Sodium ferro-pyrophosphate, Na₈Fe₂(P₂O₇)₃

Sodium ferro-pyrophosphate, Na₈Fe₂(P₂O₇)₃, and sodium ferro- and ferri-metaphosphates, Na₄Fe(PO₃)₆ and Na₃Fe(PO₃)₆, have similarly been prepared.

Alkali ferro-pyrophosphates reduce solutions of gold and silver salts in the cold, the free metals being obtained in the colloidal condition. Mercuric salts are reduced, yielding first mercurous salts and finally colloidal mercury, which is grey by reflected light, but a reddish brown colour by transmitted light. Cupric salts are reduced to colloidal cuprous hydroxide, which is yellow by transmitted light and affords a delicate reaction for copper. At 100° C. metallic copper is deposited as a thin film on glass. Solutions of platinum salts are not reduced even on boiling.

Two series of complex ammonio-salts have been prepared of general formulae: -

I. [Fe₄(NH₃)_24-4n(P₂O₇)_n](P₂O₇)_3-n,

and II. [Fe₄(NH₃)_12-4m(P₂O₇)_3+m]R_4m,

where R is an acid radicle, and n and m are small, whole numbers, of the order of 2 or 3.