Determination of solubility of sodium hypophosphates, and a study of their hydrates

Abstract

Solubility diagrams for tetrasodium, and trisodium, and disodium hypophosphate have been prepared, and compared with those from previous work. The tetrasodium salt shows an inversion of solubility at 66·5°, interpreted as a change in the stable phase from Na₄P₂O₆, 10H₂O at low temperatures to the anhydrous salt above 66·5°. The trisodium salt shows an inversion at 58·5°, attributed to a change from the low-temperature form, Na₃HP₂O₆, 9H₂O, to the anhydrous high-temperature form. The disodium salt has an inflection in the solubility curve at 80·5° corresponding to the change of Na₂H₂P₂O₆, 6H₂O into the anhydrous salt.