Spectrophotometric determination of methimazole in pharmaceutical, serum and urine samples by reaction with potassium ferricyanide-Fe(III)

This paper describes a novel method to determine methimazole by spectrophotometry using a potassium ferricyanide-Fe(III) reaction. The study indicates that at pH 4.0 Fe(III) is reduced to Fe(II) by methimazole and in situ formed Fe(II) reacts with potassium ferricyanide to give soluble Prussian Blue which is characterized by means of XRD analysis. The absorbance of Prussian Blue is measured at the absorption maximum of 735 nm, and the amount of methimazole can be determined based on this absorbance. Beer's law is obeyed in the range of methimazole concentrations of 0.02-6.00 mu g/mL. The equation of the linear regression is A = -0.0058 + 0.49988c (mu g/mL), with a correlation coefficient of 0.9998 and RSD of 0.80%. The detection limit (3 sigma/k) is 0.015 mu g/mL, and the apparent molar absorption coefficient of indirect determination of methimazole is 5.7 +/- 10(4) L/mol cm. This method has been successfully applied to the determination of methimazole in pharmaceutical, serum and urine samples, and average recoveries are in the range of 98.6-102.4%. Analytical results obtained with this novel method are satisfactory.