Solvent extraction of silver by cyanex 272, cyanex 302 and cyanex 301

The solvent extraction of silver from nitrate media by the extractants Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid), Cyanex 302 (bis(2,4,4-trimethylpentyl)thiophosphinic acid) and Cyanex 301 (bis(2,4,4-trimethyl-pentyl)dithiophosphinic acid) is characterized. Extraction shifts to lower pH with increasing sulfur substitution in the phosphinic acid, in accordance with the increased extractant acidity imparted by the soft nature of the sulfur donor atoms. The effects of extractant concentration, aqueous-phase ionic strength and organic-phase diluent on the extraction behavior are examined. The stoichiometry of the extraction reactions and the geometry of the extracted complexes are postulated based on slope analysis and spectroscopic measurements. All three extractants behave as bidentate ligands and combine in a 1:1 stoichiometric ratio with silver. The sulfur-containing ligands form multi-nuclear oligomeric complexes in which the ligands bridge between metal centers.