Solvent extraction of copper as a thiocyanate complex. Reducing effect of thiocyanate

Extraction of copper(II) from sulphuric and hydrochloric acid solutions containing an excess of thiocyanate ions with tetraphenylphosphonium (TPP) chloride and cetyltrimethylammonium (CTMA) bromide in chloroform was investigated. The optimum conditions for quantitative extraction (over 99%) by both extractants were determined. The extraction behaviour of CTMA was found to be more efficacious than that of TPP because a low excess of thiocyanate and extractant was required and extraction was feasible even from a higher concentration of sulphuric acid. The formation of an extractable copper(II) thiocyanate complex with CTMA could also be determined spectrophotometrically by measuring the absorbance of the organic phase at 409 nm. The presence of ascorbic acid decreased the absorbance at 409 nm but the formed copper(I) thiocyanate was quantitatively extracted with CTMA, independently of the ascorbic acid concentration. Thiocyanate ions also exercised a reducing effect on copper(II) at pH > 1.3; with increasing pH and/or prolonged shaking time of the aqueous and organic phases, the reducing effect increased. The composition of the extracted copper(II) complex was determined by the distribution and spectrophotometric methods. The molar ratio Cu:SCN:TPP (CTMA) of the extracted complex was 1:4:2; accordingly, its composition was [TPP](2) [Cu(SCN)(4)] or [CTMA](2) [Cu(SCN)(4)].