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Full text of "Studies on the phenoldisulphonic acid method for determining nitrates in soils"



UNIVERSITY OF CALIFORNIA PUBLICATIONS 

IN 

AGRICULTURAL SCIENCES 

Vol. 1, No. 2, pp. 21-37 October 15, 1912 



STUDIES ON THE PHENOLDISULPHONIC 

ACID METHOD FOR DETERMINING 

NITRATES IN SOILS 



C. B. LIPMAN and L. T. SHARP 



UNIVERSITY OF CALIFORNIA PRESS 
BERKELEY 



M 



<wgr*ph 



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2. Studies on the Phenoldisulphonic Acid Method for Determining Nitrates 

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\ 

- 






UNIVERSITY OF CALIFORNIA PUBLICATIONS 

IN 

AGRICULTURAL SCIENCES 

Vol. 1, No. 2, pp. 21-37 October 15, 1912 



STUDIES ON THE PHENOLDISULPHONIC 

ACID METHOD FOR DETERMINING 

NITRATES IN SOILS 



BY 

C. B. LIPMAN and L. T. SHAEP 



Despite the fact that some careful research has been carried 
out on the colorimetric method for determining nitrates, many 
factors concerned with it have not been studied, and the some- 
what uncertain nature of the method makes it imperative to con- 
trol, so far as possible, every factor which may interfere with 
the accurate analysis of nitrate-containing material. These 
statements apply particularly to the analysis of soils for nitrates 
and the authors therefore deem the subjoined data, derived from 
a thorough investigation, deserving of the attention of every soil 
chemist. 

Among the interfering factors in the phenoldisulphonic acid 
method which have been either studied inadequately or not at 
all, are the effects of salts, the effects of agents employed to pre- 
cipitate the clay and organic matter, and the effects of decolor- 
izing agents. Cognizance must be taken of all of these factors 
by the chemist in the determination of nitrates and in the ar- 
rangement and interpretation of results. The importance of salt 
effects and their significance in this connection are emphasized 
by the fact that many soils, and particularly those of arid and 
semiarid regions, may frequently be found to a greater or less 
degree impregnated with one or more of the so-called "alkali 
salts," together with which, it often happens indeed, consider- 



22 University of California Publications in Agricultural Sciences [Vol. 1 

able quantities of nitrates are to be found. So far as the clay- 
coagulating substances are concerned, it has always been a com- 
mon practice in soil work to employ varying amounts of a satur- 
ated solution of alum to obtain a clear soil solution, and more 
recently it has been proposed by investigators who have studied 
the method under discussion to use aluminum cream for the pur- 
pose in place of alum. For decolorizing solutions both aluminum 
cream and bone black have been used. The methods for both 
clay coagulation and decolorization are obviously essential in most 
soil work, since ordinary filtration, without the use of such 
agents, can rarely be depended on to yield a clear, colorless soil 
solution, even if time be no object. The employment of the 
Pasteur Chamberland filter to remove clay has been found by 
direct investigation to involve well-defined losses of nitrates. 

It is not our purpose here to enter into a lengthy review of 
other investigations bearing on the subject in hand, but into a 
brief discussion of the more important ones which show the ques- 
tions still remaining unsolved or bring out certain results with 
which ours do not agree. 

In 1894 Gill 1 carried out a series of painstaking investiga- 
tions which, briefly, indicate (1) that for purposes of accuracy 
the phenoldisulphonic acid employed in the nitrate determination 
must be carefully prepared to insure a uniform compound for 
use as a standard; (2) that chlorine induces losses of nitric acid 
both when the solution containing nitrate is evaporated on the 
water bath and when the residue is treated with the reagent; 
(3) that Na 2 C0 3 added to the nitrate-containing solution to pre- 
vent escape of nitric acid during evaporation induces losses of 
nitrates varying in quantity from four to six per cent; (4) that 
alumina may be used to precipitate colloidal material for obtain- 
ing a clear solution ; (5) that silver sulphate, if free from nitrate, 
may be employed to precipitate chlorine, thus removing an im- 
portant interfering agent. 

