A N ELECTROLYTIC METHOD FOR THE DETERMINATION BY
OF TIN I N CANNED FOOD PRODUCTS .iLLIIRTON s. CUSHMAK A K D EVERETT B. XVETTESGEL Received November 18, 1912
The issuance by the United States Department of Agriculture under date of September 30, 1910, of Food Inspection Decision, No. 126, governing salts of tin in food, which allows no more than 300 mg. of tin to the kilogram of material, has been thk occasion of a number of laboratories being called upon to make a special study of the subject. These investigations have necessitated thousands of determinations of tin in every conceivable form of canned food. The usual analytical method for estimating very small quantities of tin contained in foods is difficult, tedious, and requires the evaporation of from 1 5 0 to z o o cc. of strong acids. When a considerable number of determinations have to be made, the question of a hood and draft, which will take care of such large quantities of acid fumes, presents a very difficult problem. The gravimetric method as used in the laboratories of this Institute is briefly as follows: The entire contents of a can is pulped by maceration in a large porcelain mortar: 50 grams of the sample are transferred to a pinch evaporating dish. From 50-150 cc. of strong nitric, and from 20-50 cc. strong sulfuric acid are added, according to the nature of the sample. (Beets, spinach, asparagus, etc., require less acid than beans, sweet potatoes, corn, or fruit, to which a large amourit of sugar has been added.) The mixture in the evaporating dish is covered with a watch glass and heated over a strong Bunsen flame until sulfuric acid fumes begin to come off. Nitric acid is added repeatedly until the solution is colorless. The solution is diluted with a large volume of water and neutralized with strong ammonium hydroxide, making a total volume of 300-500 cc. The solution is made slightly acid with HCI, transferred to a flask
FIG. FRONT ELEVATIOS
and a slon- stream of H,S passed through i t for two hours. The flask is heated on the water bath for an hour or allowed to stand over night to settle the tin sulfide. The solution is then filtered and the precipitate washed alternately with a solution of ammonium acetate and water. The tin sulfide is dissolved through the filter with three successive portions
of yellow ammonium sulfide and reprecipitated with glacial acetic acid. The tin sulfide together with some precipitated sulfur is filtered on a weighed gooch. dried in a water oven. ignited over a Meker burner, and weighed as stannic oxide, SnO,. Factor for metallic tin, 0 . 7 8 8 2 . If occasion requires, a single determination can be rushed through by the above method in one working
FIG
2-sIDE
ELEV&TIO\
day, but in the usual routine in which the samples are worked in batches, from two to three days are consumed in getting out results. The hoods and flues suffer great damage from the acid fumes and the cost of the acids consumed in total destruction of the organic matter is very high. These objections to the current method of analysis led us to investigate all possible ways of shortening and improving the method with the principal object in view of avoiding the destruction of the organic matter by long digestion in nitro-sulfuric acid. I n the preliminary experiments, the expedient was tried of cooking the slightly acid sample in a n aluminum pan with the object of separating the tin from the organic matter preliminary t o its re-solution in stronger acid and preparation for separation by electrolysis. I t x a s believed that this method n-ould aid in the separation of tin from the organic colloids in which it was held, but subsequent experiment showed that the preliminary cooking in an aluminum utensil was unnecessary and the simple method as finally worked out is as follon-s: j o grams of the pulped material are placed in a 600 cc. beaker and brought to a slow boil with 50 cc. of concentrated hydrochloric and 2 5 cc. of nitric acid. The mixture is stirred continuously and the boiling continued five minutes unless there is danger of foaming, in which case the flame is removed and the material allom-ed to digest for ten minutes. The solution is then diluted with abour. an equal quantity of water, made alkaline with strong ammonia and 25 cc. of saturated ammonium sulfide. The mixture is digested for a few minutes with thorough stirring, and all insoluble organic matter filtered out on a ribbed filter. The washing is done with boiling water containing a little ammonium sulfide. About 150 cc. of wash water are used in five separate m-ashings,
T H E J O L ' R - V A L OF I A Y D C S T K I . 4 L A S D EL\-GI.\-EERI.\-G
2 18
making the total solution t o 400 cc. The solution is then electrolyzed hot, using 1.5 amperes a t 6 volts. A rotating cathode is used. The end of the revolving spindle carries a rubber stopper over which a clean weighed platinum crucible is slipped. The apparatus which has been found convenient for carrying out the electrolysis of a number of determinations a t the same time is shown in front elevation in Fig. I and in side elevation in Fig. z . From one t o four hours is necessary to complete a n electrolytic run, two hours being generally sufficient, except in cases in which the tin content is very high. At the end of a run the crucibles are cleaned by heating in a solution made by mixing I O O cc. of I O per cent. oxalic acid with I O O cc. of concentrated nitric acid. Numerous experiments were madewith the electrolytic apparatus which showed t h a t tin could be recovered with a fair degree of accuracy from a n ammonium sulfide solution. Some of these results are given in Table I. TABLEI Tin added Tin found mg mg
Sample NO.
