ORIGINAL PAPER New version of calculation of stability constant of metal–fulvate complexes on the example of zinc fulvate G. Makharadze 1 • G. Supatashvili 1 • T. Makharadze 1 Received: 9 December 2016 / Revised: 22 March 2017 / Accepted: 18 September 2017 Ó Islamic Azad University (IAU) 2017 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 sol- ubility 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 for- mation process equals to 1252. This part of the associate of fulvic acids was conventionally called an ‘‘active associ- ate.’’ The average molecular weight of the ‘‘active asso- ciate’’ was used for determining the composition of zinc fulvate complex, the concentration of free ligand and sta- bility constant, which equals to 1:6  10 4 . Keywords Active associate  Average molecular weight  Fulvic acids  Natural water Introduction Fulvic acids (FA) are one of the first macromolecular organic substances, which were discovered in natural waters. They take an active part in complex formation and sorption processes and stipulate migration forms of heavy metals and radionuclides in natural waters and soils (Rey- Castro et al. 2009; Town et al. 2012; Shizuko 1981; Sasaki et al. 2012; Ephraim 1992; Schnitzer and Skinner 1966; Bertoli et al. 2016; Kirishima et al. 2010; Tochiyama et al. 2004; Varshal 1994; de Castro Ramalho et al. 2007; Wang et al. 2016; Linnik et al. 2013). In spite of researches, experimental data on stability constants (b) of complex compounds of FA with zinc (II) are heterogenous and they differ in several lines from each other (Schnitzer and Skinner 1966; Shizuko 1981; Ephraim 1992; Wang et al. 2016; de Castro et al. 2007). This con- dition is mainly stipulated by ignoring the average molecular weight of the associates of FA, which value in its turn depends on pH and finally causes the wrong result. Therefore, it is difficult to investigate complex forma- tion processes, taking place in natural waters, identify migration forms of zinc and evaluate and assess chemical– ecological condition of natural waters. The objective of the work was to obtain the pure samples of FA, to investigate complex formation processes between the pure samples of FA, isolated from natural water and Zn(II), and to calculate b of zinc fulvate complex. Complex formation process was studied by the solubility method. Zinc oxide suspension was used as a solid phase. Complex formation process was studied at pH = 8.0. Editorial responsibility: M. Abbaspour. & G. Makharadze gogimakharadze@yahoo.com 1 Department of Chemistry, Ivane Javakhishvili Tbilisi State University, 3 Chavchavadze Ave., 0128 Tbilisi, Georgia 123 Int. J. Environ. Sci. Technol. DOI 10.1007/s13762-017-1576-8