Chromium(II) Complexes with Hydroxyethylidenediphosphonic Acid

The complex formation of chromium(II) with hydroxyethylidenediphosphonic acid (HEDP, H₅L) in an aqueous solution is studied by spectrophotometry. The pH region is determined where the HEDP complexonates of chromium(II) are stable. The effect of pH on the complexation and the composition of the coordination sphere of the complex ions is shown. The optimum conditions for the practical application of HEDP as a stabilizing ligand are chosen. The instability constants for tetra-, tri-, di-, and monoprotonated and neutral chromium(II) hydroxyethylidenediphosphonate complexes were calculated, whose pK were found to be 1.21, 2.43, 6.87, 12.60, 19.40, respectively. A new complex of chromium(II), Na₄Cr(H₂L)₂ · 2H₂O] · 4H₂O, was isolated from a solution and characterized using elemental, IR spectroscopic, and thermographic analyses.