Adsorption of tin by proteins and its relation to the solution of tin by canned foods

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ADSORPTION 0? Ti;\" BY PROTEINS AND

ITS RELATION TO THE SOLUTION OF

TIN BY CANNED FOODS

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Adsorption of Tin by Proteins and

Its Relation to the Solution of

Tin by Canned Foods

A DISSERTATION

presented to the

Faculty of Princeton University

IN Candidacy for the Degree

of Doctor of Philosophy

BY

B. C. GOSS

Accepted by the Department of Chemistry, June, 19 14.

Adsorption of Tin by Proteins and

Its Relation to the Solution of

Tin by Canned Foods

B. C. GOSS

189515

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ADSORPTION OF TIN BY PROTEINS AND ITS RELATION
TO THE SOLUTION OF TIN BY CANNED FOODS

The presence of tin in foods which have been packed
in tin cans has long been known and a great amount of
work has been done on this subject, especially since
1878, when Menke published an article on "Tin in
Canned Foods. "^ This work has, however, been al-
most entirely concerned with the mere presence of
tin, determination of total tin present and with meth-
ods for recovering it.'*'^* The general procedure is to
destroy first the organic matter. This is done by wet
or dry oxidation or a combination of the two.

In the dry oxidation, the food is evaporated and the
dry mass charred and oxidized in a muffle furnace, a
small amount of potassium nitrate or nitric acid as-
sisting in the operation. The tin is left in an insolu-
ble form as stannic oxide. It is then rendered solu-
ble by fusion with sodium carbonate and sul-
fur or with caustic potash, giving, respectively,
sodium sulfostannate or potassium stannate. Also
the stannic oxide may be reduced to metallic tin by
a stream of hydrogen gas at red heat or fused with
potassium cyanide and the metal dissolved in hydro-
chloric acid.

The moist incineration processes involve oxidation
of organic matter by nitric acid, hydrochloric acid
and potassium chlorate, sulfuric acid and potassium
sulfate or by a mixture of nitric and sulfuric acids.
In the latter two cases the tin is left in soluble form, as
stannic sulfate, without any volatile compounds
being formed which might cause a loss of part of the

> Chem. News, 38, 5.

* Analyst, 1880, p. 218.
' Chem. News, 48, 257.

* Chem. Ztg.. 24, 263.

» Z. Nahr. Genussm., 3, 246.

• Chem. Ztg.. 23, 854.
' Arch. Hyg.. 46.

» Z. Nahr. Genussm., 7, 676.

• U. S. Dept. Agr.. Bureau of Chemistry, Bull. 107, 61.

!• Report No. 7, Local Government Board, Gt. Britain (1908).

" U. S. Dept. Agr., Bureau of Chemistry, Bull. 137.

" Ibid., 67.

«" J. Ind. Eng. Chem., 5 (1913), 3.

" 8th Intern. Congr. Appl. Chem., 18, 35.

tin. Having destroyed the organic matter and hav-
ing the tin ixi solution, the amount may be determined
either gravimetrically or by one of several volumetric
methods, all of which depend upon the conversion of
stsLTinous to stannic salts. We have adopted, for the
purpose of this investigation, the method worked out
by H. A. Baker, now of the American Can Company,^
and used with slight variations by the Bureau of
Chemistry, American Can Company and the National
Canners' Association.

A new method may be mentioned here which was
tried for determining the tin in our solutions. We
have found that the organic matter may be easily
and quickly destroyed by perchloric acid at its boil-
ing point where the approximate composition* is
HCIO4.2H2O, or, especially by a mixture of perchloric
and nitric acids from which the nitric acid may then
be easily driven off. The salts of perchloric acid
are perfectly stable, readily soluble and not reduced
by electrolysis so it was thought that the tin might
be very accurately determined by electrolysis of this
perchloric acid solution. We found that by using
a mercury surface of 200 cm^. as the cathode, with
which the tin is easily amalgamated while the over-
voltage of the hydrogen is at a maximum, tin ions
could be completely and quickly removed from large
volumes of dilute solution. In the removal of the
mercury by distillation, however, difficulties were
encountered, owing to the tendency of the tin to oxi-
dize and stick to the walls of the flask. We expect to
do more work along this line.

