Abstract
A rapid and precise method for the determination of L-cysteine in the presence of L-cystine was developed by amperometric titration using a rotating (2000 rpm) platinum indicator electrode at the potential of + 0.6 V vs. SCE.L-Cysteine could be titrated at ca. 45°C by adding 0.025 ml portions of potassium iodate solution at intervals of (25) s in the presence of 0.4 M potassium bromide and 0.5 M sulfuric acid. L-cysteine {(0.21) mg} was determined with a relative error and a coefficient of variation of less than 0.2 %.L-Cysteine in ten-times excess amounts of L-cystine could be determined with a relative error of less than 1 %. The titration required only 3 min. The recommended procedure is as follows. Place 5 ml of sample solution containing (0.21) mg of L-cysteine into the titration cell. Add 5 ml of 5 M sulfuric acid, 5 ml of 4 M potassium bromide and 35 ml of water. Titrate the resultant solution with (10-35 × 10-3) M potassium iodate standard solution amperometrically.
