The combinations of Iron with Oxygen are somewhat numerous. There are two distinct Oxides which form Salts, viz. the Protoxide of Iron, containing an atom of Oxygen to one of metal; and the Peroxide, with an atom and a half of Oxygen to one of metal. As half atoms however are not allowed in chemical language, it is usual to say that the Peroxide of Iron contains three equivalents of Oxygen to two of metallic Iron.
Expressed in symbols, the composition is as follows:—
Protoxide of Iron, Fe O.
Peroxide of Iron, Fe2O3.
The Proto- and Persalts of Iron do not resemble each other in their physical and chemical properties. The former are usually of an apple-green colour, and the aqueous solutions almost colourless, if not highly concentrated. The latter, on the other hand, are dark, and give a yellow or even blood-red solution.
The Protosalts of Iron are alone useful in Photography; but the following experiment will serve to illustrate the properties of both classes of salts:—Take a crystal of Protosulphate of Iron, and, having reduced it to powder, pour a little Nitric Acid upon it in a test-tube. On the application of heat, abundance of fumes will be given off, and a red solution obtained. The Nitric Acid in this reaction imparts Oxygen, and converts the Protosulphate entirely into a Persulphate of Iron. It is this feature, viz. the tendency to absorb Oxygen, and to pass into the state of Persalts, which makes the Protosalts of Iron useful as developers.
There are two Protosalts of Iron commonly employed by Photographers: the Protosulphate and the Protonitrate of Iron.
a. Protosulphate of Iron.—This salt, often termed Copperas or Green Vitriol, is an abundant substance, and used for a variety of purposes in the arts. Commercial Sulphate of Iron however, being prepared on a large scale, requires re-crystallization to render it sufficiently pure for Photographic purposes.
Pure Sulphate of Iron occurs in the form of large transparent, prismatic crystals, of a delicate green colour: by exposure to the air they gradually absorb Oxygen and become rusty on the surface. Solution of Sulphate of Iron, colourless at first, afterwards changes to a red tint, and deposits a brown powder; this powder is a basic Persulphate of Iron, that is, a Persulphate containing an excess of the oxide or base. By the addition of Sulphuric or Acetic Acid to the solution, the formation of a deposit is prevented, the brown powder being soluble in acid liquids.
The Crystals of Sulphate of Iron include a large quantity of water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectly anhydrous, in which state it forms a white powder.
b. Protonitrate of Iron.—This salt is prepared by double decomposition between Nitrate of Baryta or of Lead and Protosulphate of Iron. It is an unstable substance and crystallizes with great difficulty; its aqueous solution is pale green at first, but very prone to decomposition, even more so than the corresponding Sulphate of Iron.