Trisodium ferri-trisulphate, Na₃[Fe(SO₄)₃]

Trisodium ferri-trisulphate, Na₃[Fe(SO₄)₃].3H₂O, occurs in nature as ferro-natrite, in Chili. It may be prepared by heating sodium sulphate с deeahydrate, Na₂SO₄.10H₂O, until it melts in its combined water, and then adding sulphuric acid and ferric sulphate. It separates as a white substance, which is slowly dissolved by water with decomposition. The salt may also be obtained by allowing a mixture of acid sodium sulphate and ferri-disulphuric acid to remain in contact with a moist atmosphere for several months, when the complex salt crystallises out in greenish white hexagonal crystals. It is completely dehydrated at 100° C.

A basic salt, 2Na₂O.Fe₂O₃.4SO₃.7H₂O, has also been prepared.

When barium chloride is added to sulphuric acid or soluble sulphate solutions containing dissolved ferric salts, the precipitated barium sulphate is contaminated with iron. This has been variously attributed to adsorption of ferric compounds, and to the formation of solid solutions of ferric salts and barium sulphate. Smith, however, does not regard either of these explanations as satisfactory, and attributes the phenomenon to the formation of insoluble barium ferri-disulphate, Ba[Fe(SO₄)₂]₂.xH₂O, which is admixed with barium sulphate.