Stability of iron(III)-thiocyanate complexes and the dependence of absorbance on the nature of the anion

Abstract

The dependence of the colour intensity of the iron(III)-thiocyanate complex formed at two different ratios of thiocyanate to iron on the nature and concentration of the acid anion was studied. Four different acids were tested over the concentration range 0.05–1.0 N: perchloric acid, nitric acid, hydrochloric acid and sulphuric acid. Fading of the iron(III)-thiocyanate colour was ascribed to a specific effect of the anion caused by progressive masking of iron(III) ions of the highly dissociated iron(III)-thiocyanate by the more strongly complexing anion ligand. The results were interpreted in the light of the calculated values of side-reaction coefficients between iron(III) and the added ligand, as well as on the magnitude of conditional stability constants of iron(III)-thiocyanate in the various media. That the order of effectiveness of the added anions towards complexing iron(III) ions is SO₄^2– > Cl^– > NO₃^– ClO₄^– was further substantiated.