Ferric pyrophosphate, Fe₄(P₂O₇)₃

Ferric pyrophosphate, Fe₄(P₂O₇)₃.9H₂O, is obtained as a white precipitate on adding ferric chloride solution to an equivalent amount of sodium pyrophosphate. It is soluble in ferric chloride, in excess of sodium pyrophosphate, and also in acids.

When precipitated ferric hydroxide is dissolved in a 48 per cent, aqueous solution of phosphoric acid, in the proportion of one molecule of Fe(OH)₃ to seven of H₃PO₄, a solution is obtained from which: -

1. With excess of cold water, a greyish yellow precipitate is obtained, of composition 4FePO₄.Fe₂(HPO₄)₃.
2. With boiling water, a precipitate of formula 6FePO₄.Fe₂(HPO₄)₃ results.
3. On boiling the filtrate from (ii.), the precipitate has the formula 4FePO₄.2Fe₂(HPO₄)₃.
4. On precipitation with alcohol, a substance 2FePO₄.3Fe₂(HPO₃)₃ is obtained.

Are these to be regarded as definite compounds ? - Probably not.

Complex pyrophosphates analogous to ferri-orthophosphates in which atoms of iron enter the negative radicle, have been prepared. Thus, sodium ferri-pyrophosphate, Na₆[Fe₂(P₂O₇)₃].9H₂O, analogous to sodium ferricyanide, is deposited at 30° C. as a pale violet micro-crystalline precipitate from a 15 per cent, solution of sodium pyrophosphate saturated with ferric pyrophosphate. The copper and silver salts, Cu₃[Fe₂(P₂O₇)₃].12H₂O and Ag₆[Fe₂(P₂O₇)₃].4H₂O respectively, have been obtained from the sodium salt by double decomposition.