Differential-pulse adsorptive stripping voltammetry of food and cosmetic synthetic colouring matters and their determination and partial identification in tablet coatings and cosmetics

Abstract

Sixteen food and two cosmetic synthetic colouring matters were shown to adsorb strongly on a hanging mercury drop electrode and to be amenable to determination by differential-pulse adsorptive stripping voltammetry. When a 2-min accumulation time was used, the increase in sensitivity over differential-pulse polarography at a dropping mercury electrode was between 8- and 100-fold, depending on the colouring matter determined. Using longer accumulation times, 1 × 10^–10M concentrations of some food colours could be determined. The addition of tetraphenylphosphonium chloride shifted the reduction potentials of some colouring matters to more negative values and either decreased or increased the size of the peaks obtained. This makes it a useful reagent for partially identifying individual colouring matters even when they are present at very low concentrations. Increased sensitivity was obtained with carmoisine, tartrazine and quinoline yellow by its addition.

Procedures for applying the method to the determination and partial identification of colouring matters in tablet coatings and in a lipstick have been developed. The need to dilute more concentrated sample solutions has the advantage of eliminating matrix effects, which are sometimes apparent when differential-pulse polarography is applied without dilution.