Symbol, FeO SO{3} + 7 HO. Atomic weight, 139.
This salt, often termed copperas or green vitriol, is a most abundant substance, and used for a variety of purposes in the arts. Commercial sulphate of iron, however, being prepared on a large scale, requires recrystallization 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 color: by exposure to the air they gradually absorb oxygen and become rusty on the surface. Solution of sulphate of iron, colorless 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.
Aqueous solution of sulphate of iron absorbs the binoxide of nitrogen, acquiring a deep olive-brown color: as this gaseous binoxide is itself a reducing agent, the liquid so formed has been proposed as a more energetic developer than the sulphate of iron alone.
Iron, Protonitrate of.
Symbol, FeO NO{5} + 7 HO. Atomic weight, 153.
This salt, by careful evaporation in vacuo over sulphuric acid, forms transparent crystals, of a light green color, and containing 7 atoms of water, like the protosulphate. It is exceedingly unstable, and soon becomes red from decomposition, unless preserved from contact with air.
The following process is commonly followed for preparing protonitrate of iron:—
Take of nitrate of baryta 300 grains; powder and dissolve by the aid of heat in three ounces of water; then throw in, by degrees, with constant stirring, crystallized sulphate of iron, powdered, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble sulphate of baryta.