1. . . . . . . . . . . . . . 2.............. 3.............. 4 .............. 5 .............. 6 . ............. 7. ............. 8.
.............
1.0 1.0 10.0 10.0 10.0 10.0 25.0 50.0
1.1 1 9.8 9.9 10.0 10.0 24.6 50.5
.o
1 hour 1 hour 21/, hours 21/4 hours 21/3 hours 2'/9 hours 23/4 hours 4 hours
No. 1 2 3 4
.......
.......
9.9 9.2 10.1 9.9
10
10
. . . . . . . . . . . . . . . 10 . . . . . . . . . . . . . . . 10
Time of electrolysis 2 hours 2 hours 2 hours 2 hours
I n run KO. 2 the cQrk blew out of the bottle during TABLEI11
---
Tin found Mg. per kilo I\IATERIAL
Run I
Apple butter A . . . . . . . . . . . . . . . . . . . . . . . . Apple butter B . . . . . . . . . . . . . . . . . . . . . . . . Apple butter A ................ Apple butter B ................ Blackberries ........................... Rhubarb A . . . . . . . . . . . . . . . . . . . . . . . . . . . . String beans.. . . . . . . . . . . . . . . . . . . . . . . . . . Beef soup(a) (solid). . . . . . . . . . . . . . . . . . . .
...
........
............ (solid). . . . . . . . . . . . . . . . . .
178 142 168 142 28 170 72 18 74 28
66 22 20 40 8
Time, hours
Run 11, 1so 142 168 142 26 174 64 22 74 26 76 36 20 42 10
.
3 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
Vegetable soup Mock turtle SOUP (solid). . . . . . . . . . . . . . . . Baked beans with tomato sauce.. Baked beans without tomato sauce.. ..... ( 0 ) All the soups except the bouillon and tomato were solid and contained large amounts of meat and vegetables.
........
processing and i t was known t h a t a slight amount of the contents had been lost. These results were very
M a r , I913
encouraging as the determination of small amounts of tin in the presence of an excess of organic food material must be but a fair approximation of the truth a t the best. The next step was to try the method on a large variety of food products. The results of a number of duplicate runs are given in Table 111. The results given in Table I11 show that it is possible to produce satisfactory check results when working in duplicate by the method, and, in fact, the checks are generally much closer than can be expected when working with the longer gravimetric method. In each analysis the insoluble residue. consisting of food pulp, was carefully examined for tin, but if 'any was present it wa%in such small amount that i t could not be found by qualitative tests. I n Table IV the results are given of a series of analyses of various foods when the gravimetric and electrolytic methods were both used. T.4BLE I V Tin found Mg. per kilo
MATERI.4L
Time taken
The next step was t o study the effect of organic food material on the method. Tomatoes were .put into glass bottles with 50 cc. of a ten per cent. sodium chloride solution, and I O mg. of tin in solution were then added to each bottle. The bottles were corked and autoclaved under pressure in a manner similar t o the processing of canned foods. The results of analysis are given in Table 11. TABLE11 Tin added Tin recovered mg. mg.
CHE.llISTRY
E'lectrolytic Gravimetric method method
Tomato soup . . . . . . . . . . . . . . . . . . . . . . . . . 145 Rhubarb.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Squash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 Spinach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 String beans (liquid portion). . . . . . . . . . . . 94 String beans (solid portion). . . . . . . . . . . . . 340 229 String beans (total).. . . . . . . . . . Beets (solid portion). . . . . . . . . . . . . . . . . . . 70 Cuthbert raspberries. . . . . . . . . . . . . . . . . . . 248 Sweet potatoes.. ........... 40 Pork and beans., . . . . . . . . . . . . . . . . 64 Sauer-kraut . . . . . . . . . . . . . . . . . . . . . 12 Apple-butter . . . . . . . . . . . . . . . . . . . . . . . . . 142
....................
27
130 102 314 22
70 356 226 27(d 222 39 73 13 112 17
(a) Result probably low.
An inspection of Table IV shows a generally satisfactory agreement between the results obtained by the two methods, and it should be stated that these are not selected results but are given directly in the order in which they were obtained by two different workmen. Only about three hours are required to get out a series of results by the electrolytic method, the number in the batch being limited only by the number of electrolytic cells available in the laboratory. With the gravimetric method usually the best part of three days is required to produce a batch of determinations, the number being in this case limited by the hood and flue space available. I n the saving of time, acids and other reagents consumed and in destruction t o flues and the laboratory generally, the advantage is all on the side of the electrolytic method. If considerable work of this kind has to be done, the first cost of the electrolytic apparatus is fully justified. THEISSTITUTE O F INDUSTRIAL RESEARCH WASHINGTON, D. C.
POTASSIUM PERMANGANATE IN T H E QUANTITATIVE ESTIMATION OF SOME ORGANIC COMPOUNDS' By C. M. PENCE
Potassium permanganate has been most generally used in the volumetric estimation of iron. Some uncertain ties formerly existed since i t was impossible t o obtain a chemically pure article and insufficient data were 1 Read before the Indiana Section of the American Chemical Society, May 10, 1912.