More recently Chamot and his coworkers 2 have prosecuted an 
even more thoroughgoing investigation than the preceding, in 
which the most emphasis has been placed, however, on the mode 
of preparation of the tripotassium salt of nitrophenoldisulphonic 



i Jour. Am. Chem. Soc. vol. 16, p. 122. 1894. 



1912] Lipman- Sharp : Phenoldisulphonic Acid Method 23 

acid used as the reagent. Their results indicate (1) that in 
order to obtain the phenoldisulphonic acid free from the mono 
and tri-phenolsulphonic acid a careful digestion of the phenol 
and sulphuric acid under certain constant conditions must be 
assured; (2) that the mono and tri-phenolsulphonic acids intro- 
duce other colors which interfere with the readings in the colori- 
meter; (3) that the tri-potassium salt of nitrophenoldisulphonic 
acid gives the characteristic color employed in the determination 
and should always be used as a standard; (4) that heating the 
dry residue of nitrates even for several hours on the water bath 
occasions no losses; (5) that aluminum cream is the best pre- 
cipitating agent for organic matter of several used and occasions 
no losses of nitrates; (6) that 2 c.c. of the phenoldisulphonic acid 
should be used in uniform amounts in all determinations; (7) 
that KOH was to be preferred to NaOH and NH 4 OH, as the 
alkali employed; (8) that chlorides induced losses of nitrates; 
(9) that carbonates and organic matter did likewise; (10) that 
temperature, concentration, and length of exposure to reagent 
greatly affect results; and (11) that there have been other minor 
effects of iron, magnesium, and nitrites. 

Reference must also be made here to the brief investigation of 
Stewart and Greaves 3 pertaining to the effect of chlorine in de- 
termining nitrates in soils, both because the work is recent and 
because it is the only one published which is derived from re- 
searches on soils. This investigation and those above reviewed 
cover most completely the questions involved and reference will 
be made in the discussion of our experimental work below to 
those questionable points which were considered settled but which 
our work shows were far from being so. 

The Interference of Salts with the Nitrate Determination 

As has been above indicated the salt accumulations which 
occur in the soils of California, Nevada, Utah, and other arid 
or semi-arid regions frequently contain considerable quantities 
of nitrates and the determination of the latter in the presence 
of the "alkali salts" is, as has been found, frequently attended 



2 Ibid., vols. 21, p. 922; 32, p. 630; 33, p. 366 

3 Ibid., vol. 32, p. 756. 



24 University of California Publications in Agricultural Sciences [Vol. 1 

with losses of nitric acid. While all the investigations above re- 
viewed have pointed out the interference of chlorine and chlorides 
with the nitrate determination, and while some of them have 
also considered the losses occurring through the use of Na 2 C0 3 , 
no mention is made of the effects of the most common and widely 
spread of the alkali salts, Na„S0 4 , or Glauber salt. It seems fur- 
ther to have been taken for granted that Na 2 C0 3 and Na 2 S0 4 
should, for obvious reasons, have the same effects on the nitrate 
determination by the phenoldisulphonic acid method. Our re- 
sults do not, however, bear out this opinion. Under this head 
were also studied the effects of the kation as well as the anion 
of salts on the same determination. 

Varying quantities of the salts tested were here added to the 
same amounts of nitrates in solution, and uniform quantities of 
salts were also tested as to their effects on varying quantities 
of nitrates. Everyone of the following tables gives the effects 
of one of the salts tested in accordance with the scheme above 
indicated and in some cases also shows how the nitrate deter- 
mination is affected by varying the quantities of both the nitrates 
and other salts. The residue containing the salts and the nitrates 
was treated with 2 c.c of phenoldisulphonic acid thoroughly 
stirred for about two or three minutes, 25 c.c. of nitrate-free dis- 
tilled water was added, and then strong ammonia drop by drop 
until the odor of ammonia persisted and the color was per- 
manent. The solution was then diluted as necessary and com- 
pared in the Sargent-Kennicott colorimeter with a standard 
solution similarly and always freshly prepared, whose strength 
was in every case carefully tested. The results of these experi- 
ments are given in the following tables. 



1912] 



IApman-Sharp : Phenoldisulphonic Acid Method 



25 





TABLE I 








Effects of NaC 


1 






NaCl added 


N. added as 


N. found as 




nigs. 


nitrate mgs. 


nitrate mgs. 


