THE SELECTIVE SOLVENT-EXTRACTION OF CADMIUM BY MIXTURES OF CARBOXYLIC-ACIDS AND TRIALKYLPHOSPHINE SULFIDES .1. THE ORIGIN AND SCOPE OF THE SYNERGISTIC EFFECT

The effect of neutral organophosphorus compounds on the solvent extraction of cadmium and zinc by solutions of various carboxylic acids in xylene is reported. Sulphur-donor compounds, especially trialkylphosphine sulphides, were found to cause strong synergistic shifts in the extraction of cadmium (up to 2.2 pH units for 0.25 M of each extractant), and considerably smaller shifts for zinc (up to 0.7 pH units), thus enabling the separation of these metals. The extent of the synergistic shifts depends markedly on the identity of the carboxylic acid used: for simple alkanoic acids the effect increases in the order primary < secondary < tertiary acid. Larger shifts are obtained with substituted acids, such as 2-bromo-3,5,5-trimethylhexanoic acid (BTMHA) and, especially, 3,5-diisopropylsalicylic acid (DIPSA). Slope analysis studies showed that the cadmium complexes extracted by mixtures of these acids (HA) with triisobutylphosphine sulphide (L) have the probable stoichiometries CdA(2)(HA)(3)L and CdA(2)(HA)(2)L(2) for BTMHA and DIPSA, respectively. In contrast, the complexes extracted by DIPSA alone are dimeric, with stoichiometries of (CdA(2)(HA)(2)(H2O))(2) and (ZnA(2)(HA))(2). Oxygen-donor organophosphorus compounds, such as trialkyl phosphates and trialkylphosphine oxides, were found to give relatively small synergistic shifts (up to 1.0 pH unit) in the extraction of cadmium and zinc by DIPSA and antagonistic effects (up to 0.5 pH units) in the extraction of these metals by BTMHA.