Preparation of Iron(III) Phosphates by the Reaction of Iron(III) Oxide with Orthophosphoric Acid

Abstract

The preparation of iron phosphates by the reaction of Fe₂O₃ with H₃PO₄ was investigated under varyingreaction conditions, such as a mixing ratio P₂O₅/Fe₂O₃ (R) of phosphoric acid to iron oxide, heating temperature, heatingtime, heatingrate and water vapor pressure in the heatingatmosphere.Crystalline iron phosphates formed in the reaction of Fe₂O₃ with H₃PO₄ at a temperature range of 100-700°C consisted of 8 kinds of compounds : A and B types of iron(III) dihydrogen orthophosphate Fe(H₂PO₄)₃, iron (III) hydrogen pyrophosphate FeHP₂O₇ neutral iron (III) pyrophosphate Fe₄(P₂O₇)₃, II type of iron(III) dihyrogen triphosphate FeH₂P₃O₁₀ A and C types of iron (III) polyphosphate Fe(PO₃)₃ and an unknown compound. 1) The A and B types of Fe(H₂PO₄)₃ were formed at R=3-4 at a low temperature below 200°C. The A type was especially formed at R=4 at 125°C, while the B type at R=3 at 125°C. 2)Pyrophosphate FeHP₂O₇ was easily formed at R=3 by heating for 5h in an electric furnace, in a sealed tube, in a water vapor, and in dry air. Further, Fe₄(P₂O₇)₃ was easily formed at about R.1.5 at 250-300°C. 3) The II type of FeH₂P₃O₁₀ was formed at R=3-4 in a temperature range 275-400°C, but only as a mixture with iron (III) pyrophosphate or iron (III) polyphosphate, not in a pure form. 4) The temperature at which Fe(PO₃)₃ formed was dependent on the mixing ratio R; with increasing mixing ratio, the temperature of its formation shifted to a lower temperature. At R=4, Fe(PO₃)₃ began to form at about 275°C. At 500°C, the C type of Fe(PO₃)₃ waseasily obtained ateither in an electric furnace/or a sealed tube or in a water vapor. On the other hand, the A type of Fe(PO₃)₃ was obtained atR=3 byheating in a vacuum, and its yield closely related with heating rate in the primary heat treatment. 5)At R.1-2 in a temperature range 150-250°C, an unknown compound was formed, and it was always obtained as a mixture with Fe₂O₃.