into it. The precipitated
silver sulphate was drained in the centrifugal machine. It was then dis-
solved in hot concentrated sulphuric acid in a platinum dish, and the solu-
tion was boiled for several minutes to expel nitric acid. On cooling, the
acid sulphate crystallized in large crystals. This acid sulphate was very
thoroughly whirled, placed in a platinum dish, and treated with purest
water. Heat was evolved and the normal sulphate crystallized out as a
fine powder. This powder was washed by decantation with the purest
water until the wash-waters were no longer acid.2 All the water used in
this work was purified in the manner described in previous communica-
tions. During this washing the action of light on the wet silver sulphate
produces a slight violet color. This, however, does no harm, as any slight
decomposition is remedied during the subsequent fusion. It was then

iRichards, J. Amer. Chem. Soc., 27, 104 (1905).

2Unless this precaution is taken the silver sulphate can not be dried thoroughly in
the air-bath; in which case it would be impossible to clean properly the receiving-
end of the tube into which it was afterwards introduced. (See p. 70.)

THE PREPARATION OF PURE MATERIALS. 75

dried as much as possible in the centrifugal machine, and the drying was
completed in an air-bath at 110°. These operations were carried out as
much as possible under the protection of a very large inverted funnel, and
the dish was kept covered with a large watch glass, to avoid the introduc-
tion of dust. This sample was called A and was used in the preliminary
series and in experiments -1 and 5 of the final series of quantitative experi-
ments.

Sample A was dissolved in concentrated sulphuric acid, boiled, crystal-
lized as acid sulphate by cooling, centrifugally drained, converted into
the normal sulphate by adding water, washed, and dried as before. This
sample was called sample B. Since the mean of the results with this sam-
ple is the same as the mean of experiments 4 and 5, in which sample A
was used, it is evident that the boiling of the sulphuric acid solution fol-
lowed by a crystallization in two different crystalline forms did not affect
the result ; therefore further purification seemed unnecessary.

HYDROCHLORIC ACID.

Hydrochloric acid, which was only used as a gas, was made from two
sources. In experiments 5 and 6 the hydrochloric acid was made by drop-
ping pure sulphuric acid on pure ammonium chloride. In the other ex-
periments the best commercial concentrated hydrochloric acid was used.
Richards and Wells1 have shown that this acid contained no other halogen
or arsenic ; it was therefore suitable for our purpose.

THE FUSION OF ARGENTIC SULPHATE.

Much thought was expended on the devising of a piece of apparatus
which should be suitable not only for the fusion of argentic sulphate, but
also for the quantitative conversion of the sulphate into the chloride.
Finally the simple symmetrical tube shown in the diagram was found to be

\1

Fig. 3. — Quartz tube used to contain the argentic falls. One-half actual size.

best. It was the outcome of several trials and suggestions, and consisted
of a thin cylindrical tube of fused quartz with smaller quartz tubes fused
upon the ends.1 The tube was very light, weighing less than 6 grams. A
very fine platinum wire was wrapped many times around the constricted

Richards and Wells, loc. cit.

2We are greatly indebted to Professor Baxter for his kindness in making this
apparatus from a fine quartz tube and a quartz test-tube. To Mr. F. B. Coffin also
we are indebted for a suggestion which led to a modification of the original shape
of the tube.

76 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

portion of the tube (A) ; by pulling this the tube could be readily rotated.
It was hung on the balance in a horizontal position by means of a platinum
wire. The same tube was used throughout the research, the cylindrical
shape giving it sufficient strength to withstand all the strains to which it
was subjected.

The tube remained remarkably constant in weight in spite of the very
vigorous treatment it received. During the entire fifteen experiments the
loss in weight was only 0.41 mg., and during the twelve experiments of
the final series the loss was only 0.16 mg. This slight loss is probably
.accounted for by a slight solubility of the quartz in the concentrated
ammonia or potassic cyanide solution used to remove the argentic chloride
after each determination.

