The complexing of polymetaphosphate anions with calcium cations: A potentiometric (pCa) study

Calcium cation complexing by polymetaphosphate anions was studied by direct potentiometric measurements with a calcium-ion-sensitive electrode. The moderately stable neutral chelate (Ca₂P₄O)_n is formed in (CaP₂O₆)_n solutions according to the equilibria logK₁₂ values for complexing with polyanions of M_av ? 1200 and 2500 (at C = 0.0003_M) were 5.86 and 6.26, respectively; the K₁₂ values then decreased with increasing polyanion concentration and were reduced by addition of equivalent sodium chloride. The very stable chelate anion (CaP₄O)_n is then formed in (Na₂CaP₄O₁₂)_n solutions according to the equilibrium logK₁₄ values for complexing with polyanions of M_av ? 1200 and 2500 (at C = 0.0003_M) were 7.48 and 8.08, respectively; these K₁₄ values also decreased with polyanion concentration. A less stable complex anion (CaP₆O)_n is formed in more concentrated solutions at PO₃/Ca ratios > 6.