Uniform quantities 


.25 


.050 


.045 


KNO — varying 


.50 


.050 


.041 


amounts NaCl 


1.00 


.050 


.035 




2.50 


.050 


.026 


Varying quantities 


1.00 


.050 


.038 


KNO -uniform 


1.00 


.100 


.070 


amounts NaCl 


1.00 


.250 


.215 




1.00 


.500 


.460 




1.00 


1.000 


.940 




1.00 


2.500 


2.300 


Varying quantities of 


.25 


.050 


.046 


both KNO^ and NaCl 


.50 


.100 


.078 




1.00 


.250 


.230 




1.50 


.500 


.460 




2.00 


1.000 


.900 




2.50 


2.500 


2.300 


Uniform amounts KNO 


.01 


.050 


.051 


small amounts NaCl 


.05 


.050 


.051 




.10 


.050 


.049 


Color blanks 


.00 


.050 


.050 


on both salts 


2.50 
TABLE II 


.000 


.000 


Effects of Na 2 S0 4 








Na 2 S0 4 added 


N. added as 


N. found as 




nigs. 


nitrate mgs. 


nitrate mgs. 


Amounts of nitrate 


1.000 


.0500 


.0480 


uniform and sulfate 


5.000 


.0500 


.0420 


varying 


10.000 


.0500 


.0400 




20.000 


.0500 


.0280 




30.000 


.0500 


.0270 


Amounts of sulfate 


15.000 


.1500 


.1420 


uniform and nitrate 


15.000 


.5000 


.4950 


varying 


15.000 


1.0000 


.9000 




15.000 


2.0000 


1.9500 




15.000 


3.0000 


2.8500 


Amounts of both salts 


1.000 


.1500 


.1420 


varying 


5.000 


.5000 


.4800 




10.000 


1.0000 


.9200 




20.000 


2.0000 


1.9300 



26 University of California Publications in Agricultural Sciences [Vol. 1 





TABLE 


III 








Effects of 


Na 2 CO ; 


t 






Na 2 C0 3 added 


N. added as 


N. found as 




mgs. 




nitrate mgs. 


nitrate mgs. 


Amounts of nitrate 


1.00 




.1000 


.0995 


uniform and carbonate 


2.50 




.1000 


.1040 


varying 


5.00 




.1000 


.1010 




10.00 




.1000 


.1010 




20.00 




.1000 


.1020 




30.00 




.1000 


.1020 


Amounts of carbonate 


10.00 




.1000 


.1000 


uniform and nitrate 


10.00 




.2000 


.2100 


varying 


10.00 




.5000 


.5000 




10.00 




1.5000 


1.4900 


Amounts of both salts 


1.00 




.1000 


.1010 


varying 


5.00 




.2000 


.1970 




10.00 




.5000 


.5050 




20.00 




.5000 


.5100 




30.00 




1.5000 


1.4900 



The results set forth in tables I. II, and III leave no room 
for doubt as to the effects of "alkali" salts on the nitrate deter- 
mination by the colorimetric method. Both NaCl and Na 2 S0 4 
induce large losses of nitrate, and especially is this true of NaCl, 
which may be responsible for losses equivalent to forty-five per 
cent and more of the total nitrate present as indicated in Table 
I. While Na 2 S0 4 induces smaller absolute losses than NaCl, they 
are none the less marked, and where large amounts of the sulfate 
are present very considerable losses of nitrate occur. 

Perhaps the most striking feature of the foregoing results is 
what appeals to one at first sight as the singular difference in 
the behavior of Na 2 S0 4 and Na 2 C0 3 . Whereas the former is 
always responsible for losses in the determination of nitrates, 
the latter is the only one of the salts tested which has no effect 
and the presence of which in a long series of tests has never, 
except in one case, decreased the amount of nitrate present as 
shown by the colorimeter readings. It was naturally assumed 
that Na 2 CO ;! . after the addition of the phenoldisulphonic acid, 
would be converted in the presence of an excess of sulphuric 
acid into Na 2 S0 4 and should therefore show the same decreases 
in the nitrate content as the latter salt. To clear up these rather 
puzzling facts, as above given, we decided to run a special series 
of experiments based on a suspicion which we had as to the 



1912] Lipman-Sharp : Phenoldisul phonic Acid Method 27 

nature of the action of the salts in question. The results of 
these experiments, which will be given below, make entirely clear 
what seemed at first quite puzzling. 

