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Organic Developers 347

solution, image, developer, developing, hydroquinone, development and fog

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ORGANIC DEVELOPERS 347. The Developing Function. With the exception of certain derivatives of hydroxyl amine and hydrazine already mentioned (§ 345), and of which none is used in practice, all organic developers belong to the aromatic series (deriva tives of benzene, toluene, naphthalene, and other coal-tar substances). Laws making it possible to say whether an aromatic body of known constitution is, or is not, a developer i were formulated simultaneously and independently in IS91 by A. and L. Lumiere, in France, and M. Andresen in Germany. Since then they have only received modifications and additions of secondary 348. Normal Constituents of a Developer. Although it is possible to obtain faint traces of an image (L. P. Clem, 1909 ; E. Cousin, 1912) by treating a sensitive film in a very dilute pure aqueous solution of some developers (diamino phenol, paraphenykne-diamine, and its deriva tives, etc.), it is usually necessary, in order to obtain a vigorous image in a reasonable time, to render the developing solution alkaline, so that the hydrobromic acid resulting from the reduction of the silver bromide may be neutral ized as it is formed. A pure aqueous solution of hydroquinone does not develop, but a solution containing 5 per cent of hydroquinone and 5 per cent of caustic soda develops a satisfactory image in about three minutes, without fog or appreciable stain (C. E. K. Mees and C. NV, Piper, 1911). The caustic alkali may, moreover.

be replaced by the salt of a weak acid, easily displaced by the hydrobromic acid and not itself preventing development. This is the case with the carbonates and various other salts, among which are the sulphites. With the latter, development is exceedingly slow, except with the developers capable of developing an image in an aqueoussolution (about one hour is required. to develop a complete image in a very concen trated sulphite solution of pyro). Development is completed in a normal time by adding a carbonate (of soda or of potash) to the pure aqueous solution of a developer substance, but these solutions (like nearly all others com pounded with a caustic alkali) oxidize in the air with very great rapidity, thus giving rise, during the time necessary for development, to brown oxidation products, which strongly stain the gelatine.

In x882 H. B. Berkeley observed that the addition of a sulphite to the alkaline solution of a developer considerably delays this oxidation and opposes the formation of the highly-coloured products obtained in the absence of this preserva live. While sulphite must be considered a necessary constituent of all developing solutions compounded with organic developers, it is not entirely advantageous. While it increases the energy of a solution of paraminoplumol and carbonate (M. Andresen, 1898), it retards development with a solution of hydroquinone and carbonate (J. Desalme, 1921), and further more, as a solvent of silver bromide (with which it forms a soluble double sulphite) it favours the formation of fog.

Finally, in many cases a bromide, added in small quantities to the developing bath, must be used as a restrainer of chemical fog (§ 336 to § 338).

349. Mixtures of Developers. While the num ber of developers is considerable, all are not of the same importance ; the practical properties of many of them do not differ to any material extent, awl changes in the relative quantities of the various constituents of a given develop ing solution often cause greater differences than those between one developer and another.

Moreover, it is often possible, by combining two developers of different character in suitable proportion, to obtain results very superior to those obtained by using one developer alone. The type of these mixed developing solutions is that prepared with metol and hydroquinone, the important properties of which were empha sized by Liippo-Cramer in 1902. In such a de veloper the image appears rapidly, the shadows following quickly on the high-lights (character of inetol) and density then builds up rapidly (character of hydroquinone). Moreover, the chemical fog, so frequently given by metol, and the yellow fog, due to hydroquinone, are thus avoided at one and the same time.

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