ВИЗНАЧЕННЯ АСКОРБІНОВОЇ КИСЛОТИ У СУМІШАХ З ЦИСТЕЇНОМ ІЗ ВИКОРИСТАННЯМ РЕАКЦІЇ ВІДНОВЛЕННЯ 18-МОЛІБДОДИФОСФОРНОГО ГЕТЕРОПОЛІКОМПЛЕКСУ У ПРИСУТНОСТІ ІОНІВ Bi(III)

A spectrophotometric method was developed for the determination of ascorbic acid in mixtures with cysteine using the reduction of 18-molybdodiphosphate heteropoly complex. In order to decrease the influence of the 18-molybdodiphosphate reduction with cysteine, the pH of the reaction was shifted to the acidic region to 1.0 – 2.2. Under these conditions, in the presence of bismuth(III) ions, as a result of the formation of a metal-substituted heteropoly complex, the reaction rate of 18-molybdodiphosphate with ascorbic acid increases and, at pH from 1.8 to 2.2, the reduction reaction is completely shifted towards the formation of heteropoly blue and ends in less than 5 minutes. Cysteine in this pH region reacts with 18-molybdodiphosphate very slowly, but has an indirect effect on the reaction of ascorbic acid with 18-molybdodiphosphate, probably due to the binding of bismuth(III) ions. It was shown that the determination of ascorbic acid is possible with satisfactory accuracy if the excess of cysteine with respect to ascorbic acid does not exceed three times. The calibration graph for the determination of ascorbic acid at pH 1.8 and concentrations of 18-molybdodiphosphate and bismuth(III) 5∙10-5 mol∙L-1 and 1∙10-4 mol∙L-1, respectively, is linear in the range from 2.5∙10-6 to 2.5∙10-5 with a detection limit of 8∙10-7 mol∙L-1 (l = 5 cm). The accuracy of the method was evaluated by the added-found method on model mixtures.