Little or no exact information has been obtained re-
garding the mechanism of the solution of the tin by
the canned food nor the condition in which it is pres-
ent. Bigelow and Bacon' compared the acidity of a
large number of canned foods with the total tin pres-
ent, and there appears to be little relation between the
two. For example, beets packed in plain tin cans
were found, 6 months after packing, to contain 72.8
mg. of tin per 100 mg. of acid, while cherries con-
tained only i.s mg. of tin per 100 of acid. J. P.
Atkinson noticed that if tin salts were added to meats,
only a third to a half of the tin could be recovered by
electrolysis even after an artificial gastric digestion.*
We have noticed that in the electrolysis of a pulped

» 8th Intern. Cong. Appl. Chem., 18, 35.

« J. Am. Chem. Soc, 34 (1912), 1480.

» J. Ind. Eng. Chem., 3 (1911), 832.

* J. P. Atkinson. Bureau of Health. New York (unpublished).

food sample over a mercury cathode only a part of
the tin was deposited, even after a much longer time
than is usually necessary. Evidently the tin is not
entirely in solution. Some evidence on this point
was found in the experiments of linger and Bodlander,
confirmed by Buchanan and Schryver, in which the
food was roughly separated into liquid and solid por-
tions by a sieve and each analyzed separately for
tin^'^. The solid portion, of course, still contained
large amounts of liquid but the results showed an un-
equal distribution of tin between the liquid and solid
portions.

It is obvious from this brief review of the situation
that the first question to be settled is exactly the one
of how much tin is in true solution in the various
kinds of canned foods as well as the total amount of
tin present.

EXPERIMENTAL

We have succeeded in making a satisfactory separa-
tion of the tin which is in true solution from the com-
bined tin by means of dialysis. Owing to the ease with
which tin salts hydrolyze, precautions had to be taken
to avoid hydrolysis during the dialysis. The follow-
ing scheme was adopted. The bottom was cut off
from a wide, two-liter bottle and replaced by a film
of collodion which was made by pouring out the col-
lodion upon a dish of mercury and before entirely
hard, pressing it upon the glass.^ This makes a mem-
brane which is very strong and capable of being used
for several determinations before requiring replace-
ment and, therefore, owing to ease of preparation,
strength, and the short time required for dialysis, it
was chosen in preference to gold-beaters' skin and thin
parchment which were also tried. The acidity of the
sample of food was determined directly on removal
from the can by titrating 20 cc. of the filtered juice,
using phenolphthalein as indicator, against N/io
sodium hydroxide. If the juice was too darkly col-
ored, azolitmin on a spot plate was used.* Informa-
tion regarding the character of the acid was obtained
in most cases from the work of Bigelow and Dunbar,
"Acid Content of Fruit Juices."^ In most berries
the acidity is due chiefly to citric acid while in the
stone fruits, such as cherries, plums, peaches, apples,

• Beckurts, Jahresber., 46.

2 Report No. 7, Local Government Board, Gt. Britain (1908).

3 Bigelow and Gemberling, Amer. Chem. J., 29 (1907), 1576.

♦ Bigelow and Dunbar. "Acid Content of Fruits" (unpublished).

8

apricots and most pears, the predominating acid is
malic.

One liter of an acid solution of the same kind and
strength as that of the liquid of the canned food was
placed in a high crystallizing dish and the dialyzer
suspended in this solution. A weighed sample of the
pulped fruit was placed inside and constantly stirred
so as to present a fresh surface to the membrane. A
battery of 8 dialyzers was stirred from a central re-
volving shaft. It was found that in about 48 hours
the equilibrium was established, although in some cases
a longer time was allowed, and after this interval
the dialyzer was raised and the volume of the contents
inside and out measured. The large volume of the
solution outside the membrane was evaporated and
transferred to a Kjeldahl flask and the residue of pulp
inside to another: 100 cc. of concentrated nitric acid
were added to each and the mixtures let stand. If
the food sample contained much sugar, rapid oxida-
tion began almost at once and the flasks were left
until brown fumes ceased to come off when 50 cc.
of concentrated sulfuric acid were added and heat
applied, thus avoiding too violent action. After
heating until dense fumes of sulfuric acid appeared,
the flasks ^ere cooled and in case the solution was
not colorless, small portions of nitric acid were
added successively and heating repeated. The finally
clear solution, from which all nitric acid had been ex-
pelled, was cooled, diluted with water and the acid
neutralized with concentrated ammonia, testing with
litmus paper and then the solution was acidified
slightly with hydrochloric acid, heated to boiling
and hydrogen sulfide passed in until the tin was all
precipitated. The precipitates were allowed to set-
tle and filtered in pairs, by suction, through asbestos,
using false bottom Gooch crucibles. The precipi-
tates were washed with hot water which had been
saturated with hydrogen sulfide. The tin sulfide
was dissolved in Erlenmeyer flasks by boiling with
concentrated hydrochloric acid, to which successive
small portions of potassium chlorate were added and
the chlorine expelled at the end of the addition of a
gram of aluminum foil. The flasks, four at a time,
were placed upon a hot plate and attached to a car-
bon dioxide generator. After all the air had been
displaced by carbon dioxide, the tin was reduced to
the stannous condition by the addition of about 2
g. of aluminum foil. The solutions were boiled for a

