Multiple Equilibrium in Humic Acid-Metal System: Determination of Successive Stability Constants of Cu(II) and Ca-Complexes with the Aid of Ion Selective Electrodes

Multiple equilibria in humic acid-Cu(II) and humic acid-Ca systems in 0.1 N KNO₃, have been studied at different pH's using ion selective electrodes. At pH 4.8, Ca has been found to form only MA⁺, where as Cu(II) forms both MA⁺ and MA₂, complexes. Significant negative co-operative effect has been observed in all the systems studied, which is more prominent in Ca-humic acid complex at low pH. Logarithms of intrinsic stability constant, K₀, ranged from 7.5 to 8.5 for Cu(II) and 5.8 to 6.0 for Ca-complexes. Results are compared with those for the two synthetic model humic acid-metal systems.