Extraction and formation of iron(III) thiocyanate complexes: Application for spectrophotometric determination of iron

Extraction of iron(III) from sulphuric and hydrochloric acid solutions containing an excess of thiocyanate ions with tetrapherylphosphonium (TPP) chloride and cetyltrimethylammonium (CTMA) bromide in chloroform was investigated. Optimum conditions for extraction by both extractants were determined. The extraction behaviour of CTMA was found to be more efficacious than that of TPP because less thiocyanate and extractant was required. Better extraction efficiency of CTMA can be attributed to surface-active properties of this substance. Formation of an extractable iron(III) thiocyanate complex with CTMA or TPP was also determined spectrophotometrically by measuring the absorbance of the organic phase at 473 nm or 506 nm, respectively. The composition of the extracted iron(III) complexes was determined by distribution and spectrophotometric methods. The molar ratio Fe:SCN:TPP of the extracted complex was 1:4:1; accordingly, its composition was [TPP][Fe(SCN)(4)]. The molar ratio Fe:CTMA of the extracted complex was 1:3. The molar ratio Fe:SCN in the complex extracted with CTMA could not be determined because of the turbidity in solutions containing less than 0.01 mol dm(-3) thiocyanate ion, but it was concluded that the composition of the extracted complex was [CTMA](3)[Fe(SCN)(6)].