Spectrophotometric study of complexation of cobalt (II) with HEDP in aqueous solutions

The complexation of cobalt(II) with HEDP (1-hydroxyethylidene-1,1-diphosphonic acid) in aqueous solutions was studied by spectrophotometry over a wide range of pH values, including strongly acidic and strongly alkaline solutions. The formation of 2:1, 1:1, 1:2 and 1:3 Co(II)-HEDP complexes was proved by decomposing the electronic absorption spectra recorded in strongly alkaline solutions into individual bands using Gaussian functions and plotting changes in absorbance against the metal:ligand mole ratio at increasing values of the ligand concentrations. Species composition dynamics in the 1:4 Co(II)-HEDP system in alkaline solutions was determined by the change in the metal:ligand mole ratio with time. A decrease in the amount of bis- and the predominant formation of tris-phosphonate species with preservation of the amount of the 2:1 and 1:1 complexes was observed after three months. The modeling of complexation in the Co(II)-HEDP system and the calculation of stability constants (beta) were carried out on the basis of experimental dependences of the absorbance of the solutions on pH. High thermodynamic stability was found for the binuclear chelates [Co2HHEDP], [Co2HEDP](-), [Co-2(OH)(2)H2HEDP](-), [Co-2(OH)(2)HHEDP](2-), log beta are equal 18.90, 7.11, 23.85 and 31.45, respectively. Speciation diagrams of complexes in the 1:1 and 1:4 Co(II)-HEDP systems were calculated for freshly prepared solutions. (C) 2019 Elsevier B.V. All rights reserved.