Complex Iron Carbonates

Complex ammonium iron carbonates have been prepared in solution by adding excess of ammonium carbonate to ferrous or ferric salts in aqueous solutions. Ferric ammonium carbonate solution is blood-red in colour, and stable when kept in a closed vessel, but deposits ferric hydroxide on evaporation. Ferrous ammonium carbonate yields a colourless solution, which, upon oxidation in a limited supply of air, yields doubly refracting green prisms. of basic ferroso-ferric ammonium carbonate, NH₄.CO₃.Fe.CO₃.Fe.CO₃.FeO.2H₂O. This, when treated with alkali hydroxides, yields magnetic ferroso-ferric oxide.

A basic carbonate is obtained by grinding crystals of ferric chloride with a slight excess of crystallised sodium carbonate until no more carbon dioxide is evolved. The now liquid mixture is mixed with water, and the precipitate allowed to settle, and dried. The proportion of carbon dioxide present is liable to vary, as washing the precipitate induces hydrolysis. A substance of composition approximating to 7Fe₂O₃.CO₂.8H₂O has been obtained.