In further general discussion of the tables above given, it 
must be added that the decreases in the nitrate content of the 
solutions tested as induced by the presence of salts never oc- 
curred in accordance with any definite law, the losses at times 
being greater with smaller amounts of salts than with larger 
amounts, the amounts of nitrates being constant. On the other 
hand, with a given amount of nitrates not exceeding one-tenth 
of a milligram the salts seemed always to induce larger per- 
centage losses than they did in the case of the larger amounts of 
nitrates. Our results not only give good opportunity for a com- 
parison of the effects of varying quantities of salts on the same 
nitrate content, but point out all the relationships between the 
salts and nitrates where first the former, then the latter, and 
finally both, are varied. There are two other points, also, which 
they would not seem to confirm ; indeed they give entirely different 
evidence on these than was obtained by other investigators. The 
first is that small amounts of NaCl do not induce losses of 
nitrates, as claimed by Stewart and Greaves, and Table I indi- 
cates that amounts of NaCl below .1 milligram do not occasion 
any losses. The other point of difference between our results and 
those of the others mentioned is that Na 2 CO a does not decrease the 
amounts of nitrates, no matter to what extent it is used, as shown 
in Table III. This is in entire disagreement with the results of 
Gill and Chamot and his coworkers, who claimed that Na 2 C0 3 
and other carbonates induced losses of nitrates, in the determina- 
tion outlined. It must also be added here that the effects of 
Na^SO.4 as given in Table II constitute the first published results, 
so far as we are aware, on the effects of Glauber salt on the 
nitrate determination, and they have indeed been indirectly re- 
sponsible for the discovery of one or two other points of interest 
which will be discussed below. 

The results above given indicate the effects of each of the 
salts taken singly on the nitrate determination. To make the 
data more complete it was thought desirable to test various mix- 
tures of the same salts and note their effects. Table IV gives 
the results obtained. 



28 University of California Publications in Agricultural Sciences [Vol. 1 







TABLE 


IV 






Effects 


of Mixed 


Alkali Salts 




Na 2 C0 3 


Na,S0 4 


NaCl 


N. added as 


N. found as 


mgs. 


mgs. 


mgs. 


nitrate mgs. 


nitrate mgs. 


1 


1 


1 


.1000 


.055 


5 


5 


5 


.1000 


.026 


10 


10 


10 


.1000 


.021 


20 


20 


20 


.1000 


.017 


1 


1 




.1000 


.082 


5 


5 




.1000 


.081 


10 


10 




.1000 


.075 


20 


20 




.1000 


.077 


1 




1 


.1000 


.061 


5 




5 


.1000 


.060 


10 




10 


.1000 


.055 


20 




20 


.1000 


.028 




1 


1 


.1000 


.050 




5 


5 


.1000 


.042 




10 


10 


.1000 


.033 




20 


20 


.1000 


.030 


10 


10 


10 


.2000 


.086 


10 


10 


10 


.5000 


.125 


10 


10 


10 


1.000 


.360 


10 


10 


10 


2.000 


1.140 



The same marked losses in nitrates occur here as where the 
salts are employed singly. NaCl seems to be responsible again 
for the greatest losses, Na 2 S0 4 is next in order, and Na 2 C0 3 
seems to have little or no effect. Since these salts occur together 
in alkali soils, however, the results in Table IV possess consider- 
able significance and interest, especially since they point out what 
enormous losses of nitrates occur where such large amounts as 
ten milligrams of each of the salts are added to the nitrate-con- 
taining solution. 

