Solvent extraction separation of divalent cobalt and nickel from sulphate solutions was studied using OPPA dissolved in n-dodecane and the optimum conditions for separation of cobalt and nickel were determined. It was found that extraction of both the metal ions increases with increase in equilibrium pH. Cobalt was preferentially extracted over nickel in all pH range, Delta pH(0,5) value of 1.8 indicates the possible separation of both the metal ions from each other. Almost complete extraction of cobalt is observed at pH 4.5 where separation factor (beta = D-Co/D-Ni) is approximately 1400, above this pH co-extraction of nickel increases and reduction in separation factor was observed. The composition of extracted species determined by slope analysis of the plots of log D vs. log[OPPA](2) and log D vs. equilibrium pH indicates formation of the species MA(2) (HA)(2) for both the metal ions. Temperature dependence studies of the extraction constant K-ex, showed that the extraction is enthalpy driven with unfavorable entropy. OPPA/dodecane was employed for cobalt/nickel separation from raffinate solution obtained during solvent extraction process of spent ammonia cracker catalyst leach solution. 1.0 M OPPA/dodecane in counter-current extraction at O/A of 1.0 was efficiently used to separate cobalt from nickel. (C) 2012 Elsevier B.V. All rights reserved.