Abstract
The complexes of magnesium, calcium and strontium ions with tetraphosphate ion have been identified and their complexity constants have been evaluated on the basis of the pH lowering due to complex formation during the titration of tetraphosphate ion with hydrogen ion. In the presence of an excess of metal ion, the (1 : 1) complexes, to which one proton could be added, were predominant, but it was possible to add a second metal ion. The complexes and the logarithms of their stepwise formation constants at 25° with the ionic strength adjusted to unity with tetramethylammonium chloride in terms of concentrations of aquo metal ions and free or protonated ligand, or second metal ion are 6·04 for MgP 4O 13, 3·74 for Mg(HP 4O 13) 3− and 2·19 for Mg 2P 4O 13 2−, respectively, for calcium, the corresponding values are 5·46, 3·54, and 3·07, respectively, and for strontium they are 4·82, 3·49, and 3·42, respectively. The stabilities of the complexes and the solubility of the metal ions are greater than those of pyro-or triphosphate.
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