The sulphate of silver prepared as described above must have contained
a little included water and excess of sulphuric acid, since it had been crys-
tallized from an acid solution.1 The only certain method of removing
mother liquor included within crystals is to fuse the salt. This part of the
problem caused considerable difficulty ; for it was found that argentic sul-
phate decomposes slightly when fused in air, becoming yellowish-brown
in color. In order to decrease this thermal dissociation, the experiment
was made of fusing the salt in a current of dilute sulphuric oxide, with
complete success. The salt is then pure white in color.

The small crystals of argentic sulphate were pushed into the tube with
a platinum rod, and the narrow ends were freed from loosely adhering
argentic sulphate by means of a clean feather tip, which had been previously
washed with alcohol and ether to free it from grease. The tube was sup-
ported, on hooks of hard glass, in front of the apparatus for delivering
sulphur trioxide. The very fine platinum wire needed for rotating it had
been wound around the tube in the first place.

The apparatus for delivering a current of pure dry air or sulphur tri-
oxide or hydric chloride is shown diagramatically in figure 4. A cur-
rent of air was first driven over red-hot copper oxide (A) to remove or-
ganic matter, and subsequently through an Emmerling tower (B) con-
taining beads moistened with a strong caustic potash solution. This tower
was closed by a rubber stopper at the top, which introduced no impurity,
since nothing but pure air passed over it. From this point, however, the
apparatus contained no rubber connections or stoppers. All stopcocks
were lubricated with phosphoric or sulphuric acid.

The air was dried by passing through two Emmerling towers (C and D)
containing beads moistened with concentrated sulphuric acid. The cur-

1 Richards, Proc. Amer. Phil. Soc., 42, 28 (1903).

THE FUSION OF ARGENTIC SULPHATE. 77

rent of air could then be either used in a pure dry state by passing through
L or F, or else charged with sulphur trioxide by bubbling twice through
fuming sulphuric acid in the bottle E. This acid was kept saturated by
an excess of solid sulphur trioxide. A current of air mixed with any
desired proportion of sulphur trioxide was thus delivered through G into
the quartz tube H. The delivery tube (G) and the quartz tube (H)
touched each other, but no attempt was made to secure a tight joint.

Fig. 4. — Apparatus for delivering appropriate gases into the quartz tubes.

The argentic sulphate contained in the quartz tube was then fused with
a fish-tail Bunsen burner held in the hand. A red heat is required. When
all of the sulphate was fused, the stopcock (P) delivering sulphur trioxide
was closed and a rapid current of air was admitted through F. As soon as
the fumes of sulphuric acid could no longer be seen escaping from the end
of the quartz tube, the flame was removed and the tube was slowly rotated
by drawing out the platinum wire wrapped around one end. This causes
the argentic sulphate to solidify in a thin sheet around the tube. Unless
care is taken to keep the salt at least 1 cm. distant from either of the end
tubes, difficulty is experienced during the conversion into chloride.

The final solidification in the current of pure dry air was carried out
very rapidly, in order to avoid decomposition and obtain a perfectly white

78 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

sample. If the argentic sulphate was kept fused 10 or 15 seconds too long
after the sulphur trioxide current was stopped, a yellowish-brown product
was obtained. This indicates that the salt contained either metallic silver
or argentic sulphide. Richards, Wells, and Forbes1 have shown that a
slight discoloration of a fused silver salt is a very delicate test for im-
purity. The loss of weight due to this discoloration was very slight, as
will be shown on page 79.

The entire tube was later heated in a current of pure air at a tempera-
ture above the boiling-point of sulphuric acid for about 5 minutes, in order
to drive out any possible accidental trace of acid in the tube. While still
warm it was placed in a desiccator, and later weighed. The tube was kept
horizontal until the end of the experiment, to avoid any chance of mechan-
ical loss.

The device of spreading the salt in a thin layer around the tube by rotat-
ing the tube during the cooling had four very important advantages :
First, the chance of breaking the very fragile quartz tube during the solid-
ification and cooling was greatly reduced by the more even distribution of
the strain ; secondly, the salt was agitated and spread out in a thin layer
while still fused in a current of pure air, thus facilitating the escape of any
possible trace of dissolved sulphur trioxide ; thirdly, the cooling was made
more uniform and rapid, so that the argentic sulphate had no time to
decompose before it had solidified; and fourthly, a much larger surface
was exposed to the later action of the hydrochloric acid.

