EFFECT OF SOLVENTS TO COORDINATION COMPOUNDS OF d- METAL IONS WITH ACID MALEIC

Abstract

The effect of aqueous ethanol solvent compositions on the stability of coordination compounds of copper (II), nickel (II) and cobalt (II) ions with acid maleic (H₂L) was studied by method potentiometric titration. The first and second dissociation constants of  H₂L  and logarithms of stability constants for complexes of Cu²⁺, Ni²⁺, Co²⁺ with maleat anion have been determined at ionic strength 0,1M at T=. The pK values of stepwise H₂L dissociation increase with the increase of EtOH content in solution. Complexes of Cu²⁺, Ni²⁺ and Co²⁺ ions with maleat anion become stronger in water – ethanol mixtures then in water solution. In the solvent of the studied compositions, Co²⁺, Ni²⁺ ions form less stable complexes than Cu²⁺ ion that corresponds to the Irving-Williams series established for aqueous solutions. The effect of solvent composition has on the stability of coordination compounds of copper (II), nickel (II) and cobalt (II) ions with maleic acid anion (L^2–) is analyzed using the thermodynamic solvation approach. It is shown that the main contribution to the shift of the acid-base equilibria of maleic acid and complexes with Cu²⁺, Ni²⁺, Co²⁺ ions in aqueous ethanol comes from the weakening of the solvation of maleat anion (L^2-).