Two spectrophotometric methods have been developed for the determination of thiols by use of two pairs of reagents, p-aminophenol (PAP)-Fe(III) for cysteine, cystamine hydrochloride, glutathione, β-mercapto-propionic acid, β-mercaptoethanol, thioglycolic acid, thiomalic acid and thiosalicylic acid and Metol [p-(methylamino)phenol sulphate]-Fe(III) for the above plus p-aminothiophenol, o-aminothiophenol and thiobarbituric acid. The methods are simple, sensitive, reproducible and accurate to within ±1.5% and have been applied to the assay of cystine after reduction and cystine in groundnut protein.
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Sastry et al. (1985) studied this question.