Kinetic study of solvent extraction of nickel(II) from aqueous solution by some heterocyclic substituted-hydrazones

Solvent extraxction of nickel(II) complexes with five heterocyclic hydrazones(containing pyridine- or/and quinoline-nitrogen atoms as the coordinating sites) into chloroform or benzene has been studied kinetically at 25°C and μ=01 by means of a spectrophotometric method. The kinetics of extraction of nickel(II) ion is first order in each of the aqueous nickel(II) concentration and the hydrazone concentration in the organic phase. The observed second-order rate constant, kobs'-pH profiles suggest three rate-determining steps to be considered in the aqueous phase: Ni2++ H2L+→, Ni2++HL→and Ni(OH)++HL→ The rate constant for the formation of 1:1 complex with 2-pyridylmethanone 2-pyridylhydrazone (PAPH) was consistent with those for bipyridine and terpyridine. Replacement of pyridyl group by quinolyl group affects the distribution constant(Kd) of the ligand between the two phases rather than the rate constants for rate-determining steps, and consequently the difference in kobs is chiefly governed by Kd. The influence of the structure of ligands on the rate constants is also discussed.