COMPARISON OF THE STABILITY-CONSTANTS OF CD(II)-HUMATE, CU(II)-HUMATE, AND PB(II)-HUMATE COMPLEXES

A comparative investigation of the complexations of divalent metal ion s (Cd, Cu, and Pb) by a well characterized soil humic acid (HA) from O kchun Metamorphic Belt was carried out in 0.05 M KNO3 and pH of 4.5 us ing ion selective electrodes. A continuous distribution model based on the Scatchard Plot was used to determine the stability constants, bec ause the constants obtained by this modeling technique takes the varia tions in binding energies into consideration without regards to the ma nner in which M(II) ion is bound on HA. The mean value of log K(i) wer e 4.05+/-0.60, 4.92+/-0.36, and 5.63+/-0.34 l mol-1 for Cd(II)-, Pb(II ), and Cu(II)-humate complexes respectively. The values of intrinsic c onstant (log K(int); binding at strongest site) were 7.12+/-0.30, 6.59 +/-0.32, and 5.07+/-0.56 l mol-1 in the order Cu(II)>Pb(II)>Cd(II) ion .