Mono- and polynuclear of heteroligand complexonates cobalt (II) in aqueous solutions of 2-aminopropanoic acid

Сoordination equilibria in binary and ternary systems containing Co(II) salt, ethylenediaminetetraacetic (EDTA) and 2-aminopropanoic (alanine, Ala) acids were investigated by absorption spectrophotometry at different molar ratios of the reactants over a wide pH range. The mathematical processing of the obtained data based on Fisher's criteria allowed to evaluate the possibility of the existence in the solution of a wide spectrum of complex particles and to identify those which are sufficient to reproduce the experimental results. Stoichiometry of polynuclear complexes Co(II)–Ala–EDTA (2:2:1, 2:4:1, 3:6:1, 4:8:1), the pH range of their existence and stability constants were determined. Studying of the absorption spectra as well as the dependency A = f (pH) and A = f (C) revealed the formation of mononuclear complexes [CoEdta]$^{2-}$ (lg$\beta$ = 16,31 $\pm$ 0,06), [CoAla]$^+$ (lg$\beta$ = 4,38 $\pm$ 0,09), [CoAla$_2$] (lg$\beta$ = 8,06 $\pm$ 0,06), [CoAla$_3$]$^-$ (lg$\beta$ = 9,64 $\pm$ 0,06), [CoAlaEdta]$^{3-}$ (lg$\beta$ = 21,17 $\pm$ 0,05), binuclear complexes [(CoAla)Edta(CoAla)]$^{2-}$ (lg$\beta$ = 34,52 $\pm$ 0,06), [(CoAla$_2$)Edta(CoAla$_2$)]$^{4-}$ (lg$\beta$ = 44,25 $\pm$ 0,10), trinuclear complex [(CoAla$_2$)$_2$Edta(CoAla$_2$)]$^{4-}$ (lg$\beta$ = 58,66 $\pm$ 0,15) and tetranuclear complex [(CoAla$_2$)$_2$Edta(CoAla$_2$)$_2$]$^{4-}$ (lg$\beta$ = 71,55 $\pm$ 0,26) (all given constants are valid for a temperature of (20$\pm$2) $^{\circ}$C and the ionic strength of the solution I = 0,2 (NaClO$_4$)). Possible structures of the formed complexes are given.