few minutes after the aluminum disappeared and then
cooled in ice-water, still in an atmosphere of carbon
dioxide, removed one at a time, tubes and stoppers
washed down with air-free water and titrated with
N/ioo iodine solution, using starch as indicator.
Each time a series of titrations was made the iodine
was standardized against a tin solution, i cc. of which
contained i mg. of tin. Knowing the amount of tin
in the solution outside the membrane, from the rela-
tive volumes of the acid solution and of the food pulp,
the total amount of tin which was in true solution
was calculated and, by .difference, the tin which was
in an insoluble form. The determinations were car-
ried out in pairs and the average of results given. In
some cases, as that of rhubarb, the agreement was
exceptionally close, the pair yielding, respectively,
8.9 and 9.17 mg. of insoluble tin in a 75-g. sample.
Where a large percentage of the tin was in an insolu-
ble form, however, the agreement was not so close,
due partly at least to the impossibility of getting two
samples having just the same proportions of liquid
and solid, and therefore in which the insoluble tin
compound was equally distributed. A determina-
tion of the total tin in the sample of food used was also
made in the usual way.

It will be noticed that in Table I the foods examined
are arranged in the order of their increasing acidity
as shown in column 9. It is obvious that neither
the total tin nor the tin which is in solution are directly
proportional to the acidity and it is evident that the
amount of tin which is removed from the can is de-
pendent also upon other factors. In calculating the
amount of tin in solution in the pulp, from the concen-
tration of tin outside the membrane after dialysis
and the volume of the pulp it was assumed that the
tin in actual solution was free to diffuse throughout
the whole volume of the pulp; that is, that the space
occupied by the solid particles of the food did not les-
sen the volume over which the soluble tin could dis-
tribute itself. In order to determine the maximum
possible error which might arise from this source, the
volume of the solids was determined in the case of two
of the foods which contained the highest percentage
of tin in solution, for it would be in such foods that
the error must be greatest. A weighed sample of
rhubarb, similar to the one used in the dialysis, was
sucked dry of liquid in a Buchner funnel and the solid
residue immersed in a measured amount of water,

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noting the increase in volume. This was found to
be only 0.3 cc. so that the volume of the pulp inside
the membrane through which tin could diffuse was
189.7 instead of 190.0 cc. Calculating the amount
of tin in solution, on this basis, in the pulp we found
26.22 instead of 26.26 mg., a difference of 0.04 mg.,
which is negligible. Next to rhubarb, beets contain
the highest percentage of tin in solution, and the
difference found in the amount of tin in solution in
the pulp, when the volume of the solid in the pulp
was taken into consideration, was 0.02 mg. out of a
total of 9.63 mg. From these results we have con-
cluded that the volume actually occupied by the solid
in the pulp may be neglected and the tin in the solu-
tion in the pulp calculated as if it were equally dis-
tributed over the total volume inside the membrane.

It will be observed from Table I that rhubarb, which
was the first of the fruits examined, showed a small
percentage of tin in an insoluble form, while pumpkin,
squash, string beans and other foods high in proteins,
contained a large amount of tin which was no longer
in solution. We expected then that in the case of
the berries, which are rather strongly acid and con-
tain almost no protein matter, the greater part of the
tin would be found in solution as was determined for
rhubarb. When we came to examine raspberries,
however, we were much surprised to find so high a per-
centage of the tin, about 81 per cent, in an insoluble
form. The same was true in varying degrees for other
similar fruits, strawberries, gooseberries, currants,
cherries, etc. Since there appeared to be some rela-
tion between the amount of protein matter and the
part of the tin which was insoluble, and since the only
proteins in berries are in the nuclei of the seeds,
some of these seeds were analyzed for tin.

