ANALYTICAL EDITION
260
Vol. 3, No. 3
Quantitative Estimation of Amaranth and Tartrazine in a Food Color Mixture' 0. L. Evenson and R. H. Nagel COLOR CERTIFICATION LABORATORY,
FOODA N D DRUGADMINISTRATION, u. s.
D E P A R T M E N T OP
AGRICULTURE, WASUINGTON, D.
c.
A quantitative estimation of certain food dyes, boiling. The sodium bitarespecially amaranth and tartrazine, which have praccolor mixtures it is gentrate also serves as a buffer in tically the same solubility in organic solvents and erally possible to sepathe titration of the tartrazine therefore cannot be separated by the extraction rate the various components with standard titanium trimethod, may be made by employing selective reducfrom one another, p r i o r to chIoride solution, which was tion. In a mixture of amaranth and tartrazine, the a quantitative estimation of the next step. amaranth is reduced by ammonium sulfide, leaving each, by taking a d v a n t a g e I t was o b s e r v e d t h a t the tartrazine, which is then estimated by means of of their different solubilivariations i n t e m p e r a t u r e titanium trichloride. Another method, based upon t i e s . I n t h e case of amanoticeably affect the length spectrophotometric measurements, is also given. ranth (Colour Index 184) of time necessarv to reduce and tartrazine (Colour Inthe a m a r a n t h . To illusdex 640) this method has not been found feasible owing trate this and to show the magnitude of the error introto the fact that their solubilities are very nearly the same in duced by not stopping the reaction between the ammoall of the many solvents that have been tried. Since ama- nium sulfide and the dye till long after all the amaranth had ranth and tartrazine are two of the most widely used food col- been destroyed, experiments were performed as summarized ors, it is quite important to be able to analyze quantitatively in Table I. The temperature of the reaction was regulated by mixtures containing them. Jablonski (6) has recently partially submerging the flask in a large vessel of water of the developed an ingenious method for separating these two dyes, required temperature. In Table I, determinations A and B which, however, is very time-consuming. on mixture 3, and determinations B and D on mixture 1, show In the first method recorded here, advantage is taken of the that a change of only 10" C. approximately doubled the time difference in stability of amaranth and tartrazine in the of reaction. Determinations A, B, and C on mixture 1 presence of a reducing agent. It was found that amaranth and B and C on mixture 3 show that the time elapsing before is quickly destroyed by ammonium sulfide, while tartrazine the reaction is stopped can vary considerably and conis relatively stable. When ammonium sulfide solution is cordant results may still be obtained. It has been found added to a mixture of these two dyes, the amaranth is first that i t is not difficult to judge by the color changes when reduced. The point a t which the amaranth has been com- the reaction should be stopped and that there is quite an pletely destroyed can be readily ascertained by careful ob- interval before appreciable amounts of tartrazine are deservation, and the reaction stopped. The tartrazine is then stroyed. Determinations B, mixture 2, and D, mixture 3, determined in the usual manner, using standard titanium illustrate the error introduced by allowing the reaction t o continue for an unreasonably long time. If the reaction is trichloride solution. stopped too soon, a distinct red color will appear when the Materials sodium bitartrate and water are added, indicating the presThe ammonium sulfide solution was made by saturating ence of undestroyed amaranth, Table I gives definite inconcentrated ammonium hydroxide, specific gravity 0.9, formation as to temperature and time on mixtures of various with hydrogen sulfide. The titanium trichloride solution composition, and may be used as a guide until the analyst was made approximately 0.05 N , and standardized by two obtains the little experience necessary to judge correctly the separate methods (1). The dyes were commercial, certified color changes involved. food colors from various manufacturers. I n all titrations Table I-Effect of Temperature and A m o u n t of A m a r a n t h upon Time of R e d u c t i o n mentioned, the contents of the reaction flask were stirred ARANTX TARTRAZINE TIME vigorously by means of a mechanical stirrer, and kept under DBTN. M I X T U R E TEMP. A MPresent Present Found a n atmosphere of carbon dioxide. Min.
I
N THE analysis of food
Experimental Procedure
Several mixtures containing known amounts of amaranth and tartrazine in varying proportions were made. From these mixtures aqueous solutions containing one gram of dye per 100 cc. of solution were prepared. Twenty-five cubic centimeters of this solution were placed in a 500-cc. widemouth Erlenmeyer flask, and a small quantity (1 to 10 cc.) of ammonium sulfide solution added. When the amaranth had been destroyed as evidenced by the color change from red to yellowish orange, the reaction was stopped by adding an excess, 10 to 30 grams, of sodium bitartrate, 75 cc. of water, and boiling the mixture. The sodium bitartrate, because of its acidic properties, reacts with the remaining ammonium sulfide, liberating hydrogen sulfide which is driven off by 1 Received February 28, 1931. Presented before the Division of Dye Chemistry a t the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930.
2
A B
32 32
56.3 56.3
43.7 43.7
44.1 42.4
6l/s 12
3
A B C D
35 25 25
25.0 25.0 25.0 25.0
75.0 75 0 75.0 75.0
74.25 74.25 74.12 72.76
9 11 20
'il/i
After many trials the method described below was found to give the best results. Selective Reduction Method Using a dye mixture known to contain only amaranth and tartrazine, a 1 per cent aqueous solution was prepared. Twenty-five cubic centimeters of this solution were placed in a 500-cc. wide-mouth Erlenmeyer flask, and 2.5 cc. ammonium sulfide solution added. The contents of the flask were thoroughly mixed by imparting a gentle swirling motion,
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INDUSTRIAL AND ENGINEERIA'G CHEMISTRY
July 15, 1931
