Examination of the Reaction Products in the Aqueous-Phase Preparation of Hydrated Calcium Diphosphates

Abstract

The formation of by-products during the preparation of hydrated calcium diphosphates(Ca₂P₂O₇·nH₂O, n=2 or 4) was examined in samples which were obtained through the literature methods. When CaH₂P₂O₇ was used as the reactant (2CaH₂P₂O₇+nH₂O→Ca₂P₂P₇·nH₂O+H₄P₂O₇), the product contained not only hydrated calcium diphosphate but also calcium ammonium diphosphate and/or acidic calcium diphosphate, depending on the reaction medium(0.05M NH₄OH, 2.2M Ca(CH₃COO)₂, or water). When K₄P₂O₇ and CaCl₂ were reacted in an aqueous solution of KCl, a significant amount of diphosphate ion (ca. 57%) was hydrolyzed to orthophosphate ion. The product was amorphous when the molar ratio of the reactant Ca²⁺ to diphosphate ion was higher than 1.0. Several kinds of crystalline calcium potassium diphosphate were found in the product when the ratio was lower than 1.0. Therefor, it was concluded that hydrated calcium diphosphates prepared in the aqueous phase by the literature methods are generally contaminated with several kinds of by-product.