Preparation of double pyrophosphates in aqueous systems chromium(III) salt−sodium pyrophosphate

Abstract

The specifics of preparation of chromium double pyrophosphates in systems Cr(NO₃)₃–Na₄P₂O₇–H₂O, CrCl₃–Na₄P₂O₇–C₂O, and KCr(SO₄)₂–Na₄P₂O₇–H₂O are discussed. Changes in pH of the mentioned systems have been studied depending of the component ratio. The effects of the deposition duration and sodium pyrophosphate concentration on pH have been studied for system Cr(NO₃)₃–Na₄P₂O₇–H₂O. Phase composition has been determined for compounds prepared in systems Cr(NO₃)₃–Na₄P₂O₇–H₂O and KCr(SO₄)₂–Na₄P₂O₇–H₂O. It has been found that the acid residue anions do not substantially affect the formation of a continuous series of compounds consisting of hydroxy and hydro combinations of chromium ions and pyrophosphate ion, whereas cations can have an effect. System KCr(SO₄)₂–Na₄P₂O₇–C₂O is less sensitive to reaction conditions than systems in which normal salts Cr(NO₃)₃ and CrCl₃ act as a source of Cr³⁺ cation. A product stable over time can be prepared in system KCr(SO₄)₂–Na₄P₂O₇–C₂O; after heat treatment at 900°C, it is a mixture of KCrP₂O₇ and a-CrPO₄ phases.