[Photometric determination of iron contamination of drugs and biological matrices].

The Ph. Eur. detects colorimetrically the limiting value of iron by formation of a complex with thioglycolic acid in ammoniacal solution. This reaction was rechecked by experiments. Contrary to the literature a solvated Fe(II)-complex will be formed due to the reduction of Fe(III) by the present thioglycolic acid. A supposed oxidation of the Fe(II)-complex by atmospheric oxygen does not occur. For the routine determination of traces of iron in medicinal substances and biological matrixes a VIS-method was developed based on the measurement of the absorption of the complex at 534 nm in the range of 10-100 ppm after graded oxidative decomposition with conc. sulfuric acid/H2O2 30%. This assay needs a calibration mixture or in the case of a higher demand of accuracy a calibration curve with at least five measuring points. Because of the contamination of commercially available reagents a blank value must be determined.