The Interference of Precipitants of Clay and Organic 
Matter on the Nitrate Determination 

It is very singular that analytical chemists have for so long 
a time been employing such materials as saturated alum solu- 
tions, aluminum cream, and bone black for precipitating clay 
and organic matter in obtaining the soil solution to be used for 
nitrate determinations without ever having attempted to ascer- 



1912] 



Lipman-Sharp : Phenoldisulphonic Acid Method 



29 



tain if such materials in any way affect the accuracy of the 
determination. Indeed Chamot and his coworkers have recom- 
mended the use of aluminum cream for removing suspended 
material from the solution, and claim to have had very satis- 
factory results in the use of that material. Our experiments 
in this series were intended to clear up this question and the 
following results show very strikingly that none of the materials 
mentioned may be employed in the nitrate determinations with- 
out incurring very serious losses. Table V gives results obtained 
in the use of potash alum, and Table VI gives results obtained 
in the use of bone black and aluminum cream. 







TABLE V 








Effects of K 2 A1,(S0 4 ) 4 










K 2 A1 2 (S0 4 ) 4 


X. added as 


X. found as 






added 


nitrate 


nitrate 






mgs. 


mgs. 


mgs. 


Amounts of nitrate 




5.00 


.050 


.040 


uniform and alum 




12.50 


.050 


.036 


varying 




25.00. 


.050 


.033 






50.00 


.050 


.031 






100.00 


.050 


.034 






150.00 


.050 


.040 


Amounts of alum uni 


form 


45.00 


.050 


.035 


and nitrate varying 




45.00 


.100 


.075 






45.00 


.250 


.168 






45.00 


.500 


.345 






45.00 


1.000 


.675 






45.00 


2.500 


1.800 


Amounts of both 




5.00 


.050 


.040 


salts varying 




12.50 


.100 


.074 






25.00 


.250 


.175 






50.00 


.500 


.335 






100.00 


1.000 


.690 






150.00 


2.500 


1.850 


Color blanks on 




.00 


.050 


.049 


both salts 




.00 
100.00 


.500 


.480 



30 



University of California Publications in Agricultural Sciences [Vol. 1 



N. added as 


N. found as 


nitrate 


nitrate 


mgs. 


mgs. 


.5000 


.254 


1.000 


.648 


2.000 


1.460 


.5000 


.100 


1.0000 


.300 


2.0000 


1.180 


1.0000 


.135 


2.5000 


.650 


5.0000 


2.200 



TABLE VI 

Effects of Aluminum Cream and Bone Black 



Sufficient aluminum cream to 
clear solution. Five minutes 
exposure 

Twice the amount of aluminum 
cream used above. Exposed one 
and one-half hours 

Sufficient bone black to clear 
and decolorize solution 



The data in Tables V and VI are clearly very striking. The 
enormous losses of nitrates sustained through the use of a satur- 
ated solution of alum, varying quantities of aluminum cream and 
bone black, make these substances entirely unfit for use as 
precipitants for clay, or organic matter, or both, when nitrates 
are to be determined. While bone black occasions the largest 
losses, and potash alum the smallest, of any of the substances 
above described, the losses of nitrates brought about through the 
use of all the precipitants are too great to permit of their con- 
tinuance in a method for nitrate determinations which is none 
too accurate under the best of conditions. It is therefore evident 
that nitrates are lost not merely through the loss of nitric acid, 
as is the case where salts are used, but that there is a loss of 
nitrates mechanically through adsorption on the part of the 
colloidal material of the precipitant, as must be the case where 
such substances as aluminum cream and bone black are used. 
The large amounts of colloids possessed by these substances, with 
the accompanying large surface areas, evidently prevent some of 
the nitrate in solution from going through the filter. 

On casting about for a method to precipitate clay or organic 
matter, we first tried the Briggs filter pump, but found that open 
to two objections. First, the losses of nitrates through what we 
look upon as adsorption on the part of the clay filter, though not 
very large, were nearly equal to those induced by small amounts 
of sulfates. Second, while the filter pump yields a clear solu- 
tion, it does not serve to decolorize solutions. After several fur- 



1912] Lipman-Sharp : Phenoldisid phonic Acid Method 31 

ther attempts to find a coagulating and decolorizing agent which 
might promise well for this method, it struck us that quicklime, 
being the best coagulating material for clay, might perhaps also 
serve to remove organic matter and yet might not decrease 
seriously the amount of nitrates in the solution to be tested. 
Accordingly, tests were carried out by adding lime to solutions 
containing known amounts of nitrates, to soils containing known 
amounts of nitrates and to soils with unknown amounts of 
nitrates, in which latter a comparison was also especially made 
between lime and aluminum cream. We found in these experi- 
ments that the losses of nitrate through the use of lime were not 
only very small or negligible, but that the action of lime in 
precipitating both clay and organic matter was equal to or better 
than that of the best of the coagulating and decolorizing agents. 
Its coagulating action on clay has of course always been recog- 
nized in soil physics. The results of the experiments are given 
in Table VII. 

