Spectrophotometric determination of iron with pyrrolidinecarbodithioic acid

Abstract

Pyrrolidinecarbodithioic acid (hereinafter abbreviated to PDTC) produces a sparingly soluble precipitate with ferric iron. However, ferric iron reacts with PDTC to form a blackish purple water-soluble complex in the presence of surface active agent such as Triton X-100 (alkylaryl polyether alcohol). It is assumed that the complex dissolves into micelles of surface active agent formed in water. This blackish purple solution has three absorption maxima at the wavelengths of 360, 515 and 600nm.
The recommended procedure for the determination of ferric iron with PDTC is as follows: Take into a 50-ml beaker an aliquot of sample solution containing less than 120 μg of ferric iron. Add 2 ml of 2 N HCl, 2 ml of 0.275% (v/v) Triton X-100 solution and 2 ml of 6.1×10^-3M PDTC solution. Control pH of the solution with 0.5 N NH₄OH solution by using the pH-meter. Add distilled water to make the total volume of the solution 20 ml. Final pH of the solution must be adjusted to 1.05±0.05. Allow it to stand for 40 to 45 minutes at ambient temperature. Measure the absorbance at 360 or 600 nm against the reagent blank. Beer's law was obeyed over the range of concentration from 0 to 120 μg Fe(III)/20 ml at 360 nm and from 0 to 160 μg Fe(III)/20 ml at 515 or 600 nm. The molar absorptivities of the complex at 360 nm and, at 515 or 600 nm were 1.1₄×10⁴ and 3.7₀×10³, respectively.The sensitivities at 360 and 600 nm were 0.005 μg Fe/cm² and 0.015 μg Fe/cm² for log(I₀/I)=0.001, respectively.
This method was applied to determine the concentration of iron in natural waters. This result agreed very closely with that obtained by the ο-phenanthroline method (Table IV). The apparent solubility product of Fe(PDTC)₃ at pH=1 and [Cl^-] =0.2 M, K_f= [Fe]·[PDTC]³, was estimated to be 4.2×10^-17 from Fig. 4. When PDTC solution was added to the solution containing ferrous iron instead of ferric iron, a blackish purple solution was obtained and the absorption spectrum of this solution agreed with that of ferric iron. Absorbances at the wavelengths of absorption maxima decreased by an addition of reducing agent such as hydroxylamine (see Table V). This can be understood if we assume that the blackish purple complex is a complex of ferric iron and that ferrous iron is easily oxidized by the air in the presence of PDTC because of the high stability of Fe(III)-PDTC complex.