The possibility just mentioned that the colorless argentic sulphate might
contain an excess of sulphur trioxide needed careful consideration. Un-
fortunately a direct test for acid seemed to be impracticable, owing to the
slight solubility of silver sulphate, hence light upon the question was
sought in several distinct ways.

Weber2 has found that in order to prepare a compound Ag2S,O7 argen-
tic sulphate must be heated with sulphur trioxide under pressure in a
sealed tube. This, together with the fact that argentic sulphate decom-
poses so easily when fused, indicates that it does not have a great tendency
to retain sulphuric oxide, and that the vapor tension of the trioxide in the
disulphate is far above that maintained in the present experiments. In
order to obtain quantitative evidence, the pure white sulphate in six of the
final experiments, after being weighed, was fused again for about 10
to 15 seconds in order to decompose it slightly. This gave a slight but
unmistakable dark color to the salt. The resulting losses of weight are
tabulated on page 79.

iRichards and Wells, Carnegie Inst. Pub. 28, 31 (1905) ; Richards and Forbes, the
present publication, p. 55.

2 Weber, Ben, 17, 2503 (1884).

THE FUSION OF ARGENTIC SULPHATE.
The Loss of Weight Caused by Slight Decomposition.

No. of

White

Darkened

experiment.

Ag2S04.

Ag2S04.

Difference.

9

5. 27714

5. 27709

0. 00005

10

5. 16313

5. 16302

0.00011

11

5. 08383

5.08377

0. 00006

12

5. 13372

5. 13367

0. 00005

13

5. 16148

5.16138

0. 00010

15

5. 37436

5. 37425

0.00011

Average...

0 00008

The loss in weight was thus on the average only 0.08 mg. or 0.0015 per
cent. In seems certain that at least part of this loss in weight was due to
a deficiency of either oxygen or sulphur trioxide in the darkened sulphate,
and not to the loss of an illegitimate excess of the latter substance. There-
fore, although it may be a debatable question as to whether it is safer to
take the weight of the white or darkened samples as the true weight, it
seems probable that the weight of the white sample was more trustworthy.
Even at the worst the uncertainty can not have been greater than 0.001
per cent. This result agrees with the earlier conclusions already cited con-
cerning the obvious effect of very slight decomposition on the color of sil-
ver salts. It appears that in the present case, as in the others, the slight
stability of these compounds is a real assistance in the production of a
typical compound, instead of a hindrance to precise quantitative work.

For weighing the argentic sulphate, and of course the chloride also,
the Troemner balance which had served in many similar researches was
used.1 The Sartorius platinized brass weights were standardized by the
usual Harvard method.2

As usual, all weighings were made by substitution, a make-weight being
placed on the right-hand balance pan. In order to avoid as far as possible
any error due to changing meteorological conditions, a substituting coun-
terpoise of the same material and approximately the same surface and
weight was used. Every weighing was repeated, and the successive values
seldom differed more than 0.03 mg. No difficulty was experienced from
hygroscopic adsorption of water by either argentic sulphate or argentic
chloride, except on a few days when the air was unusually humid. In these
cases the tube was heated to about 330° and allowed to remain in a desic-
cator until the conditions were more favorable. The tube was always
allowed to remain at least three hours in a desiccator near the balance

iRichards, Proc. Amer. Acad., 26, 242 (1891).
2Richards, J., Amer. Chem. Soc., 22, 144 (1900).

80 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

before making a weighing and during this time was covered by a black
cloth to protect the sensitive salts from the light.

In order to correct the weights to the vacuum standard, the specific
gravity of argentic sulphate is needed. Following are the published results
concerning this datum:

Density of Argentic Sulphate.

5.341. Karsten, Schweigger's J., 65, 419 (1832).

5.322. Playfair and Joule, Mem. Chem. Soc., 2, 430 (1845).

5.410. Filhol, Ann. Chim. et Phys. [3], 21, 417 (1847).

5.425. Schroder, Pogg. Ann., 106, 245 (1859).

5*54'} Petterson, Upsala, Nova Acta [3], 9, 35 (1874).

After a study of the original papers, the value 5.45 was provisionally
adopted as the most probable value; but, as there was some doubt about
its accuracy, this was verified by experiment.