TABLE VII 

Effects of Lime 
A — Solutions of known nitrate content 



CaO present 


N. added as nitrate 


N. found as nitrate 


grms. 


mgs. 




mgs. 


1 


1.0000 




1.0150 


3 


1.0000 




.9800 


5 


1.0000 




.9550 


3 


5.0000 




4.6500 


B- 


-Soils of known 


nitrate 


content 




CaO present 


N. present as nitrate N. foun< 




grms. 




mgs. i 


Lime ground with 








soil and water 


2 




3.280 ; 


Lime added to muddy 






suspension 


2 




3.280 



3.150 



3.200 



C — Comparison of lime and aluminum cream on soil of unknown nitrate 

content 





CaO present 


N. found as nitrate 




grms. 


mgs. 


Lime ground with 






soil and water 


2 


1.210 


Lime added to 






muddy suspension 


2 


1.225 


Sufficient aluminum 






cream added to 






clear solution 




.800 



32 University of California Publications in Agricultural Sciences [Vol. 1 

It would seem from these results therefore that lime can 
yield a clear, colorless solution without decreasing the quantity 
of nitrates present in the solution appreciably, and that it is 
therefore the only one of the coagulating agents above tested 
which can be safely used in the work. We commend it to soil 
chemists and others making nitrogen determination under similar 
conditions. Only where very large quantities of lime are em- 
ployed, and they are not necessary, have we found definite losses 
of nitrates. We find that 2 grams of CaO is sufficient to 
coagulate the clay in 100 grams of loam soil and to remove 
whatever color may be present at the same time. 

While lime has been used by some chemists in accordance 
with the method above outlined, its use has by no means been 
general and no data prior to this existed with reference to its 
effects on the nitrate determination. J. G. Lipman and P. E. 
Brown give directions in their laboratory manual on Soil Bac- 
teriology for the use of 2 grams of lime to precipitate the clay 
in the 100 gram samples of soil used in nitrification experiments, 
but we have never seen any published statements beyond that 
as to the advisability or feasibility of employing lime. It is cer- 
tainly surprising that those who have tested the method for 
nitrate determination should not have tried and urged the use 
of lime as a substitute for alum or aluminum cream. 

Other Experiments on Salt Effects 

It appeared interesting, when the results in Tables I, II, and 
III were obtained, to ascertain if the kation as well as the anion 
of salts was responsible for losses of nitrates. Accordingly a 
series of experiments was instituted in which the effects of 
NaCl, KC1, and MgCl 2 could be compared. The following re- 
sults were obtained. 



1912] Lip man-Sharp: Phenoldisulphonic Acid Method 33 







TABLE 


VIII 








Effects of Ions 










X. added as 


N. found as 


KC1 


MgCl 2 


NaCl 


nitrate 


nitrate 


mgs. 


mgs. 


mgs. 


mgs. 


mgs. 


1 






.1000 


.070 


5 






.1000 


.063 


10 






.1000 


.055 


20 






.1000 


.050 




1 




.1000 


.057 




5 




.1000 


.028 


.... 


10 




.1000 


.016 




20 




.1000 


.011 






1 


.1000 


.065 






5 


.1000 


.043 






10 


.1000 


.035 






20 


.1000 


.038 



It is evident from Table VIII that the chlorine and not the 
base is the interfering element, and while the amounts of chlorine 
were not so proportioned as to be equivalent in the case of the 
two monovalent bases, the effect is clearly seen of the smallest 
and the largest amounts of chlorine present in the salts, which 
can be calculated from the molecular weights. The negative 
ion therefore seems to be the active agent in setting free nitric 
acid, but the decreases, depending as they do on other conditions 
such as evaporation on the water bath and length of exposure, do 
not take place in accordance with any definite law. 

