Correlation of the extraction constants of the system Cd(II)–H3PO4/Cyanex 302–kerosene at different ionic strengths

Bromleyʹs theory for calculating activity coefficients in order to correlate the values of cadmium extraction constant by Cyanex 302 from phosphoric acid solutions at different ionic strengths has been applied. A chemical model for the aqueous phase including the species H3PO4, H2PO4−, H5P2O8−, H6P2O8, CdHPO4 and CdH2PO4+ has been considered. The increase observed for the extraction constant value when increasing the phosphoric acid concentration is probably due to the significant increase of the cadmium activity coefficient. A reaction extraction including water as a component has been proposed, and the value of the thermodynamic extraction constant of log K0=7.02 for the formation of CdR2(HR) species, HR being the major component of Cyanex 302, has been obtained.