The density of the toluol to be displaced by the salt was determined by
means of an Ostwald pycnometer at 29.2° to be 0.8566 (the mean of 3
concordant determinations) ; 6.067 grams of previously fused silver sul-
phate were found to displace 0.9532 gram of this toluol, and therefore
occupied 1.113 ml. (the mean of two determinations). Hence the density of
argentic sulphate is 5.45, as supposed. This involves an additive correc-
tion of 0.0000775 gram to each apparent gram of the salt, a value which
is decreased 0.0000011 by an increase of a centimeter of atmospheric
pressure, or decreased 0.00000026 by an increase of a degree of tempera-
ture. The correction to the weight was applied accordingly.

THE CONVERSION OF ARGENTIC SULPHATE INTO CHLORIDE.

The next step in the process was the conversion of the carefully
weighed fused sulphate into chloride by heating it in a current of dry
hydrochloric acid gas. This reaction has been observed by Hensgen.1

The hydrochloric acid generator used for the previous work was con-
structed entirely out of glass. Pure ammonic chloride or concentrated
hydrochloric acid was placed in the flask (/) shown in figure 4, and con-
centrated sulphuric acid was dropped upon it slowly. The gas was dried by
passing through the tower (/) containing beads moistened with concen-

1Hensgen, Recueil des Travaux chimiques de Pays-Bas., 2, 124 (1883). "Le
sulfate d' argent absorba 2 mol. HC1 a la temperature ordinaire, avec un degagement
de chaleur notable, et se changea completement en chlorure. En chauffant, meme
jusqu a 300°, la reaction inverse (observee par M. H. avec le sulfate de cuivre) n'eut
point lieu, mais 1'acide sulphurique fut chasse completement par un courant d'air.
Parmi les sels susdits, le sulfate d'argent est celui qui d' apres les donnees thermi-
ques, doit se changer en chlorure avec le plus grand degagement de chaleur." (See
p. 83.)

THE CONVERSION OF ARGENTIC SULPHATE INTO CHLORIDE. 81

trated sulphuric acid.1 The gas then passed through the stopcock (K) to
the delivery tube (Af). The ground-glass stopper at the top of the tower
(/) acted as a safety-valve when the stopcock (K} was closed. Pure dry
air might also be delivered at N by means of the stopcock (L), in order to
sweep out the excess of acid at the conclusion of the reaction.

The quartz tube containing the argentic sulphate was supported by
means of hooks of hard glass in front of the delivery tube (N), as before,
without making a tight joint.

In seven of the final experiments a condenser tube of quartz (O) was
placed over the exit end of the tube (M) in order to condense the sul-
phuric acid and retain any silver which might escape. As will be shown,
only very small traces of silver were found in the distillate. The conden-
sation of sulphuric acid in the narrow tubes on the end was prevented by
keeping them hot by means of a small fixed fish-tail burner.

A slow current of hydrochloric acid was generated and the tube warmed
gently. The reaction took place readily and quietly, the only difficulty
being that the argentic chloride formed was inclined to creep over the walls
of the vessel. This was probably clue to the liberated sulphuric acid hav-
ing dissolved undecomposed argentic sulphate ; the acid sulphate was then
transported by surface tension and converted into chloride in another place.
In two of the experiments one of the narrow end tubes was completely
blocked up in this manner, making a successful completion of the experi-
ment impossible ; but in other cases the difficulty was avoided by starting
with the argentic sulphate in a band in the middle of the tube.

The sulphuric acid must be evaporated at a temperature below its
boiling-point in order to avoid loss by the projection of small particles.
The tube was heated by a flame held in the hand, the heating being regu-
lated by watching the escaping fumes of acid and also the color of the
argentic chloride in the tube. It is well known that as the temperature
increases, silver chloride acquires a deeper and deeper yellow color; and
after acquiring the necessary experience, this change of color proved very
helpful in regulating the temperature.

