AMMONIUM PERSULPHATE (Fr., Per sulfate d'ammoniaque; Ger., Ueberschwe felsaures ammonium) Molecular weight, 228. Solu bility, t in 2.5 water. It takes the form of colourless crystals, which are obtained by electro lysis. It is principally used as a reducer, and is especially valuable in that it reduces the high lights more than the shadows. The following formula may be used : Ammonium persulphate 48o grs. so g.
Distilled water to 20 OZ. 1,000 CCS.
The negative should be immersed in this until the reduction is nearly complete, and then rapidly washed. A stop bath of sodium sulphite (5 per cent. solution) is sometimes recommended, but it occasionally gives rise to stains, and it is better to use water only. The chemical action which takes place is supposed to be Ag (NH4), S208 = (NH4)2 + Silver Persulphate Ammonium Silver sulphate sulphate The silver sulphate dissolves in water. Namias states that a solution of persulphate acidified with nitric acid acts like the ordinary " hypo " and ferricyanide reducer.
The addition of o•5 to i per cent. to the normal platinotype developer shortens the scale of gradation, and is thus useful for over-exposed prints.
H. W. Bennett has made a special study of ammonium persulphate as a reducer, and his special formula will be found under the heading, " Bennett's Reducer." The addition of ammo nium sulphocyanide has been recommended, this causing the persulphate reducer to act in the opposite way—namely, to clear the shadows first after the manner of the ferricyanide and " hypo " reducer, which makes it particularly suitable for negatives of line subjects. The formula for the persulphate reducer often re ferred to as " Puddy's reducer" is : water i oz., ammonium persulphate 25 grs., and ammonium sulphocyanide (to per cent. solution) i2o minims. Namias advocates a 5 per cent. solution of per sulphate made acid with t per cent. of sul phuric acid for developing over-exposed carbon prints. Ammonium persulphate has also been suggested as a " hypo " eliminator, stain remover, and as an addition to the oxalate developer for platinotypes ; to i per cent. added to the normal oxalate developer shortens the scale of gradation and saves over-exposed prints.