In photography

Na2S20j. Obtained NaOH solution saturated SO2 at 100 C. Forms 7 and 6 hydrates. Used in photography. 

Na COj and Na2S with SO2 or from Na2S03 plus sulphur. Forms many hydrates. Used-in photography ( hypo ) because it dissolves silver halides. Also used in tanning, preparation of mordants, as a fermentation preventative in dyeing and in chemical manufacture. 

Hydrogen peroxide in aqueous solution has many uses, because the products from its reaction are either water or oxygen, which are generally innocuous. The chief use is bleaching of textiles, both natural and synthetic, and of wood pulp for paper. Other uses are the oxidation of dyestuffs, in photography and in the production of... 

The reducing action of sulphurous acid and sulphites in solution leads to their use as mild bleaching agents (for example magenta and some natural dyes, such as indigo, and the yellow dye in wool and straw are bleached). They are also used as a preservative for fruit and other foodstuffs for this reason. Other uses are to remove chlorine from fabrics after bleaching and in photography. 

It is used as the fixer in photography under the name hypo . 

All the silver halides are sensitive to light, decomposing eventually to silver. In sunlight, silver chloride turns first violet and finally black. The use of these compounds in photography depends on this (see below). (All silver salts are, in fact, photosensitive—the neck of a silver nitrate bottle is black owing to a deposit of silver.)... 

Iodine compounds are important in organic chemistry and very useful in medicine. Iodides, and thyroxine which contains iodine, are used internally in medicine, and as a solution of KI and iodine in alcohol is used for external wounds. Potassium iodide finds use in photography. The deep blue color with starch solution is characteristic of the free element. 

TARLR VI-l.S. PROPOSED USES OF 2-AMINOTHIAZOI.HS (Al AND 2-IMINO-4-THIAZOI,INES (B) IN PHOTOGRAPHY... 

The use of sensitizing dyes in photography has been the subject of many studies and constitutes. still now. one of the most studied areas in specialized periodic publications (125, 126) or in textbooks (88. 127). It can be ascertained that one hundred years after Vogel s discovery of spectral sensitization, the basic mechanisms of action of dyes on their silver halide support still remain not fully understood. However, the theoretical reasons explaining why among many other dye families practically only cyanine methine dyes appear to be spectral sensitizers (128) are better known. 

They are used principally as intermediates for cyanine dyes that serve in photography (5, 6. 35. 36. 50-52). 3-Ethyl-2,4-dimethylselenazolium iodide (m.p. 157-8 C) was prepared as colorless crystals in 87% yield by Brooker et al. (6) by heating 2,4-dimethyls el en azole with an excess of ethyl iodide for 48 hr. 

ThiO 4-oxoselenazolidines substituted in the 3-position, have been described in a number of patents, (79, 80). They are used as intermediates in the manufacture of pharmaceuttcal products and color sensitizers in photography. They are obtained by action of isothiocyanates on a-hydroselenoacetic acid in the presence of strong base and in the absence of air (Scheme 77). 

Carbocyanine or merocyanine dyes prepared from selenazole or selenazoline rings have a particular interest in photography. 

Hydrazono-5-(p-dialkylaminophenylimino)seienazoles, pyrazolone azomethin selenazoles, quinoid dyes, and formazans are other important dyes used in photography and the color industry (cf. previous sections). 

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