After from 2.5 to 4 hours no more fumes of sulphuric acid could be seen
issuing from the tube. The argentic chloride was then fused very slowly
and quietly and kept in the fused state for 20 minutes in a current of hydro-
chloric acid. The tube was gently agitated in order to expose a fresh

JIn one of the preliminary experiments the hydrochloric acid was not dried. The
sulphuric acid first formed absorbed considerable water, thus becoming diluted and
nearly filling the tube with liquid sulphuric acid in which a large part of the silver
sulphate dissolved. After heating for some time, the whole mass was solidified.
This gave a non-porous mixture of silver sulphate and chloride. The hydrochloric
acid had no appreciable further action until the mixture was fused, and then the
action became very vigorous. The sulphuric acid which was formed boiled, and
caused spattering and therefore danger of mechanical loss.

82 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

surface, at intervals of about 1 minute, by grasping the exit end with
platinum forceps. The agitation can be performed without danger of loss,
because of the high surface tension of fused argentic chloride. This mate-
rial when liquefied does not adhere to quartz, and therefore is not drawn
into the end tubes by capillarity. At the expiration of 20 minutes the
current of acid gas was stopped by means of the stopcock (K), and a cur-
rent of pure dry air was passed through the tube through L, the argentic
chloride being still maintained in the liquid state for at least 5 minutes
with occasional agitation. The tube was rotated while subsequently cool-
ing in a similar manner to that employed with the sulphate.

The condensed sulphuric acid evolved by the reaction and the condenser
tube were tested for silver by means of the nephelometer.1 The acid was
transferred to a small flask and the tube was rinsed with ammonia, which
was added to the acid. The excess of ammonia was then neutralized with
nitric acid, and hydrochloric acid was added to precipitate any trace of
silver present as an opalescent cloud of argentic chloride. This opalescence
was compared in the nephelometer with a standard which had been made
in a manner very similar to the unknown solution.2 A measured volume of
a standard silver solution was taken ; to it were added approximately the
same amounts of sulphuric acid, ammonia, and nitric and hydrochloric
acids as were present in the unknown solution ; and it was made up to the
same volume (about 30 ml.).

The greatest amount of argentic chloride thus found in any experiment
was 0.00009 gram and the average amount 0.0000-1 gram. The trace
found in this way was of course added to the weight of the silver chloride.
In three of the experiments (Nos. 4, 5, and 7) this determination was not
made, but the average amount is added in these cases. The probable rea-
son for the escape of this trace of silver will be discussed later.

The question as to whether or not this reaction is complete is, of course,
of fundamental importance. There are four pieces of evidence bearing
on this point.

In the first place, the argentic chloride was fused for twenty minutes in
a current of almost pure hydrochloric acid. Since the temperature was
far above the boiling-point of sulphuric acid, this product of the reaction
was driven off as fast as formed. Therefore according to the law of
concentration-effect it is to be expected that through the agency of the
continually renewed hydrochloric acid all the argentic sulphate would be
decomposed and all the sulphuric acid would be driven off. This would
be hastened by the fact that fresh surfaces were continually exposed
through agitation.

iRichards and Wells, Amer. Chem. Jour., 31, 235 (1904),
2Richards, Amer. Chem. Jour., 35, 510 (1906),

THE CONVERSION OF ARGENTIC SULPHATE INTO CHLORIDE.

83

Again, in this case the law of concentration-effect is assisted by the rela-
tive affinity, indicated approximately by the large amount of heat evolved
by the reaction. It can be calculated from Thomsen's data that the reac-
tion gives out 40,400 grams calories,1 or 170 kilojoules. Rarely, if ever, is
the difference between total-energy and free-energy changes in a reaction
of this kind as great as this, hence it is safe to infer that there is a con-
siderable preponderance of driving tendency in the desired direction, aris-
ing from the mutual affinities concerned.

Thirdly, constancy of the weight on continued treatment indicated the
completion of the reaction. In seven of the experiments the argentic
chloride was fused a second time in hydrochloric acid for 15 minutes with
occasional agitation, followed by 5 minutes in air. The following table
gives the changes in weight found in this way:

The Effect of Continued Treatment ivith Hydrochloric Acid.

No. of
experiment.

AgCl, first
weighing.

AgCl, second
weighing.

Difference.

