Colorimetric determination of some amino acids containing a sulfur group

A simple and rapid colorimetric method for the determination of some amino acids containing a sulfur group is described. The studied compounds are cysteine, cystine and methionine. The method is based on the formation of ferroin, from the reaction of the studied drugs with a mixture of iron(III) and 1,10-phenanthroline and measurement of the absorbance at 512 nm. The procedure has been successfully applied to the assay of the pharmaceutical preparations of the studied drugs after thin-layer chromatographic separation and the results of the studied compounds compare favourably with the official methods.