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Inorganic Developers 345

developer, oxalate, ferrous, solution, alkaline and various

INORGANIC DEVELOPERS 345. Mineral Salts as Developers. Iii addition to ferrous oxalate (which is the only one of these various developers which has come into effective practical use) and various other ferrous salts (fluoride, citrate, etc.), the following have been suggested for use as developers : clipro oxalate of ammonia (Carey Lea, 1879) ; titano oxalates and alkaline molybdo-oxalates (T.

'For instance, with a developer containing 6 parts of hriroquinone and r part of metol, the " factor " will differ very slightly from (6x)±i x 3o) 8.5 6 + r Pavolini, 1933) ; hydro-sulphite in alkaline solution (W. B. Bolton, 1893) ; alkaline per oxides (Le Roy, 1894) ; or hydrogen peroxide (M. Andresen, 1889) in alkaline solutions.

To this class of developers may be added two substances on the border-line between inorganic and organic compounds, viz., hydroxylamine (C. Egli and A. Spitler, 1884), and hydrazine (E. Votocek, 1898), both in alkaline solutions. These substances have never been used in practice as developers, but on various occasions the use of certain of their organic derivatives has been suggested (Lumire and Seyewetz, 1894 ; W. H. 19o8).

346. Ferrous Oxalate Developer. The ferrous oxalate developer, of which the present method of preparation was indicated by J. M. Eder (1879), is now scarcely ever used by either amateur or professional photographers. Its very low reduction potential does not enable it to develop satisfactorily any but amply exposed negatives. Its preparation and use demand more care than is required for organic devel and afford no material economic com pensation for these practical drawbacks. But for various scientific uses of photography this developer possesses the distinct advantage of containing no solvent of silver bromide, and of thus avoiding various disturbing factors which sometimes occur in the use of ordinary devel opers, but noticeable only in exact localization of points, and in photometric measurements of a high order of accuracy. This developer is the

only one with which chemical fog is completely avoided (Hurter and Driffield, 1890) and which produces a perfectly neutral grey image, com pletely free from all coloured oxidation products.

The ferrous oxalate developer is prepared at the time of use by pouring slowly, with constant stirring, i volume of a 25 per cent solution of ferrous into 3 volumes of a 25 per cent solution of neutral potassium This produces a limpid reddish mixture (feno potassic oxalate), which can be used as a developer without any addition of potassium Over-exposure can be corrected to a certain extent by adding potassium bromide. Being a developer with a low reduction potential (0-33, taking hydroquinone as of reduction potential = 1), the iron developer is very susceptible to the action of bromides. For correction of over exposure it suffices to add a relatively small quantity of potassium bromide to the developing bath (e.g. not more than 4 gr. per I() oz. of bath).

Sodium thiosulphate (hypo) in very minute quantity (a few drops of a i : i,000 solution) has a marked accelerating effect on the ferrous oxalate developer (it decomposes the ferric salts formed), the image then appearing much more rapidly. An excess of hypo produces heavy fog, and, finally, reversal of the image, the fog being then denser than the image proper.

On the negative impregnated with the oxalate solution being placed in the wash-water or the fixing bath, a uniform precipitation of calcium oxalate takes place in the upper layers of the gelatine, forming a white deposit. During, wash ing after fixing this deposit is readily removed by placing the negative for a few moments in a very weak solution of hydrochloric acid, e.g. I : 20o solution about i dram in 20 oz.).