Evaluation of the Stability Constants for Alkaline-Earth Metal Complexes with Dodecaoxohexaphosphate(III) Using EDTA as a Complexation-Competing Agent.

The stability constants for complexes of a ring oxoanion of phosphorus dodecaoxohexaphosphate(III), with magnesium, calcium, and strontium were evaluated in aqueous solutions at an ionic strength of 0.1. A new spectroscopic equilibrium analysis for a system involving complexes of very high stability was accomplished by introducing EDTA as a complexation-competing agent to the system. The logarithmic stability constant values for the magnesium, calcium, and strontium complexes were found to be 8.4, 8.8, and 7.5, respectively, whose stability order was different from that of a simple one expected from their ionic radii.