Direct differential-pulse polarographic determination of mixtures of the food colouring matters tartrazine-Sunset Yellow FCF, tartrazine-Green S and amaranth-Green S in soft drinks

Abstract

Tartrazine and Sunset Yellow FCF can be determined directly in orangeade by differential-pulse (d.p.) polarography on the addition of pH 9 Britton-Robinson buffer and tetraphenylphosphonium chloride. The tetraphenyl-phosphonium chloride removes the large polarographic maximum obtained with tartrazine at pH > 4 and causes the d.p. polarographic peaks of the two colouring matters to be separated.Tartrazine in limeade can be determined in a similar supporting electrolyte but these conditions are not suitable for the determination of Green S, which is usually present at a low concentration relative to the tartrazine and for which the d.p. polarographic peak is depressed by the addition of tetraphenylphosphonium chloride. Green S can be determined after adding pH 4 Britton-Robinson buffer and tetramethylammonium chloride to the limeade: the addition of tetramethylammonium chloride gives a better base line in the presence of tartrazine. The solution is then re-adjusted to pH 9 and tetraphenylphosphonium chloride is added in order to determine the tartrazine.At pH 4 the sugar present in blackcurrant syrup gives a small d.p. polarographic peak at the same potential as Green S. At pH > 6 the peak of the sugar disappears but amaranth gives a broad polarographic maximum. This maximum is suppressed at pH 7.8 by the addition of tetramethylammonium chloride. Under these conditions the Green S peak is separate but the small concentrations of Green S normally present in blackcurrant drinks can only just be detected.The procedures have been tested on soft drinks prepared with known concentrations of colouring matter.