6

7
11
13
14
15

4. 67812
4.93119
4. 67373
4. 74491
4.77995
4. 94088

4. 67809
4.93118
4. 67375
4. 74489
4. 77990
4. 94088

- 0. 00003
— 0. 00001
+ 0. 00002
— 0. 00002
— 0.00005
+ 0. 00000

Average...

— 0 000015

The constancy in weight was thus entirely satisfactory, the average loss
being only 0.0003 per cent of the weight of the chloride.

This experiment, however, does not absolutely preclude the possibility
that a small but constant amount of sulphate may remain. In order to test
this question 4.90 grams of argentic chloride which had never been con-
taminated with sulphate was fused in the tube, and then 0.00479 gram of
pure argentic sulphate was added and thoroughly mixed with the chloride
by fusion. On cooling the appearance was very different from the pure

1Thomsen, Thermochemische Untersuchungen :
2Ag + C12 = 2AgCl -f 58760. (Vol. 3, p. 381.)
Ag2SC>4 = 2Ag+O2-f SO2 — 96200. (Vol. 3, p. 382.)
SO + O -f- Ha = H2SO- -f 121840. (Vol. 2, p. 255.)
2HC1 = H2 + Cl« — 44000. (Vol. 2, p. 114.)
Therefore, Ag2SO4 + 2HC1 (gaseous) = 2AgCl 4- H2SO, (Liquid) -f 40,400.

A different set of equations gave 40,200. The result, of course, only applies to 18°.

In this connection it is worth while to note that Ag2SO4 + C12 = 2AgCl + SO2 -{-
Oj — 37440 cal. ; which indicates that chlorine would not be as suitable for our
purpose as hydrochloric acid. This expectation is confirmed by the experience of
Krutwig, Ber., 14, 306 (1881) : "Die Einwirkung ist hier (chlorine on silver sul-
phate) keine direkte; nur bei sehr holier Temperatur, nachdem das Salz geschmol-
zen ist und sich zersetz, giebt es schweflige Saiire, Chlorsilber und Sauerstoff ab."

84 MOLECULAR WEIGHT OF AfcGENTIC SULPHATE, ETC.

' •»

chloride, the mass being translucent or almost opaque, instead of transpar-
ent. It was then fused in a current of hydrochloric acid for twenty minutes
after the sulphuric acid could no longer be seen escaping from the tube, as
before. On the assumption that the sulphate was converted completely
into chloride the loss in weight would have been 0.00039 gram, while the
actual loss in weight was 0.00044 gram. The reaction was evidently com-
plete, and a fourth argument was added to the other reasons for believ-
ing that the process should yield satisfactory results.

It will be recalled that the narrow end tubes were kept very hot by small
stationary burners in order to prevent the condensation of sulphuric acid.
At the close of several of the experiments a very slight sublimate was
observed on the portion of the narrow tubes which was kept somewhat
cooler by the protection of the supporting hooks. This sublimate, although
never large in amount, appeared largest in experiments 6 and 10. There
was a smaller amount in experiments 7, 11, and 14, but none worthy of
consideration in experiments 4, 5, 12, 13, and 15. In experiments 10 and
14 a slight amount of the sublimate was visible in the condenser also, near
the end of the tube.

Although the most rational explanation of this trace of sublimate, which
was usually too slight to be weighable, was probably to be found in the
assumption that it was argentic chloride and therefore entirely without sin-
ister meaning except as suggesting the risk of the loss of other traces,
the matter was studied further. Careful tests for arsenic1 and copper were
made with negative results, and the hydrochloric acid was demonstrated
to contain no trace of anything which could be deposited in a red-hot
quartz tube. On the other hand, the sublimate was soluble in ammonia
and behaved in every way like argentic chloride, so that its nature was
considered as nearly proved as was possible with such a minute amount of
material. Having settled the nature of the sublimate, its source and signi-
ficance must be traced. Evidently it could not have come from the main
body of the silver sulphate, because it appeared at the very beginning of the
experiment, as soon as the current of hydrochloric acid was started and
the end tubes heated, before the heat was applied to the main body of the
argentic sulphate. Moreover, the mass of the argentic chloride was never
heated to a temperature high enough to volatilize weighable amounts of
this substance, as was shown by the constancy of weight on continued
heating in a current of gas.2

In view of these considerations, it seemed probable that a few invisible
crystals of argentic sulphate had been left in the end tubes by the feather

1This test was kindly made by Mr. O. F. Black.

