ANALYTICAL
VOLUME5 NUMBER 3
EDITION
Industrial Chemistry AND ENGINEERING
MAY15, 1933
PUBLISHED B Y T H E AMERICAN CHEMICALSOCIETY HA~RISON E. HOWE,EDITOR
Use of Stannous Chloride in Evaluation. of Dye Mixtures J. A. KIME Color Certification Laboratory, Food and Drug Administration, U. S. Department of Agriculture, Washington, D. C.
I
K T H E evaluation of dye mixtures advantage is often
taken of the difference in stability or ease of reduction in the presence of reducing agents. Titanous chloride is widely used in the estimation of dyes and under generally favorable conditions most dye mixtures are reduced with but alight evidence of selective action. Holmes (S), however, investigated a number of dye mixtures and demonstrated that under less favorable conditions the reduction of one dye often takes precedence over the other, or the one dye may not be reduced a t all. Certain of the permitted food colors also exhibit this property. The azo dyes appeared to take precedence over the triphenylmethane group under the influence of titanous chloride, but as Holmes pointed out with several dye mixtures, the difficulty in the visual detection of the end point limited the practical utility. From this apparent difference in ease of reduction it appeared that many simple food dye mixtures might be evaluated by means of a set of reducing agents. In employing two such reagents there are two limiting conditions: (1) one agent shall be selective in its action, and (2) the reaction products of the first reduction shall not interfere with the action of the second agent. Evenson and Nagel (a) utilized ammonium sulfide in selectively reducing amaranth (C. I. 184) in presence of tartrazine (C. I. 640), the excess being destroyed by lowering the pH with sodium bitartrate, which also acted as a buffer for the estimation of the tartrazine with titanous chloride. Stannous chloride and titanous chloride fulfil the requirements for a desirable set of reducing agents. The former is selective in its action and the stannic ions which are formed in a reduction do not react with the titanous ions in the subsequent titration of the second dye. Knecht and Hibbert (5) state that stannous chloride will reduce diamine sky blue (6. I. 518) in the presence of chrysophenin (C. I. 365), two azo dyes, and (4) that titanous chloride does not appear to react on stannic chloride nor stannous chloride with titanium tetrachloride, yet so far as the author is aware, these properties have never been employed in the evaluation of dye mixtures. Among the permitted food dyes tested in this laboratory, it was interesting that the triphenylmethane group was sufficiently stable and four of the azo dyes sufficiently susceptible to the action of stannous chloride to permit evaluation of a simple mixture, one from each group. I
MATERIALS For convenience, the stannous chloride was made up approximately normal; 113 grams of tin chloride (SnC12.2HzO) were dissolved in an equal number of cubic centimeters of hydrochloric acid and diluted to one liter. The dyes were of certified grade, made by various food color manufacturers. The titanous chloride solution was made and Standardized according to the usual laboratory procedure (4). The sodium bitartrate used as a buffer was U. S. P. or c. P. quality.
SELECTIVE REDUCTION EXPERIMENTS One per cent pure dye solutions of the food colors were made and standardized. Amaranth, ponceau SX,orange I (C. I. 85), and sunset yellow FCF were found to be rather readily reduced with stannous chloride. The reduction may be completed a t room temperatures, and increasing the temperature only serves to speed up the reaction rate. Fast green FCF, guinea green B (C. I. 666), light green SF yellowish (C. I. 670), and brilliant blue FCF are but slightly attacked by stannous chloride, even on continued heating. Amaranth, particularly, reduced more rapidly in the presence of hydrochloric acid, but sodium bitartrate proved generally better. Buffers such as borax cannot be used in these dye mixtures, as the triphenylmethane dyes are not sufficiently stable toward stannous chloride a t such an alkaline pH. From the standardized pure dye solutions, mixtures were made and the following procedure adopted: Samples of a known mixture were pipetted into 1-liter widemouth Florence flasks. Fifteen grams of sodium bitartrate and water to bring the volume to 200 cc. were added. The solution was then heated and the number of cubic centimeters of 0.1 N titanous chloride for the reduction of both dyes determined under an atmosphere of carbon dioxide. To one sample was added approximately 100 per cent excess of the amount of stannous chloride equivalent to the azo dye present (component A). The reaction was carried on according to the conditions given in Table I. The triphenylmethane dye (component B) was then determined by the usunl titanous chloride titration. In order to approximate the excess for the azo dye, a trial reduction was first made, using twice the quantity of stannous chloride that would be necessary for the reduction of both dyes as indicated by the titanous chloride equivalent of the mixture. For example, 25.0 cc. of 0.1 N titanous chloride are required for a given mixture. Therefore, 5.0 cc. of norinal
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ANALYTICAL EDITION
152
Vol. 5, No. 3
TABLEI. TYPICAL RESVLTS BY SELECTIVE REDUCTION SERIBB
1
2
C O ~ P O N BDYES NT
A, ponceau SX B, guinea green B A aunset yellow FCF
B: brilliant blue FCF 3
A orange I B: light green SF yellowish
4
A, amaranth B, fast green FCF
1% DYE
SOLUTION
STANNOUS CHLORIDE REDUCTION N SnCh added Time Temperature
cc.
co.
Min.
30 30 50 50 100 100
4.5 3.8
3 3 3 3 3 3
c.
