Fading spectrophotometric determination of methimazole using Fe(III)-potassium thiocyanate

In acidic condition with pH=3.6, Fe3+ can react with potassium thiocyanate to form the red complexes of Fe(SCN)2+. Hydrosulfuryl(-SH) in methimazole molecule can reduce Fe3+ to Fe2+, which results in the fading of the red complexes of Fe(SCN)2+, and the fading degree is directly proportional to the addition of methimazole. So, the content of methimazole can be indirectly measured by measuring the decrease value of absorbance of the red complexes of Fe(SCN)2+. Accordingly, a new method for the fading spectrophotometric determination of methimazole using Fe(Ш)-potassium thiocyanate has been established. The method has been applied to the determination of methimazole in drugs, and the results are basically consistent with the results of pharmacopoeia.