Sulphite of soda tends to dissociate itself, at least in very weak proportions, into soda bisulphite and caustic soda, thus permitting of its use in preparing " non-alkaline " developing solutions with basic developers which oxidize rapidly in alkaline solutions.
Any factor which increases this dissociation, and particularly the addition of a substance capable of combining with soda bisnlphite, e.g. acetone and the aldehydes (Lumiere and Seyewetz, 1896), accelerates development in the case of developers working without an alkali, and even renders it possible to dispense with an alkali in developers that usually require it. This particular action does not manifest itself fully except with the phenolic developers (pyro, in particular), which form a phenolate with the alkali available. Of these various mixtures capable of replacing the alkalis in developing solutions, the only one used in practice is that of soda sulphite and acetone.
355. Effect of Soluble Iodides. The addition of a small quantity of an alkali iodide (0-02 per cent to per cent) to a developing solution, causes, in the case of low-potential developers, and particularly hydroquinone, a very consider able acceleration of the appearance of the image and, in some cases, also an acceleration of devel opment itself (A. Lainer, 1891), this particular action being often referred to as the Lainer effect. The addition greatly modifies the Watkins " factor," in some cases. With hydroquinone (made up with carbonate), the " factor " rises from 7 to 28, but the addition is without effect in the case of metol (S. E. Sheppard and G. Meyer, 1920).
The same action is observed when the iodide, instead of being mixed in the developer, is used as a preliminary bath, followed by prolonged rinsing in pure water, and the fact indicates that the action of iodide is on the grain of the emul sion and not on the developer itself (Lippo Cramer's hypothesis of nucleus denudation). The mechanism of this action is not yet under stood.
The presence of an iodide in the developer prevents the formation of black abrasion marks (Liippo-Cramer, 1914), but not of white marks, and lessens the tendency to dichroic fog. The use of iodide also prolongs fixation because of the formation, in the film, of yellow silver iodide, which is less rapidly dissolved than silver bromide.
In more concentrated solutions iodide pro duces a weakening of the image equivalent to a reduction of sensitivity, and may produce fog, particularly in a developer containing much sulphite.
356. Use of Various Neutral Salts. Gelatine swells less in a bath if the latter contains salts. The excessive swelling of gelatine, which occurs in warm climates during development and causes various failures, may be avoided by adding to the bath a neutral salt which has only a neglig ible effect in development (L. J. Bunel, 1910).
Among the various salts which may be used (sulphates, nitrates, phosphates, oxalates, etc.), soda sulphate is usually chosen on account of its cheapness. In moderate quantities, these salts slightly accelerate development with very dilute hydroquinone developing solutions, have no action on high-potential developers, and slightly retard the action of pyro solutions. At the very high strengths required to prevent the swelling of gelatine they retard development to an appreciable extent, a fact which is an advan tage, for in this way it is possible to compensate in some measure for the acceleration of develop ment due to high temperature.' At various times certain advantages have been attributed to the use of the alkali fermi cyanides mentioned in some old formulae for developers. The complete uselessness of this addition has been shown, in particular by P. Mercier (1892).
357. Various Additions. Soda thiosulphate (hypo), the accelerating action of which on the ferrous oxalate developer has been mentioned (§ 346), retards development in alkaline devel opers, decreases the density of the image, and causes the formation of fog, unless used in such large quantity that, like all solvents of the haloid salts of it leads to the production of dichroic fog. Under certain conditions, it is, however, possible to increase the amount of hypo up to the point when development and fixation take place together (§ 392).
On various occasions it has been suggested that development be slowed by increasing the viscosity of the developing solution by adding various thickeners, such as glycerine, glucose (W.
de W. Abney, 1897), sugar (L. Backeland, 1899). By retarding to a certain extent the interchange of liquids between the substance of the gelatine and the bath in which it is immersed, these additions tend to cause the image to be formed to a greater extent at the surface of the film, thus reducing contrast. The action, however, is not always very appreciable.
The addition of alcohol has been sometimes advised to avoid the precipitation of the developer in concentrated developer solutions, or to facilitate wetting. In developers containing hydroquinone, if ethyl alcohol and, more sev erely, methyl alcohol, are added in 'appreciable proportion, they give rise to fog. Developers may also contain antiseptics (§ 35r) and wetting agents (§ 369).
In addition to desensitizers which can be added to the developing bath, and of which some hinder the production of aerial fog (§ 331 to § 332), some commercial developing solutions contain an inert dye (usually cosine) serving to disguise the slight yellow or brown tinge, due to oxidation products of the developer, produced during preparation and bottling.