Determination of cysteine, 3-mercaptopropionic, and mercaptosuccinic acid with neutral hexacyanoferrate(III)

Two procedures are proposed for the determination of -SH groups. In both a known amount of hexacyanoferrate(III) is added to the sample and the excess is then determined. In one procedure an excess of ascorbic acid is added and the excess of that is titrated with iodine. In the other, the excess is determined by direct titration with ascorbic acid. Both procedures avoid errors resulting from side-reactions of iodine which takes place if the excess of hexacyariofcrrate(III) is reacted with potassium iodide.