COMPONENT COMPOWENT COMPONENT A PRESBNTB PRESENT B FOUNDDIFFERPNCB
so- 100 80-100 80-100 80-100 80-100 80-108
%
%
%
91.2 75.2 54.2 25.5 10.3
8.8 24.8 45.8
74.5
3.5
89.7 96.5
11.0 25.8 48.4 74.4 89.8 96.5
%
f2.2
40 110
5.1 1.7
18 hr. 18 hr.
20-25 20-25
75.8 9.5
24.2 90.5
90.5
fl.O f2.6 -0.1 +o. 1 0.0 f1.4 0.0
60
2.0
24 hr.
20-25
19.3
80.7
79.3
-1.4
50 100
1.2
6.0
12 12
80-100 80-100
90.9
9.1 90.9
11.0 91.7
f1.9 +0.8
4.6
2.1 1.8 0.6
stannous chloride are used in the trial reduction. Subsequently, this trial reduction showed that component B used 15.0 cc. of 0.1 N titanous chloride and component A, by difference, needed 10.0 cc. Therefore]2.0 cc. of 1.0 N stannous chloride represented roximately 100 per cent excess. The amounts of stannous %l%ide used, appearing in Table I, were ascertained by such a procedure. In a mixture containing less than 40 to 45 per cent of component B, an additional step was necessary, because (1) estimations of small amounts of triphenylmethane dyes with titanous chloride invariably give high results; and (2) if a larger sample of mixture is taken, the high concentration of the stannic ions formed in the reduction of the azo dye restores the color of component B so rapidly after reduction by titanous chloride that quantitative estimation is uncertain. A known amount. of Component B was therefore added prior to the stannous chloride and subsequently subtracted from the final titration value obtained with titanous chloride.
CONCLUSION The general utility of the set of reducing agents, stannous chloride and titanous chloride, in the evaluation of dye mixtures, has been investigated. Experimental evidence showed
9.1
25.6
that mixtures of azo and triphenylmethane food dyes were satisfactorily analyzed. No doubt there are many dye mixtures whose components differ in ease of reduction in such a manner as to permit evaluation by a similar procedure. The author wishes to thank 0. L. Evenson, of this laboratory, for his helpful suggestions in performing and presenting this work.
LITERATURE CITED (1) Ambler, J. A., Clarke, W. F., Evenson, 0. L., Wales, H., U. 8. Dept. Agr., Bull. 1390, 26 (1925). (2) Evenson, 0. L., a n d Nagel, R. H., IND.END.. CHEM.,Anal. Ed., 3, 260 (1931). (3) Holmes, W. C., Am. Dyestuf R e p t r . , 14, 415 (1925). (4) Kneoht a n d Hibbert, “New Reduction Methods in Volumetric Analysis,” 2nd ed., p. 6, Longmans, 1925. (5) Ibid., p. 7. RECEIVEDNovember 25, 1932. Presented before the Division of Dye Chemistry at the 85th Meeting of the American Chemical Society, Wasbington, D. C., March 26 to 31, 1933. Contribution 10 from the Color Certification Laboratory, Food and Drug Administration, U. S. Department of Agriculture, Washington, D. C.
Volumetric Determination of Nitroglycerol and of Nitroglycerol and Dinitrotoluene in Admixture WALTERW. BECKER,Experiment Station, Hercules Powder Company, Wilmington, Del.
N
ITROGLYCEROL may be conveniently determined is distilled into standard acid. I n the method of Muraour by means of the du Pont nitrometer; the results ( 6 ) the nitroglycerol in acetone solution is treated with obtained, however, are usually slightly low. Certain hydrogen peroxide, sodium hydroxide, and sodium perborate; aromatic nitro compounds, such as dinitrotoluene, do not after standing overnight Devarda’s alloy is added, and the resulting a m m o n i a d i s t i l l e d i n t e r f e r e in this nitrometer into standard acid. Muraour d e t e r m i n a t i o n ; compounds Practically theoretical results f o r nitroglycerol obtained results which closely such as diethyldiphenylurea or diphenylamine, however, do inand for ethylenegh‘ol dinitrate may be obtained checked t h e theoretical. terfere, b e c a u s e they undergo by dissolving the sample in acetic acid, adding Excellent results on the analya n excess of ferrous chloride, hydrochloric acid, sis of nitroglycerol were also n i t r a t i o n in the decomposing boiling, and then titrating the resulting ferric Obtained in this laboratory, but bulb, and cause low results. because of the time r e q u i r e d iron with standard titanous chloride solution, The nitrate groups in nitroglycerol m a y be r e d u c e d by for a determination, the method suitable reagents, and the niusing ammonium thiocyanate as the indicator* was not exactly suited to control trogen determined as ammonia. W h e n present in admixture, 2,4-dinitrotoluene analysis. does not interfere, and may be accurately deD i c k s o n a n d Easterbrook I n t h e method of S i l b e r a a d termined in the residual solution by reduction (2) determined dinitrotoluene (?’) the nitroglycerol i n e t h e r is partially reduced with titanous chloride in the regular way. in admixture with nitroglycerol w i t h sodium e t h y l a t e , then by reducing the latter with W h e n present in small amount, dielhyldiferrous chloride in strong metallic zinc and ironareadded, and the r e s u l t i n g a m m o n i a phenylurea or diphenylamine does not interfere. hydrochloric acid solution, then
