Abstract: The stability constants (logK_PbLA^Pb) of the mixed ligand complexes, PbLA[L=2,2'-bipyridyl(bipy)、1,10-phenanthroline(phen)、5-nitro-1,10-phenanthroline(Nphe), A=malonic acid(mal^2-)、 benzyl malonic acid(pmal^2-)、 phenylethylmalonic acid radicals(pmal^2-)] have been determined at 25 ℃ in 10% ethanol with ionic strength I =0.1(KNO₃) mol/L by potentrometric titration. It is found that the ΔlogK、logX values of the mixed ligand complexes are obviously higher than the ΔlogK(s)、logX(s). The stability caused by the complexes has been discussed.

Key words: bipyridyl, phenanthroline, nitrophenanthroline, phenylalkyl malonate, lead(Ⅱ) mixed complex, stability