362. Paraphenylene Diamine. This developer 's sold either as the free base or as salts (usually the hydrochloride). The free base and the water-soluble salts may cause eczema wherever their dust touches the skin. This does not occur with the aqueous solutions of these salts, and it can be avoided, even with the dry product, by using it in the form of sulphate or other salt insoluble in pure water and soluble only in sulphite or alkali solutions.
The free base of paraplienylene diamine occurs in colourless crystalline plates, melting at 147 U. (296-6 F.), slightly soluble in water, fairly soluble in ether, and very soluble in alcohol. It is a weak solvent of silver bromide. The hydrochloride occurs in the form of colourless crystalline plates, decomposing without inciting when heated. It is very soluble in water, slightly soluble in alcohol, and insoluble in ether.
In a plain solution of sulphite it develops the latent image, but only very slowly, development ceasing before the image has acquired sufficient contrast. As only a part of each grain has been developed, the image is formed by a deposit of very finely divided silver the density of which is usually insufficient.
In a solution made alkaline with an alkali carbonate it is possible to obtain an image of normal density, but such a developer has little energy and does not develop the shadow detail of a correct exposure completely. (hi the other hand, a very energetic developing solution results from the use of a caustic alkali, and even with a low sulphite content this solution keeps fairly well. The normal oxidation products formed during development are colourless.
363. Addition Products of Developers. Mete tptinone. This definite combination, which must not be confused with a mixture of metol and hydroquinone, occurs in colourless crystalline flakes melting at C. (257° F.), slightly soluble in cold water (r per cent at to° C., 50° F.), more soluble in hot water, very soluble in alcohol, and slightly soluble in ether or benzene. This compound is decomposed by mineral acids, even in dilute solutions, freeing the hydroquinone and yielding a salt similar to (or identical with) metol. Metoquinone dis solves well in alkaline solutions, and with addition of sulphite these solutions keep well.
Metoquinone develops slowly in a non-alkaline sulphite solution. This solution does not oxidize readily ; for rapid development it must be ren dered alkaline, usually with an alkali carbonate.' hloranol and II are no longer manufactured.
364. Sulphites, Bisulphites, and Metabisul phites. Neutral Sulphites. The only neutral sulphite commonly used in preparing developing solutions' is the neutral sulphite of soda, or sodium sulphite, supplied either as anhydrous or crystallized. One part of the former is equal to two parts of the crystals, both forms being assumed to be pure. 4 Commercial sodium sulphite crystals gener ally contain from 8o to 90 per cent of the pure substance, i.e. 40 to 45 per cent of the pure anhydrous salt, whereas the anhydrous sulphite rarely contains less than 90 per cent of real substance. The anhydrous sulphite is supplied in a fine powder, like flour, and is not so heavy and bulky as the crystals. In the dry state it always keeps better than the latter (the powder clots together and prevents the circulation of air) the pieces of which are always more or less dehydrated, and are sometimes partially con verted into carbonate on the surface. 3 Both forms contain a small amount of sodium sulphate and dithionate, produced by oxidation of the sulphite, which have no effect on photographic operations, and traces of hyposulphite and of sodium chloride. This sulphate is formed especially by oxidation in damp air.
Sodium sulphite is very soluble in cold water, and still more SO in tepid water (about 23 per cent at 68° F. and 44 per cent at 104° F., these percentages being for the anhydrous salt). When boiled, concentrated solutions of sulphite deposit the anhydrous salt. No attempt should be made to dissolve sulphite in water which is too hot, for it will only dissolve after partial cooling. Contrary to what occurs with the crystals, anhydrous sulphite does not lower the temperature of the water in which it is being dissolved. This fact, and the rapidity with which it dissolves, cause the anhydrous salt to be preferred whenever the developing solu tion is being compounded for immediate use.