The Polarographic Reduction of Amaranth

Abstract

Amaranth, a food color, has been studied polarographically over a pH range of 2.0 to 10.0 in aqueous solution and was found to give a well-defined wave, diffusion controlled and suitable for quantitative work. Irreversibility was indicated by the behavior of the half-wave potential to changes in pH and co~icentration. With pH, a direct relationship of -0.080 v. per pH unit held between pH 2.0 and 7.0, while with concentration, the half-wave potential was found to apparently vary directly with the logarithm of concentratioa. Using a coefficient of diffusion value obtained by coilductance measurements the value of n for the reaction was calculated to be 4, which corresponds to a ruptl~re of the azo group t o yield the corresponding amines.In connection with the work of this laboratory, a study of the polarographic behavior of the artificial food colors was begun with the object of determining the possible value for qualitative and quantitative analysis. This paper is the first of a series ancl describes the reduction of amaranth a t the dropping mercury electrode.A consideration of the formula of amaranth reveals that two liltely possibilities for reduction exist: ( a ) the azo group accepts two hydrogen atoms to form an hydrazo group, and (b) the azo group accepts four hydrogen atoms to yield the corresponding amines.Polarographic work on several con~pounds similar to amaranth suggests that the former reaction should take place. The azo-hydrazo reaction has been shown to occur with azobenzene and its derivatives (5, 6), although some doubt still exists as t o the reversibility of the reaction (7). T h e azo dyes, orange I1 and metanil yellow, are claimed to follow a two electron reduction lManuscript received J u l y 30, 1954.

Organic Polarography

“…In other studies on azo compounds reductions are reported for Evans blue (T-1824), for the same dye in the presence of plasma (272), for the food coloring dye amaranth (263), for the azoproteins made by coupling diazonium salts -with egg albumin (40), and for methyl orange in the presence of albumin (37).…”

Reduction of the azo food dyes FD and C Red 2 (Amaranth) and FD and C Red 40 by thermally degraded D-fructose and D-glucose

“…Complete reduction by SnCl2 is irreversible, as it involves scission of the -N=Nbond, but reduction to the hydrazo form is generally regarded as easily reversed by atmospheric oxygen (McKeown and Thomson, 1954). In contrast to expectations, partial reduction by sulfite was only incompletely, and slowly, reversible by atmospheric oxygen or even H202 in acidic solution.…”

“…The concentration of HMF was calculated to equal 6.7 x 10'5 M after 30 to 40 min. The observed rate of reaction with Red 2 at low pH would thus seem to be caused by some species in the HMF degradation pathway and not by HMF itself or fructose, particularly since Red 2 is less stable to reduction with decreasing pH (McKeown and Thomson, 1954). Alkaline pH.…”