Kinetics of humic acids interactions with proton and calcium ions were studied u sing modified potentiometric titration
Solutions of 1 and 2 g L -1 humic acids i n 0 1 mol L -1 NaClO 4 or KCl were titrated with 0 1 mol L -1 HClO 4 or HCl from pH 7 5 to about 2 6 at 24 ℃ under carefully controlled anaerobic conditions Th e reaction kinetics were well described by a traditional
relaxation equation Y=Y 0 EX P( -t/τ ) In each titration point occurred an initial fast relaxation of re dox p otential ( E R) and glass electrode potential ( E G), followed by furthe r slow relax ation and/or oscillations of E R And eventually, the reaction reached a s teady s tate, indicating the non-equilibrium nature of humic acid solution system The a nalyses of time-resolved data of E G for the stepwise addition of calcium ions to humic acid solutions revealed that the overall reaction rates were diffusion-c o ntrolled rather than reaction-controlled After relaxation down to a steady st at e E R showed a linear pH dependence with an almost constant slope of 45 3 ±1 mV/p H unit despite the differences of humic acid concentrations and supporting elect rolytes, which then could be applied as an index of the arrival at a steady stat e of the humic acid interactions with metal ions The characteristics of proton
reactions with humic acids were studied using the suggested index, and well repr oducible results were obtained Four types of binding sites HA,H 2A,H 3A an d HA 2 with accordingly apparent pK-values of 5 79～5 82, 3 77～3 84, 1 64～1 7 1 and 2 75～2 79 were found existing in the studied humic acids