The last phase of the salt effects studied was that above re- 
ferred to in the discussion of Tables I, II, and III, namely, the 
reason for differences in the action of Na 2 C0 3 and Na 2 S0 4 on 
nitrate-containing material. Since it was evident that Na 2 C0 3 
should react similarly to Na 2 S0 4 when the phenoldisulphonic 
acid was added to the dried residue to be analyzed, we suspected 
that the losses occurring when Na 2 S0 4 was employed came about 
on the water bath in evaporating the solution, under which con- 
ditions only, according to our work, could there have been a 
difference in the action of the two salts. 

The results given in the following table prove that our sus- 
picions were well founded. In this series the dry salts were 
thoroughly mixed with the nitrate-containing residue obtained by 
evaporating standard nitrate solutions, and then the phenoldi- 
sulphonic acid reagent was added. NaCl was similarly tested. 



34 University of California Publications in Agricultural Sciences [Vol. 1 









TABLE 


IX 










Effects of 


Dry 


Mixing of 


' Nitrates and 


Salts 














N. added as 


N. 


found as 


Na 2 C0 3 


Na,S0 4 




NaCl 




nitrate 




nitrate 


mgs. 


mgs. 




mgs. 




mgs. 




mgs. 


50 











.1000 




.097 


100 


50 
100 




50 
100 




.1000 
.1000 
.1000 
.1000 
.1000 




.102 
.103 
.102 
.080 
.062 



The data in Table IX make it quite clear that the losses due 
to Na 2 S0 4 occur only when the latter salt is present in solution 
with nitrates and the solution is evaporated on the steam bath. 
When, however, the salt is mixed dry with the dry nitrate no 
losses of the latter occur any more than they do when Na 2 C0 3 is 
added. The same is not true, however, of NaCl, as is shown in 
the last table. That salt causes losses of nitrates during both 
the evaporation on the steam bath and the reaction setting 
chlorine free in the treatment of the dry residue with phenoldi- 
sulphonic acid. This latter fact is a confirmation of work done 
by Gill and reviewed above. We have thus shown the individual 
reaction of each of the salts as related to the nitrate determina- 
tion and the causes which are responsible for the difference. 
Nitric acid is evidently set free from nitrates through the com- 
bined action of heat and the S0 4 radicle on the steam bath and 
in the evolution of chlorine when the phenoldisulphonic acid is 
added to nitrate and chloride-containing material. Na 2 CO.„ 
however, possessing only a weak and unstable acid radicle is 
powerless to set free nitric acid either through the help of heat 
on the steam bath or by its reaction with the phenoldisulphonic 
acid. 

General, Remarks 

So many factors may interfere with the determination of 
nitrates by the phenoldisulphonic acid method that it would ap- 
pear to be almost worthless, and yet it would seem to us that 
since there is no other good method to take its place which is 
nearly as simple and capable of use in very numerous deter- 
minations, it is worth while taking certain precautions to avoid 



1912] Lipman- Sharp : Phenoldisulphonic Acid Method 35 

error, and to establish the method on a firmer basis. Our results 
as above outlined show that losses of nitrates are induced by 
the presence of NaCl and Na 2 S0 4 , and such losses are indeed 
hard to avoid when working with "alkali soils." Even the 
suggestion of Chamot that AgS0 4 might be used to precipitate 
chlorides would seem, from our results, not to be useful, since 
the addition of sulfate to the solution would accomplish very con- 
siderable losses itself, even if the silver sulfate can be obtained 
nitrate-free, which Chamot claims is seldom the case. So that while 
we deem it unsafe in the presence of considerable quantities of salts 
containing chlorides and sulfates to determine nitrates by the 
phenoldisulphonic acid method and would therefore recommend 
the Street modification of the Ulsch method in such cases, it is 
likewise clear that many of the nitrate determinations made 
in soil laboratories, as is especially the case in soil bacteriological 
work, would not be interfered with by salts. In such cases the 
method can be safely depended on if potash alum, aluminum 
cream, and bone black are not used to coagulate clay and or- 
ganic matter, since they have been found in the researches above 
described to be productive of very serious errors. We recom- 
mend as a substitute for these coagulating agents the oxide of 
lime in its chemically pure state, to be employed in accordance 
with the method above given. The losses of nitrates sustained 
through its use have been shown to be very small in the work 
above reported, and it may be employed by grinding the soil 
with water or by direct addition to the muddy suspension pre- 
pared from the soil. 

