Pyro developing solutions ready for use oxidize somewhat quickly, as indicated by the yellow or brown colour which they assume. It is therefore customary to have two stack solu tions, one containing the carbonate or caustic alkali, and the other, the pyre, sodium sulphite, and a certain amount of sodium bisulphite, as pyre is relatively stable only in acid solution.
With pyre the maximum energy is obtained with a caustic alkali, but the quantity must not exceed that corresponding with the formation of the inonophenolate (E. Valenta, 1902). Alkali in excess of this proportion (45 parts of caustic potash or 32 parts of caustic soda per too parts of pyre) leads to intense chemical fog and a very rapid discoloration of the baths.
By adding acetone to a solution of pyre in sodium sulphite a developer is formed which gives pleasing warm tones on lantern plates when properly dosed with bromide. The smell of the acetone and the headaches which may result from its fumes forbid the use of this mixture except in perfectly well ventilated dark rooms.
360. Aminophenol Developers. Paramino phenol. This developer is sometimes supplied as the free base, but it is then very rapidly oxidized, so that it is usually preferred to obtain it as a stable salt, generally in the form of a hydrochloride.
Paraminaphenal hydrochloride occurs in colourless crystalline needles, decomposing with out melting on heating. It is very soluble in cold water, somewhat less soluble in alcohol, and insoluble in ether. The addition of an alkali carbonate to a concentrated aqueous solution of paraminophenol hydrochloride causes efferves cence and the precipitation of the free base, which is very slightly soluble in waterancl in solu tions of sulphite or carbonate. For this reason it is impassible to prepare concentrated developing solutions of paraminophenol when the alkali is a carbonate. With a caustic alkali the free base is converted into an extremely soluble aminophenolate, so that it becomes possible to prepare very highly concentrated developers, which are diluted from 20 to Ioa times for use (Rodinal, Agfanol and their imitations). Para minophenol developing solutions keep satis factorily.
Paraminophenol is one of those developers which have the least tendency to give chemical fog when development is done with warm solu tions. It is therefore largely used in tropical countries, but should be employed with an alkali carbonate and not with a caustic alkali.
In the United States paraminophenol is available in the form of an oxalate (Kodelon), and the suggestion has been made to use it, preferably, in the form of a tartrate ( J. Desalme,
1924) the base of which is not precipitated by carbonates, so that concentrated solutions can be made without caustic alkali and also without first having to isolate the free base, sodium tartrate being much more soluble than the chloride.
Paraminocresol, This developer has no marked advantage over paraminophenol. During the war, when stocks of German metol were ex hausted, and its manufacture was not yet perfected in England, France, and the United States, paraminocresol was suggested as a substitute for metal, to which, however, it is far inferior in energy. Its properties differ little from those of paraminophenol, and therefore do not need description.
One of its immediate derivatives, Edind, is scarcely of greater importance.
Metol. Metol, known for many years only under this German trade-name, occurs as almost colourless crystalline needles which decompose without melting when heated, are soluble in cold water and in alcohol, and insoluble in ether. Metol being one of the developers most subject to fraud, it is useful to note that it is quite insoluble in ether' and also that it dissolves completely in three times its own weight of concentrated hydrochloric acid, in which the other aminophenols likely to be substituted for or mixed with it are only very slightly soluble.
The free base of metol can be precipitated from very concentrated aqueous solutions of the sulphate by adding a carbonate. It can also be precipitated, even at much lower con centrations, by a comparatively strong solution of sulphite. Owing to its great solubility in alcohol, the latter is sometimes used instead of part of the water when a more concentrated solution (to be diluted four to five times for use) is prepared. Concentrated solutions of sulphite can also precipitate metoquinone when an at tempt is made to dissolve metol and hydro quinone simultaneously. This trouble is avoided by adding the major portion of the sulphite after these developers have been dissolved, or by not dissolving the larger part of the sulphite salt until the mixture has been rendered alkaline. The presence of a little alcohol (§ 357) also pre vents this precipitation to a certain extent. By combining these various methods it is possible to obtain metol-hydroquinone developing solu tions two or three times as concentrated as the normal strength.