Adsorptive stripping voltammetry of colouring matter and dyes

Eleven food colouring matters, alizarin fluorine blue and some indicator dyes were studied using adsorptive stripping voltammetry at a hanging mercury drop electrode. The effect of adding gelatin and phosphonium salts on the stripping peak potentials and peak currents was studied. In the case of 7 food colours 25 ppm of gelatin was sufficient to obtain a full shift and full enhancement of peak height. The largest potential shifts were obtained with Chocolate Brown, indigocarmine, Ponceau 4R, Red 2G and quinoline yellow. The effect is pH dependent and the addition of gelatin can be used in the identification of food colours. Alizarin fluorine blue (AFB) in pH 4.7 acetate buffer gives two peaks, the first due to adsorption of copper(II)-AFB and the second peak due to adsorption of AFB itself. The peak heights are dependent on pH and on electrolyte concentration. Addition of AFB to the solution containing lead(ll) enhances the peak owing to adsorption of the lead-AFB complex. Addition of lanthanum(III) eliminates the second peak and the signal is not enhanced by the addition of fluoride. Enhancement of the peak at -1.05 V for a zinc solution due to addition of AFB indicates that Zn-AFB complex can be accumulated. Compound related to AFB, namely xylenol orange was very poorly adsorbed on mercury. Studies of metal complexes of Mordant Red 74, a selective reagent for beryllium were affected by precipitation although the compound gives two large peaks at pH 4 on its own. 2-, and 4-nitrobenzyltriphenylphosphonium bromides which were electroactive were shown to adsorb at mercury. Comparison was made of the effect of these phosphonium compounds and that of those which are not electroactive on the adsorptive stripping voltammograms of tartrazine.