Complexation of Iron(III) with Thiocyanate Ions in Aqueous Solution

Abstract

Spectrophotometric and calorimetric studies of the complexation of iron(III) with thiocyanate ions have been performed in an aqueous solution containing 1mol dm^-3 NaClO₄ or NH₄ClO₄ as a constant ionic medium at 25°C. Spectrophotometric titration data were well explained in terms of the formation of [Fe(SCN)]²⁺, [Fe(SCN)₂]⁺ and [Fe(SCN)₃], and their formation constants and individual electronic spectra were determined. The [Fe(SCN)]²⁺, [Fe(SCN)₂]⁺ and [Fe(SCN)₃] complexes exhibit absorption maxima at 450-500nm. Enthalpies and entropies for the formation of the [Fe(SCN)]²⁺, [Fe(SCN)₂]⁺ and [Fe(SCN)₃] complexes were also determined from calorimetric titration data. On the basis of these thermodynamic quantities, it is postulated that the [Fe(SCN)]²⁺, [Fe(SCN)₂]⁺ and [Fe(SCN)₃] complexes have a six-coordinate octahedral structure.