352. The fact that a small quantity of the silver halide dissolves in the sulphite of soda used as a preservative of the developing solu tion against oxidation results in the solution behaving, to a certain extent, like a physical developer ; the dissolved silver salt is reduced by the developer and deposits itself, partly in the liquid and on the walls of the tanks, and partly on the image itself, which is thereby reinforced.
In those parts of the film where there is no image, which is especially the case with shadow parts of large size in an under-exposed negative, the reduced silver has not in its neighbourhood germs on which it can deposit itself selectively, and it then deposits itself in a nearly uniform manner, in the state of colloidal silver, thus constituting one of the forms of dichroic fog (§ 433). This fog occurs especially with hydro quinone developer, containing much sulphite, or that is warm, when development is unduly prolonged.
It may incidentally be mentioned that a bath containing plenty of sulphite generally avoids the local fog which sometimes tends to form on negatives developed vertically in metal frames if the metal is not one of those specified (§ 264) as being without action on the developing solutions.
353. The action of sulphite is not the same with all developers. While certain developers derived from diphcnylaniine are, after oxidation, indefinitely regenerated by sulphite (j. Desalme, I9r0), that is a special case, limited, unfor tunately, to substances which have not come into general photographic use.
Besides protecting the developer against oxidation,' an important part played by sul phite is the prevention of the formation of the highly-coloured oxidation products of developers, which are capable of staining the gelatine. This action takes place either by the sulphite reducing these coloured substances to the state of colour less leuco bases, or by reason of the oxidation of the developer, in the presence of sulphite, producing products different from those obtained in an alkaline solution containing no sulphite, 2 354. Role of the Alkalis. The alkali, whilst playing the part of an electrolyte in the develop ing bath, must also assure the neutralization of the hydrobromic acid, which otherwise would arrest development at the outset. Also, when
the developing substance is a salt (e.g. the hydrochloride or sulphate of an aminophenol or diamine), it is needed to liberate the base ; or when the developer is a phenol (a polyphenol or aminophenol) it is needed to form a salt with at least part of the phenol Owing to the disintegration of the gelatine, which is easily produced by the caustic alkalis (when present in excess of the quantity required to form the phenolate), it is customary to use in place of them the corresponding carbonates (or, sometimes, basic phosphates, borates, or silicates, in an aqueous solution, a minute proportion of the carbonate is dissociated into caustic alkali and bicarbonate (or dibasic phos phate) and can therefore play the part of a reserve of caustic alkali which is constantly replenished as long as the mixture contains available carbonate.
At the strengths generally used in developing baths (5 per cent to BD per cent) the alkaline carbonates accelerate the diffusion of the liquid into the gelatine, without affecting the gelatine or causing excessive swelling. 1 During development, a bath rendered alkaline by a carbonate becomes charged with bicar bonate, which plays practically no part as an alkali. 2 Thus, when it is desired to strengthen a bath in continuous use, a little caustic alkali may be added to convert this bicarbonate into normal carbonate.
Ammonia and ammonium carbonate can act is alkalis in developing baths, but their use is limited to a few special cases. They are solvents of silver chloride and bromide, and so favour the production of dichroic fog ; in a slow-acting developer rendered alkaline by ammonia the contrast of the image is restricted because a considerable portion of the silver is deposited on the sides of the dish, this portion being the larger as the strength of the ammonia is higher, the grain finer, and development slower ( J. Vidal, 1931). While the true amines (am monia substitutes) have never been used on account of their offensive smell, many of their derivatives have been employed in phenol or aminophenol developers : aminoacetate of soda (Hoechst Farbwerke, Igor), alcoholamines, particularly triethanolamine I) 3, and its salts (M. L. Dundon, 1932), etc.