The whole raspberries, containing 180 mg. of tin
per kg., were pulped and pressed through cloth, the
seeds being removed from the solid residue by washing
and decantation in a large crystallizing dish. In this
way perfectly clean seeds, free from pulp, were ob-
tained. These were then washed with boiling water,
dried in air and the tin determined in the usual way.
The tin in the seeds ran 805 mg. per kg. In other
words, most of the tin which is in an insoluble form
was found in the seeds. In strawberries, this reten-
tion of tin is even more marked. The seeds of straw-
berries were found to contain, roughly, six times as
much tin, weight for weight, as the whole fruit — the

Total

Per

Tin

cent

Seeds

Acidity

Mg.

Insoluble

Mg.

Per cent

per Kg.

Tin

per Kg.

Malic 0.70

222.0

67.2

448.0

Malic 0.51

90.0

83.3

321.0

Citric 0.71

180.0

81.6

805.6

Citric 0.70

416.0

55.5

2630.0

Citric 0.39

70.0

37.1

106.5

seeds gave 2630 mg. per kg. as compared to 416 for
the whole fruit. The fact that the larger part of the
tin in the berries mentioned is combined and insoluble
in the seeds is of fundamental importance in deter-
mining the physiological action of tin in canned foods,
for the seeds, and with them the adsorbed tin will
be eliminated, to a large extent at least, directly in the
feces.

On the basis of these experiments, it would appear
that the amount of soluble tin salts, rather than the
total tin present in a can of food, should be limited,
since it is the part of the tin adsorbed which deter-
mines the physiological action. A few typical de-
terminations on fruit seeds appear in Table II.

Table II — Adsorption op Tin by Seeds op Fruits

Age
No. Food Sample Yrs.

1 Red cherries 5

2 Black cherries (enamel) . 5

3 Red raspberries (enamel) 5

4 Strawberries 5

5 Tomatoes 1

We have already mentioned the fact that beets
and rhubarb, the first of the foods examined, contain
almost no protein and that in these foods we found
large amounts of tin in solution. The foods high in
proteins, such as string beans, squash and pumpkin,
next to be investigated, showed a high percentage
of tin in an insoluble form. In berries, we found that
the greater part of the tin was concentrated, with the
protein, in the seeds. It seemed from these results
that there was some connection between the amount
of protein in the food and the percentage of tin in
solution as well as the total amount of tin removed
from the inner surface of the can.

In order to get further evidence on the part played
by proteins in determining the action of canned foods
on the tin can, the following experiments were car-
ried out: Coagulated globulins, prepared by heating
the lo per cent sodium chloride extract from dried,
pulverized pea beans (soup beans), were washed and
suspended in water in contact with tin plate of 392 sq.cm.
surface and the tubes sealed. After two weeks, an
average of 0.6 mg. of tin was found combined with
the protein. Also, it was found that proteins, sealed
with dilute acid solutions in contact with tin, greatly
increase the amounts of tin going into solution (Table
III). In these experiments a coil of tin plate, having

13

Table III — Influence of Agar Jell. Proteins, Etc., on Solution of
Tin by Citric Acid

Time Mg. Tin

(Mo.) Dissolved

Citric Acid (5%) 2 17.9

7 20.5

Citric Acid (5%) + Agar Jell 2 18.3

4 25.0

7 25.3

Citric Acid (5%) + Proteins 4 32.7

4 34.6

Citric Acid (2%) 7 11.7

Citric Acid (2%) + Agar Jell 7 16.9

Citric Acid (2%) + Crushed Peas 7 35.5

Citric Acid (2%) + String Beans 7 32.5

a surface of 392 sq. cm., was sealed in contact with a
constant volume of citric acid solution (100 cc), a
part of the tubes containing citric acid alone, the
others having coagulated proteins, agar jell, etc.,
added. All were kept at the same temperature.

The simple first reaction of the acid in the can of
food is complicated by the presence of large amounts
of colloidal proteins which undoubtedly affect the
solution of tin. Albumins, globulins and other pro-
teins are negative colloids and are precipitated by an
ion of opposite charge. This is especially true of the
heavy metal ions, of which tin is an example, and this
precipitation is irreversible. It is known that when
a sol is thus precipitated, the precipitating ion is firmly
adsorbed and carried down with it. Linder and Picton
first observed this in the case of arsenious sulfide
sol and barium chloride.^ In such cases the solu-
tion remaining is found to be strongly acid and in the
same degree in which the precipitate contains the
metal ion. These precipitates hold the metal ion
very firmly and no amount of washing will remove it.
In some respects they appear to be true chemical
compounds, but the composition is too variable to
admit of this view. For example, precipitates formed
by the action of copper salts on albumins contain
all the way from 1.4 to 20 per cent of copper oxide.*

These facts observed for other heavy metals agree
closely with the facts observed in the combination of
tin with food materials. After a small amount of
tin has been dissolved from the surface of the can,
adsorption and precipitation take place. When the
tin ion is removed from solution by the proteins, the
acid ion is liberated and more tin dissolved. In this
way the tin would be constantly removed from solu-
tion and a small concentration of acid could ultimately
dissolve a very large amount of tin. If the cell walls
surrounding the colloidal proteins were unbroken,

« Chem. Soc. Jour., 67 (1895). 63.

