A study of the formation and stability of the iron(III)-thiocyanate complex in acidic media

Abstract

The formation of the iron(III)-thiocyanate complex in sulphuric and perchloric acids has been studied spectrophotometrically. Under no circumstances has any evidence been found that more than one thiocyanate ion enters the complex, contrary to widely held beliefs that up to six ions may be involved. Hydroxyl, sulphate and hydrogen sulphate ions and water molecules complete the co-ordination sphere. Decomposition products have been isolated and identified. Three ranges of acid concentration are identified, each region characterised by the manner of variation of the molar absorptivity and formation constant with change in acid concentration, and an interpretation is offered. Decomposition of both reagent and complex render thiocyanate unsatisfactory as a reagent for the precise determination of iron.