The same advantages follow in all cases of combining a developer of high reduction poten tial with one of low reduction potential, e.g. inetol and glycin, or metol and pyro.
Similar results to those obtained by these mixtures can be obtained by various definite. combinations of developers with different char acters, e.g. compounds of a polyphenol with an aminophenol or polyamine, described by Luniii.'!re and Seyewetz from 1899 to 1913 ; hydramine, metoquinone, and chloranol (§ 363), all substances developing the latent image in a sulphite solution, and considerably energized by rendering this solution alkaline. It is also possible to obtain very similar developing solutions without needing to isolate these compounds, by mixing the two separate devel opers in suitable proportions.
350. Oxidation Products of Developers. In the presence of sulphite there is not the produc tion of highly-coloured compounds, compar able to the acids (humus) which form in the oxidation of nearly all developers, and especially with the polyphenols, but all oxida turn is not prevented, and it is of course not possible to prevent oxidation due to the reduc tion of the silver bromide. The products of oxidation formed during development often attach themselves to the gelatine in the very places where they have been formed, causing a tanning of the gelatine, which takes place more completely according to the density of the siker image at the particular and sometimes forming an actual secondary frequently in conjunction with a general staining of the gelatine.
Of the developers in general use, pyre exhibits the above-mentioned properties most strongly when not protected by a relatively large propor tion of sulphite. In addition to a slight yellow brown stain extending over the whole image, there may be observed, after complete removal of the silver (e.g. with Farmer's reducer), a brown image, not of great visual intensity, but which, by reason of its highly inactinic colour, is often strong enough to allow of satisfactory prints being made from The secondary image given by the oxidation products of many developers can play the part of a mordant (§ 603) towards several basic dyes (Lumiere and Seyewetz) it can also cause the reduction of various metal salts (14'. Leiber,
351. Role of Sulphite. The quantity of sul phite to be used in a developing solution depends on the nature of the developer and of the substances used to render it alkaline, on the dilution of the bath, on the degree of keeping quality desired in partly emptied bottles or in developing tanks, and on the prevailing tem perature. 3 For instance, very little sulphite is required with glycin (the spontaneous oxidation of which is negligible) and with paraphenylenediamine (the oxidation products of which are then almost colourless), while a relatively large quantity is required with pyro solutions to avoid staining the image.
Developing solutions made alkaline with caustic alkalis tend to oxidize more rapidly than those containing carbonates ; for this reason the sulphite content should be somewhat increased with the former. All other conditions being equal, the proportion of sulphite to developer must be greater in a dilute solution than in a concentrated one.
With most developers, if the developing solution is prepared shortly before use and is used once only, a sulphite content of about per cent is sufficient. It is, moreover, at this strength that the solubility of silver bromide' in solutions of sulphite reaches its minimum, thus markedly reducing chemical fog Nees and Piper, 1912). The amount of sulphite must be increased when a developing solution has to be kept long in a partly empty container, especially when there is a large surface in contact with air (solution left in a developing tank without a floating cover) and in all cases where air has free access to the emulsion during its develop ment (development of cinema film on continuous machines or drums, and development in tanks where the developer is kept in motion by a stream of air). 2 Finally, as an appreciable rise in temperature always accelerates the atmospheric oxidation of the developing solution, it is necessary in hot climates to counteract this effect by in creasing the quantity of sulphite specified for use in temperate countries.