2See the preceding description; also Richards and Wells, loc. cit., p. 60; Baxter,
Proc. Amer. Acad., 41, 83 (1905).

THE CONVERSION OF ARGENTIC SULPHATE INTO CHLORIDE. 85

used. in cleaning it, or carried into the end tube by the current of sulphur
trioxide before fusion. As soon as the hydrochloric acid was admitted and
the end tubes heated, these small invisible crystals of silver sulphate must
have been at once converted into chloride and sublimed to the cooler por-
tion of the tube - - for the end tubes were usually raised to a red heat. This
explanation is consistent with the frequent absence of any significant sub-
limate, especially in the case of experiment 15. In this experiment the
tube had been dusted and treated with particular care, in order to test the
point.

Because the sublimate was weighed in the tube which had previously
contained the sulphate, its presence could not affect the accuracy of the
results even if it had been weighable. Nevertheless the suggestion that
another portion might have been carried out of the tube was worth con-
sidering. Doubts on this point were set at rest by the analysis of the con-
tents of the condenser tube which received the volatile products of the
reaction. The average amount of silver found in this tube was less than
0.001 per cent of that taken in each experiment, and the small appropriate
correction was easily applied.

Although it was not probable that argentic chloride which had been
fused in air for five minutes still retained any dissolved hydrochloric acid,
this point also was tested. In experiment 13, the silver chloride after the
first heating in hydrochloric acid and fusion in air for five minutes as
usual, weighed 4.74491 grams. After the second fusion in hydrochloric
acid, and finally in air, the weight was 4.74489 grams. After another
fusion for twenty minutes in a current of pure air, the weight was 4.74493
grams. These slight changes in weight can only be ascribed to errors in
the weighing ; the outcome shows that the argentic chloride after the usual
treatment did not retain any dissolved hydrochloric acid. Richard and
Wells have already shown that it does not dissolve weighable amounts
of air.1

In still another case the outside of the tube was washed with water to
make sure that nothing had deposited on it during the long exposure to
the flame and acid. The loss in weight was only 0.02 mg., which again is
not greater than the possible error in weighing.

1Richards and Wells, loc. cit, p. 60.

8C

MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

THE FINAL RESULTS.

Having shown the feasibility of the plan of operations and gained prac-
tice by three preliminary experiments, a final series was undertaken, whose
outcome is detailed below.

Sample A of argentic sulphate was used in experiments 4 and 5 and
sample B in the remainder. In experiments 5 and 6 the hydrochloric acid
was generated from ammonic chloride, and in the other experiments from
concentrated hydrochloric acid. The corrected weight of argentic chloride
was obtained by adding the trace found in the condenser to the average
weight after fusion in hydrochloric acid. In experiments 4, 5, and 7 the
correction for the argentic chloride in the condenser was not determined
directly, but the mean of the other determinations (0.00004 gram) was
added.

In experiments 8 and 9 the narrow exit tube became completely blocked
by the creeping of the argentic chloride. This made it necessary to fuse
the solid while there was still present considerable argentic sulphate; and
therefore the sulphuric acid boiled and material was lost by being pro-
jected out of the tube. The first of these was rejected, and the second not
finished. All the other determinations are recorded in full in the table.
The weighings have of course been corrected to the vacuum standard.

The Quantitative Conversion of Argentic Sulphate to Chloride.

No. of
experiment.

Weight of color-
less Ag0SO4

Weight of
total AgCl in

Parts of AgCl
obtained from
100.000 parts of

in vacuum.

vacuum.

Ag2S04.

4

5. 21962

4. 79859

91.934

5

5. 27924

4. 85330

91.932

6

5. 08853

4.67810

91.934

7

5. 36381

4.93118

91.934

10

5. 16313

4. 74668

91.934

11

5. 08383

4. 67374

91. 933

12

5. 13372

4. 71946

91.931

13

5. 16148

4. 74490

91 . 929

14

5. 19919

4. 77992

91.936

15

5. 37436

4. 94088

91.934

Average...

