Films silver bromide

During the developing process of black-and-white film, silver bromide is removed from photographic film by the fixer. The major component of the fixer is sodium thiosulfate. The net ionic equation for the reaction is... 

Bromine has a lower electron affinity and electrode potential than chlorine but is still a very reactive element. It combines violently with alkali metals and reacts spontaneously with phosphorus, arsenic and antimony. When heated it reacts with many other elements, including gold, but it does not attack platinum, and silver forms a protective film of silver bromide. Because of the strong oxidising properties, bromine, like fluorine and chlorine, tends to form compounds with the electropositive element in a high oxidation state. 

When the film is developed (the developer being a reducing agent), the unchanged silver bromide immediately surrounding these nuclei is reduced to give a visible blackening of the film. 

L.l Sodium thiosulfate, photographer s hypo, reacts with unexposed silver bromide in film emulsion ... 

Silver bromide is a major component of photographic films and paper. A film manufacturer mixed 75.0 L of a 1.25 M solution of silver nitrate with 90.0 L of a 1.50 M potassium bromide solution and obtained 17.0 kg of silver bromide precipitate. What was the percent yield of this precipitation reaction ... 

The most valuable bromine compound probably is silver bromide, which is the light-absorbing species on which most film photography is based (see Box 20.2, Chemistry and Technology, on page 895). Bromine compounds are also used extensively as fire retardants, particularly in carpets, rugs, and clothing... 

In the non-stoichiometric case where ionization of defects is the norm, the mathematics become too complicated so that the equations are not solvable. However, we can use a thermodynamic method to obtain the results we want. We will present here the case of silver bromide whose use in photographic film highlights the use of defect chemistry for practical purposes. 

It has been found that only a few photons, maybe as little as six, are needed to form the latent image. Photographic film is a very sensitive light detector. The final step in the photographic process, fixing, removes the unreacted silver bromide crystals from the emulsion, thus stabilizing the image (Fig. 2.4). 

Silver bromide is used in photographic film and plates. It also is used in photochromic glass. In medicine it is used as a topical anti-infective and astringent agent. 

In some cases it has been found that the maximum on saturation adsorption of a solute from a solution corresponds to the formation of an adsorption layer one molecule thick. Thus Euler Zeit. Elehtrochem. xxviii. 446,1922) found that a maximum adsorption of silver ions by silver and gold leaf was attained in a 0 03 A solution. It was found that 5 5 and 8 5 to 9 mgm. of silver ions were adsorbed by a square metre of metallic silver and gold respectively, such a surface concentration is practically unimolecular. The adsorption of silver ions by silver bromide (K. Fajans, Zeit Phys. Ohem. cv. 256, 1928) was found on the other hand to be not complete, for only every fourth bromide ion in a silver bromide surface was found to adsorb a silver ion. Similar conclusions as to the unimolecular character of the adsorbed film in the case of chemical charcoal as an adsorbing agent for fatty and amino acids may be drawn from the data of Foder and Schonfeld Koll. Zeit xxxi. 76, 1922). 

Silver halides employed in emulsions are the chloride, the bromide and the iodide. Negative emulsions are composed of silver bromide with a small amount of silver iodide. Positive emulsions for films and paper contain silver chloride, or mixtures of silver chloride and silver bromide in varying amounts, according to the tone, speed, and contrast desired. 

The emulsion of a typical color film has three silver-bromide layers separately sensitized by suitable dyes to blue, green, and red light (Figure 28-11). When processed (Section 28-6C), the color formed in each layer is complementary to the color to which the layer is sensitive. Thus, if unexposed film is processed, intense yellow, magenta, and cyan colors are respectively formed in the blue-, green-, and red-sensitive layers. Then, when white light strikes this processed... 

A different color coupler is used for each layer of the emulsion and, although the complete color picture is formed in this step, the film is coal black because of the metallic silver produced at the same time. The silver then is oxidized to silver bromide with dichroinate solution containing bromide ion and removed with thiosulfate solution. The final image thus contains no silver. 

What occurs in a given layer of the film is roughly as follows Activation of the silver bromide in the layer by the light to which it is sensitized, and then... 

Zhelev and Astrov (125) studied the effects of S-sensitiza1-tion on the stability of silver deposited on evaporated films of silver bromide. The bulk distribution of development centers formed on the grain surface by deposition of silver atoms from vacuum was identical with that formed by exposure to high-irrad-iance light flashes or by the action of hydrogen atoms (126) on the silver bromide. As in experiments with unsensitized sheet crystals (127), surface developability disappeared upon storage... 

PROBLEM 16.28 The "fixing" of photographic film involves dissolving unexposed silver bromide in a thiosulfate (S2032-) solution ... 

When a photographic film is developed, silver bromide is reduced by hydroquinone (the developer) in a basic aqueous solution to give quinone and tiny black particles of silver metal ... 

Production of the film or plate. A thin layer of gelatin containing a colloidal dispersion of silver bromide is placed on a film (made of cellulose nitrate or cellulose acetate) or a glass plate. After drying, the film (or plate) is ready for use. 

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