• W. W. Taylor. "Chemistry of Colloids," p. 118.

the proteins could not diffuse out into the solution,
but the tin could enter and adsorption take place.
Since practically all of the action of the food on the
container takes place after processing, which involves
heating to a rather high temperature, most of our
proteins have been coagulated, but this seems to have
little or no effect on the removal of tin from solution,
and coagulated proteins were found to take up large
quantities of tin. Beans were pulverized with sand,
extracted first with water, obtaining a solution of
proteoses and albumins, and then with a lo per cent
sodium chloride solution which removed large quan-
tities of globulins. These solutions and egg albumen
were used for the following tests: Small volumes of
each of the above solutions were added to an excess
of 2.5 and 5 per cent stannic chloride and stannic
ammonium chloride solutions and the precipitate
which was formed filtered, washed several times in
boiling water, dried at 110° C. and the percentage of
tin in a weighed sample determined gravimetrically.
Parts of the same protein solutions were coagulated
by heat and the coagulated proteins suspended in the
same tin solutions for two days. The results show a
varying percentage of tin which, however, is uni-
formly high. It was also noticed that if the precipi-
tate was filtered and washed and one part of it dried
and the percentage of tin determined, while the other
part was put back in the solution and let stand, it
continued to adsorb more tin. For example, 40 cc.
of dilute globulin solution in 10 per cent NaCl were
added to 400 cc. of 5 per cent stannic ammonium
chloride; a white precipitate formed which was warmed
to complete the coagulation and let stand for a day,
then filtered, washed and dried; 0.409 g. gave on anal-
ysis 0.227 g- of till or about 55.5 per cent. A part
of. the same precipitate was left in contact with the
solution for a week before filtering, when 0.777 g-
showed 0.441 g. of tin or 60.6 per cent. Using a 5
per cent stannic chloride solution the percentage of
tin in one case ran to 69.2. As might be expected,
the percentage of tin increases with the concentra-
tion of the solution. This is shown in Table IV.

Tablb IV — Adsorption op Tin by Coagulated Proteins
Original

concentration Final Per cent

Gm. of Tin concentration Tin

(as SnCh) Gm. of Tin in Dried

per cc. per cc. Protein

A 0.000310 0.0000019 4.92

B 0.000517 0.0000465 9.90

C 0.001610 0.0009730 20.50

D 0.004680 0.0041600 35.60

15

Stannic chloride solutions, of varying concentrations,
were made up and concentrated hydrochloric acid
added to each to prevent hydrolysis. A constant
weight of coagulated protein^ 1.5 g., was suspended
in 350 cc. of each solution and left for a week, after
which a portion of the clear liquid was withdrawn
with a pipette and analyzed for tin, and the protein
was filtered off, washed with several portions of boil-
ing water, dried at 110° C, and the percentage of tin
determined. It will be noticed that in each case
tin was left in solution.

Experiments were made with the insoluble tin com-
pound from several of the canned foods which, al-
though but slightly acid, contained large amounts
of tin and it was found that here, too, the tin is very
firmly bound. Squash is a good example of this.
Samples of squash, which had been packed in tin cans
and contained 300 mg. of tin per kg., were boiled for
about 5 hours with water and the three protein sol-
vents, 10 per cent NaCl solution, 70 per cent alcohol
and 2 per cent HCl, and filtered through hardened
filters. In the first three cases — water, alcohol and
salt solutions — only 32.5, 32.8 and 35.8 per cent,
respectively, of the tin was found in solution. The
hydrochloric acid seems to break up the tin com-
pound slowly on boiling and after 5 hours 25.4 per
cent of the tin was still found combined with the solid
residue. The question as to whether the tin which
is adsorbed by these proteins passes through the pro-
cesses of digestion without being absorbed is of first
importance. We have mentioned this point in re-
gard to the tin which was found combined in the seeds
of berries and in addition have performed the follow-
ing experiments to obtain further information.

