The Spectrophotometric Determination of Iron(III) with Methylthymol Blue

Abstract

Abstract The reaction between iron(III) and methylthymol blue has been studied spectrophotometrically in an attempt to establish the optimum conditions for determining small amounts of iron. Depending on the acidities of a solution, iron(III) reacts with methylthymol blue to form one of two complexes. The 1-to-1 complex, which has its maximum absorbance in the acidity range from 0.07 to 1.00 m with respect to perchloric acid, has an absorption maximum at about 580 mμ against a reagent blank. The 1-to-2 (metal-reagent ratio) complex, which has its maximum absorbance in the pH range from 4.5 to 5.5, has an absorption maximum at about 520 mμ against a reagent blank. With either complex, Beer’s law is obeyed over the range from 5 to 50 μg. of iron(III) in 25 ml. of complex. The apparent molar extinction coefficients of the two complexes at their absorption maxima were calculated to be 15000 and 14000 respectively (the former, at 0.08 m HClO; the latter, at pH 5). According to Sandell’s notation, the sensitivities of the reactions are 0.004 μg. of iron(III) per cm2 at 580 mμ and 0.005 μg. of iron(III) per cm2 at 520 mμ respectively. The effect of diverse ions was examined with a solution containing iron(III) and 25 cations.