Ferrous Oxychlorides

Numerous basic- or oxy-chlorides of iron have been described at various times, but it is very doubtful if the chemist is justified in regarding these as separate chemical entities. For example, Bechamp found that, on oxidising ferrous chloride with nitric acid in the presence of a small quantity of hydrochloric acid, an insoluble, yellow residue was obtained, of composition corresponding to the formula FeCl₃.6Fe₂O₃. On treatment with water a product, 2FeCl₃. 17Fe₂O₃, resulted, whilst addition of ammonium hydroxide induced the formation of a still more basic residue, namely FeCl₃.72Fe₂O₃. Such precipitates are of little theoretical interest.

A crystalline oxyehloride is obtained when solutions of ferric chloride containing not less than 80 per cent, of the salt are heated in sealed tubes to 150°-160° C. in the presence of a few fragments of magnesium carbonate to neutralise the liberated hydrochloric acid. The crystals consist of lustrous rhombic prisms, reddish brown in colour, and of composition corresponding to the formula 2FeCl₃.Fe₂O₃.2Fe(HO)₃. These, on contact with hot water, retain their crystalline form, although they lose their chlorine as hydrogen chloride, being converted completely into oxide corresponding in composition to Goethite.

If, in the foregoing method of preparation, an 85 to 90 per cent, solution of ferric chloride is maintained at 225° to 280° C. for some time, an oxyehloride of composition FeCl₃.Fe₂O₃ is obtained as reddish brown lamellae, whilst between 300° and 340° C. large plates of brownish black 2FeCl₃.3Fe₂O₃ result.

Upon heating anhydrous ferric chloride in a slow current of carbon dioxide saturated with water vapour, the oxyehloride FeCl₃.Fe₂O₃ is produced at 275° to 300° C. in the form of reddish brown needles, whilst larger and darker needle-shaped crystals of 2FeCl₃.3Fe₂O₃ result if the temperature is raised to between 350° and 400° C.