Artificial gastric digestions were carried out upon
the solid residue obtained by boiling canned squash,
which contained 300 mg. of tin per kg. with water
and filtering. This solid residue contained about 67
per cent of the total tin in the squash sample. The
gastric juice, pepsin in 0.35 per cent HCl, was added
to the squash and the mixture kept in a thermostat
at 36° C. for 24 hrs., after which it was transferred
to a dialyzer and the tin in solution determined in the
usual way. Less than 10 per cent of the tin was found
in solution. Both gastric and tryptic digestions were
kindly carried out for us by Dr. E. N. Harvey, of the
Biology Department, on the tin protein complex,
prepared by allowing the freshly coagulated protein

i6

to stand in contact with tin solutions, after it had been
allowed to dry, and in each case only a trace of tin was
found in solution. It appears from the above results
that the tin protein combination which is formed is
very stable, and in most of the foods containing the
larger amounts of tin, the greater part is in an insolu-
ble form. The possibility suggests itself that the
part of the tin which is so firmly adsorbed will be
eliminated directly in the actual digestive processes
and not figure in the physiological action as deter-
mined for soluble tin salts.

The work of J. P. Atkinson on the electrolysis of
metallic salt solutions to which chipped beef had been
added is of interest in this connection. A known
amount of the metal in the form of a soluble salt was
added to finely divided beef and then submitted to
artificial gastric digestion for 24 hrs. at 37° after
which the solution was electrolyzed for 45 to 50 hrs.
A few typical results follow:

Added Recovered Difference Per cent

Mbtai, Gram Gram Gram Recovered

Mercury 0.0500 0.0121 —0.0379 24.1

Mercury 0.0500 0.0217 —0.0283 43.4

Tin 0.0330 0.0051 —0.0279 15.5

Tin 0.0330 0.0063 —0.0267 19.1

Zinc 0.0500 0.0561 +0.0061 100.0

Nickel 0.0492 0.0497 +0.0005 100. 0

Iron 0.0500 0.0497 —0.0003 99.7

It appears that the metals of relatively low toxicity
are least firmly bcnind and he suggests that this may
offer an explanation of the relative toxicity of metals,
in that they interfere with the metabolism of the cell.
Iron, being so easily separated, adds to this view. It
was found that the toxicity of mercury as the bichloride
was greatly diminished by adding it to chopped meat
and submitting it to an artificial gastric digestion.
One mg. of mercury as bichloride will kill a 2So-g.
guinea pig in 4 hrs. if injected subcutaneously, toxic
symptoms beginning in a few minutes. The same
quantity of mercury, after combining it with tissue
as described above, produced no toxic symptoms and
death did not follow until the fifth day. Rabbits
also were injected without apparent harmful effects.

RESUME AND CONCLUSIONS

It has been shown that the solution of tin by canned
foods is neither dependent upon, nor proportional to,
the acidity alone and, also, that in the foods of rela-
tively slight acidity which dissolve large amounts of
tin, the greater part of the tin is in the form of an in-
soluble and stable complex. The explanation which

«7

agrees most closely with the observed facts is that
we are dealing here with adsorption phenomena; that
the tin, after being dissolved from the lining of the
can, is being constantly removed from solution by
the proteins, carbohydrates and other highly porous
solid phases in contact with the solution. Whether
we regard this as an adsorption of tin ions, or whether
we consider the tin salt to be first hydrolyzed and the
resulting stannous hydroxide adsorbed, in either case
the acid would be regenerated and able to attack more
tin. The former explanation seems to be the more
probable; i. e., the tin ions are adsorbed, since tin
is taken up equally well by proteins even from con-
centrated acid solution. It will be seen from the
above results that while in several respects the ob-
served phenomena appear to be true adsorptions,
in one important respect they differ. While a true
adsorption is an equilibrium and can be approached
from either side, being reversible, this removal of tin
is not a reversible action, for if t^he tin protein complex
is transferred to an aqueous solution containing no
tin, it does not lose tin to the liquid phase. A num-
ber of cases similar to this are known and have been
called by W. W. Taylor, "Pseudo-adsorptions."*
The removal of heavy metal salts from solution by
charcoal is an example of this type of action; the first
stage may be an adsorption, since the salts of heavy
metals are strongly adsorbable, but a secondary re-
action must have taken place and the final state can-
not be put down to adsorption alone.

The author wishes to express his appreciation and
thanks for the very valuable assistance and advice
given by Dr. G. A. Hulett in connection with this
work.

Laboratory of Puysical Chemistry, Princeton, N. J.
AND Bureau of Chemistry, Washington. D C.

» W. W. Taylor. "Chemistry of Colloids." p. 252.

189515

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