Studies of Some Metal-Hypophosphate Complexes.

A method is described for the preparation of the di- and tetra-sodium salts of hypophosphoric acid and, for their conversion, by a cation exchange method into the free acid in a pure state. Using batch experiments with a cation exchange resin in the metal form and hypophosphoric acid, complex formation has been shown to occur between this acid and the divalent metals, beryllium, magnesium, calcium, strontium, barium, manganese, iron, cobalt, nickel, copper, zinc and cadmium. Similar experiments have been performed, for purposes of comparison, with orthophosphoric and perchloric acids. The results obtained in this way with hypophosphoric acid, have been supported, in most cases, by results of pH titrations of metal salt solutions with the acid. The calcium and nickel hypophosphate systems have been investigated by batch experiments using anion exchange resins in the hypophosphate form. The occurrence of negatively charged complex ions with a metal to ligand ratio of 1:2 is indicated. In addition, the calcium and cobalt hypophosphate complexes have been studied by another ion exchange technique using tracer quantities of radioactive isotopes. Results of these experiments provide further evidence of complexes, in the case of calcium, with a metal to ligand ratio of 1:2. and with a negative charge, and of a complex with a 1:1 metal to ligand ratio in the case of cobalt. Values are obtained for the dissociation constants of these complexes. The degree of complexing found with the transitional type and the non-transitional type divalent metals with hypophosphoric acid is discussed and reasons are suggested for the differences observed between these two groups and, within the groups themselves. An attempt is made to correlate the complexing power with the ionic radius of the metal. Comparison is made between the hypophosphate ion as a complexing ligand and the ortho and pyrophosphate ions