91.933

Thus 100.000 parts of colorless argentic sulphate were found to yield
91.933 parts of argentic chloride, with a vanishingly small "probable
error." If the weight of the darkened samples are used in the calculation
the result becomes 91.934, a value which certainly represents the maxi-
mum. To have reduced the chief uncertainty to within such narrow limits
was to have solved the problem as far as it need be solved at present.
The interpretation of the result alone remains.

THE FINAL RESULTS. 87

In comparing this result with that of Stas on argentic sulphate, it may
be noted that Stas found in silver sulphate 69.203 per cent of silver. Be-
cause Richards and Wells found in argentic chloride - 100 -- 75.2632

132.867

per cent, it is clear that our results indicate 0.91933 X 75.2632 -- 69.192
per cent of silver in silver sulphate, or 0.011 less than Stas's, one is forced
to the conclusion that Stas's argentic sulphate was not completely reduced
by hydrogen, and that his silver obtained in this way was no purer than
the silver used in his other work. The only test of complete reduction
used at the time was the solution of the residual metal in nitric acid ; but
this test could not reveal undecomposed sulphate and might not reveal
traces of sulphide.

THE ATOMIC WEIGHT OF SULPHUR.

The interpretation of the new results is very similar to that already dis-
cussed in the preceding paper on nitrogen and silver. In this case, as in
that, there are two uncertain ratios ; and one of these ratios, namely, that
of silver to oxygen, occurs in each. In the present case, the other uncer-
tain ratio is that of sulphur to oxygen, while in the former case the other
uncertain ratio was that of nitrogen to oxygen.

In order to obtain a complete solution of the numerical relations in either
of these cases, it is therefore obviously necessary to obtain another series
of results, bringing in such a ratio as that of silver to sulphur, or oxygen
to sulphur, or chlorine to oxygen. Such an additional ratio is not known
at present with modern accuracy. Because of the fact that silver and
oxygen are concerned in each of the cases, a single new result, properly
chosen, will solve both problems at once ; but of course many new results
with different compounds, confirming one another, are greatly to be
desired. As has been said in the foregoing paper, some of these are
already in the process of determination at Harvard College, and it is
intended to pursue the matter further at the University of Berlin as well.

For the present it is necessary to state the interpretation of the results
in a conditional manner, assuming various possible ratios between oxygen
and silver, and stating the corresponding values for sulphur. In the fu-
ture, when the assumed relationship is replaced by knowledge of the facts,
intelligent choice can be made between the alternatives.

If oxygen is taken as 16.000, the following table gives the atomic
weights of sulphur corresponding to the several atomic weights of silver.

Atomic Weight of Sulphur.

If Ag = 107.930 and 01 = 35.473, 5 = 32.113
If Ag = 107.890 and Cl = 35.460, 3 = 32.078
If Ag= 107.880 and Cl = 35.457, 5 = 32.069

88 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC.

The lowest value in this case, as well as in the case of nitrogen, is the
one supported by the recent work on the densities of gases.

In conclusion, it is a pleasure to acknowledge the generous assistance
cf the Carnegie Institution of Washington, without which the present
work could not have been performed.

SUMMARY.

The most important results of the research may be briefly summed up as
follows :

(1) A method for the preparation of pure argentic sulphate was devised.

(2) The specific gravity of argentic sulphate (previously fused) was
found to be 5.45.

(3) Indication was obtained that Stas was unable wholly to reduce sil-
ver sulphate in hydrogen.

(4) Argentic sulphate was found to be occluded by silver chloride from
solutions containing an excess of sulphuric acid.

(5) It was proved that argentic sulphate can be completely converted
into silver chloride by heating in a current of hydrochloric acid gas.

(6) 100.000 parts of argentic sulphate were thus found to yield 91.933
parts of argentic chloride.

(7) The atomic weight of sulphur as calculated from this ratio, if
oxygen is taken as 16.000, with several assumed values for silver is :

Ag = 107.93 5 = 32.113

Ag = 107.89 5 = 32.078

Ag = 107.88 5 = 32.069

Attention is called also to the summaries of the three previous papers
on pages 24, 44, and 65.

"BRAKY