SELECTIVITY-STRUCTURE TRENDS IN THE EXTRACTION OF Co(II) AND Ni(II) BY DIALKYL PHOSPHORIC, ALKYL ALKYLPHOSPHONIC,AND DIALKYLPHOSPHINIC ACIDS∗

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Abstract
The conditional equilibrium constants for the extraction reactions of Co(II) and Ni(ll) by toluene solutions of homologous series dialkyl phosphoric, alkyl alkylphosphonlc, and dialkyl-phosphlnic acids have been evaluated. From the equilibrium constants the Co/Ni selectivity factors were calculated. Absorption spectra and water extraction data for organic solutions of selected reagents which had extracted the metals were also obtained. The data have allowed us to establish correlations between the structure of the extractants and their selectivity for Co(II) over Ni(II). The two main features controlling the different extractive behavior of Co(II) and Nl(II) with the organo-phosphorus reagents appear to be the different degree of hydration of the extracted complexes and their different coordination geometry. Sterlc hindrance effects, introduced by branchings on the alkyl chains greatly reduce the extractability of Ni(II) which forms octahedral complexes in the organic phase, while the extraction of Co(II), which forms tetrahedral complexes, is less influenced by the structural variations of the extracting reagents.