New version of calculation of stability constant of metal–fulvate complexes on the example of zinc fulvate

Abstract

Natural macromolecular organic substances—fulvic acids—take an active part in complex formation processes and stipulate migration forms of heavy metals in natural waters. In spite of researches, experimental data on stability constants of complex compounds of fulvic acids with heavy metals (among them zinc) are heterogenous and they differ in several lines from each other. One of the reasons of such condition is ignoring an average molecular weight of the associates of fulvic acids, which finally causes the wrong results. Complex formation process between zinc (II) and fulvic acids was studied by the solubility method at pH = 8.0. ZnO suspension was used as a solid phase. Fulvic acids were isolated from Paravani lake by the adsorption chromatographic method. This article shows that during the complex formation process, every 1/5 part of an associate of fulvic acids inculcates into zinc’s (II) inner coordination sphere, as an integral ligand. So it may assume that the average molecular weight of the associate of fulvic acids which takes part in complex formation process equals to 1252. This part of the associate of fulvic acids was conventionally called an “active associate.” The average molecular weight of the “active associate” was used for determining the composition of zinc fulvate complex, the concentration of free ligand and stability constant, which equals to $1.6 \times 10^{4}$.