Other sources of loss such as those brought about through 
the sterilization of controls in the autoclave are unavoidable. 
They have been found at times to be distinctly appreciable, and 
especially in the presence of considerable quantities of organic 
matter. It is further of the greatest interest to learn, from the 
experiments above described, of the action of the anion of the 
salts employed in our studies and the losses of nitrates occurring 
on the water bath from solutions being evaporated there when 
either NaCl or Na„S0 4 is present. 

We should also make mention here of our attitude toward 
the use of NH 4 OH instead of KOH, which was found superior 



36 University of California Publications in Agricultural Sciences [Vol. 1 

to the former in the investigations above reviewed. While higher 
absolute results may no doubt be obtained from the use of KOH 
than from NH 4 0H, and while in addition ammonia possesses 
other objectionable features, we were not aware of the first of 
these objections when these investigations were begun and did 
not deem the others serious enough to warrant a change in the 
established method. Moreover, the same relative values would 
exist for the data above given if obtained with one or the other 
of the hydrates, and therefore our results, having been obtained 
throughout by the use of ammonia, do not in any way lose their 
value. We do intend, however, in the future to employ KOH 
exclusively in nitrate determinations made in this laboratory. 
Finally we desire to call the attention of soil chemists to the 
fact that losses of nitrates by the agencies above described never 
seem to occur in accordance with any definite law, with the 
exception of the case in which the various alkali chlorides are 
compared. In these it would appear, from calculations which 
we have made, that the losses of nitrates are proportional to 
the amounts of chlorine present. While no law can be formu- 
lated, however, in accordance with which nitrates are lost in 
the presence of salts, it may be possible to work out tables 
for the losses of nitrates incurred in the presence of varying 
quantities of chlorides and sulphates, and to make corrections, 
therefore, in samples whose composition is unknown after alkali 
determinations are made. It is true, however, that calculation 
has shown on the basis of data in Table VIII that the losses 
of nitrates induced by chlorides alone are proportional to the 
amount of chlorine present. 

CONCLUSIONS 

1. The "alkali" salts NaCl and Na 2 S0 4 induce losses of 
nitrates when the latter are determined by the phenoldisulphonic 
acid method. Na 2 C0 3 has no such effect. NaCl induces much 
greater losses than Na 2 S0 4 . 

2. Among the substances used to coagulate clay and organic 
matter from solutions in which nitrates are to be determined, 
potash alum, aluminum cream, and bone black have been found 
decidedly unreliable. They all induce large losses of nitrates. 



1912] Lipman-Sharp: Phenoldisulphonic Acid Method 37 

3. Lime has been found to be much more reliable for the pur- 
pose named than any of the other substances, the losses incurred 
through its use being very small. 

4. The reason for the difference between the action of Na,S0 4 
and Na 2 C0 3 so far as the nitrate losses are concerned is to be 
found in the fact that Na 2 S0 4 induces the loss of nitric acid 
from the solution while the latter is being evaporated, while 
Na 2 CO,. containing only a weak acid radicle has no power to 
set nitric acid free. Neither Na 2 S0 4 nor Na 2 C0 3 has the power 
to set nitric acid free from nitrates when the dry residues of the 
two are mixed prior to treatment with phenoldisulphonic acid. 

5. Losses of nitrates from solutions as induced by chlorides 
alone seem to be proportional to the amount of chlorine present. 

6. The work of Gill which showed that chlorine induces losses 
both on the water bath and in mixing the dry residue with 
phenoldisulphonic acid is confirmed. 



UNIVERSITY OF CALIFORNIA PUBLICATIONS— (Continued) 

Vol. 8. 1907-1909. 

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227. March, 1912 20 

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229-268; plates 25-31. May, 1912 *0 

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