Solvent extraction from halide solutions. Part VI. Complexing constants for ferric thiocyanates and the temperature dependence of their extraction

Abstract

Analysis of data on the solvent extraction of iron from thiocyanate solutions by trioctylphosphine oxide in carbon tetrachloride gives for the logarithms of the successive stability constants for the species Fe(SCN)_n^3–n(n= 1–6): 2·09,1·48,1·00, 0·67, 0·18, and –0·6 at 20 °C and in an aqueous phase 0·2M in acid and 2M in total perchlorate. Succession constants are assumed to be related by K_n/K_n+1=n/(6 –n+ 1)(n+ 1)(n+ 1)/k. The partition coefficient for the extracting Fe(SCN)₃ is 52·3 with a 2·2 × 1 0^–5M-phosphine oxide solution. Partition coefficients are also obtained for Fe(SCN)₃ and Fe(SCN)₄- with solutions of butyl dibutylphosphinate, dibutyl butylphosphonate, and tributyl phosphate in carbon tetrachloride. The temperature-dependence of the extraction has been measured and an attempt is made to interpret the data in terms of the various enthalpy changes involved. Comparative data on the extraction of ferric chloride and bromide have been obtained and the differences between extraction in the thiocyanate and halide systems discussed.