Chemistry of Photography.
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Chemistry of Photography While it is easy to make comparisons between the pupil of the eye and the f-stop of a camera or between the retina of the eye and photographic film, once we get past the basic similarities of the optics of the two systems, comparisons begin to rapidly break down. The eye is not only much more complex than a camera and its film, but the two imaging devices function by different chemical mechanisms. The photographer (or the automatic exposure system of the camera) regulates the f-stop opening and time of exposure of her camera to match the sensitivity of film, while the iris and retina sensitivity of the eye adjust to correspond to the light level of the scene. While science is slowly putting together the pieces to explain the functioning of our vision system, the basic nuts and bolts of classical photography have been known for years, although certain details remain the subject of some discussion. Just as in the human eye, classical photographic systems are composed of two separate, but interrelated processes - the basic black and white image structure and the finer points of color reproduction. This first installment on the chemistry of photography is intended to introduce, in a simplified way, the basic concepts of silver halide photography. It will not delve into the physics of optics, the functioning of cameras and lenses, photographic techniques, non-silver processes, or the artistic aspects of photography. Nor will it go beyond a cursory mention of color photographic processes, which will be left for the future. Anyone interested in more detail is referred to the selected bibliographic material cited at the end. A Brief History of Black and White Silver Halide Photography Perhaps the earliest reference to the concept of silver-based black and white photography is that of J. H. Schulze who observed in 1727 that a mixture of silver nitrate and chalk darkened on exposure to light.
In the preparation of a photographic emulsion the gelatin also acts as an anti-coagulant or stabilizing colloid. The silver halide formed in fluid gelatin does not precipitate out of solution but remains uniformly distributed throughout the preparation, ripening (see below), and coating processes. The gelatin is an important factor in determining the dispersity or range of grain sizes of the silver halide. By suitable regulation of the concentration of the gel, the temperature, and the rate of addition of the components, the grain size distribution can be controlled to meet specific requirements. In the silver halide dispersion, gelatin molecules adsorb at the surface of the silver halide grain, surrounding the grain and forming a barrier that stabilizes the dispersion. The adsorbed layer also, in all likelihood, affects the radiation sensitivity of the grain and makes reduction by developers, more controllable. This is important in the development process and makes it possible to obtain desired results from a given system based on easily controlled parameters such as developer chemistry, development time, temperature, etc. Photographic Emulsion Preparation The exact methods used in preparing commercial photographic emulsions are closely guarded trade secrets, but the basic procedures are well known. There are two general classes of emulsions, the characteristics of which are determined by the end use. They are "negative" emulsions that are used for exposure in cameras and produce a reversed or negative image, and print images that produce the final photograph that we show off to our friends and relatives. Negative emulsions generally must exhibit a relatively wide flexibility in terms of sensitivity since they are used under conditions that are generally beyond the control of the casual photographer. It would be rather impractical if we couldn't take our vacation pictures in bright sunlight and shade with the same camera and film. The professional photographer has the option of having several thousands of dollars of cameras dangling around her neck to suit the conditions.
Fine-grain emulsions fix in less time than those of larger grains, and paper emulsions of silver chloride fix faster than bromo-iodide negative emulsions. Thickly coated films, other things being equal, fix more slowly than those with a thin emulsion coating. The fixing time increases appreciably as the solution becomes depleted. With continued use the halide-ion concentration rises in proportion to the amount of silver halide dissolved. When the product of the silver-ion and the halide-ion activities reaches the solubility product of the least soluble silver halide present, the solution will dissolve no more of that silver halide and fixation will necessarily be incomplete It is usually desirable to harden the gelatin after development, and while this may be accomplished by a hardening stop bath prior to fixing, the usual practice is to combine hardening with fixing. The conventional fixing and hardening bath contains in addition to the fixing agent: 1. An organic acid, usually acetic, to provide the necessary acidity to stop development and create the proper pH for effective hardening. 2. Sodium sulfite, which prevents the decomposition of the thiosulfate by the acid and forms colorless oxidation products of the developer thus preventing staining. 3. Alum as a hardening agent. The hardening produced by alum is due to the reaction of the aluminum ions, Al+3, and the carboxyl groups of the gelatin with the formation of cross-linkages between chain molecules. The degree of hardening, other things such as temperature, alkalinity of the film when placed in the fixing bath, etc., being equal, depends on the pH of the solution which in turn depends on the relative proportions of acid, sulfite and alum. Since the addition of developer tends to increase the pH of the fixing bath, the solution should be buffered against an increase in pH. For this reason weak organic acids, such as acetic acid, are used in preference to a stronger acid, such as sulfuric. The addition of boric acid increases the useful hardening life of potassium alum baths and reduces the tendency of the bath to form a sludge.
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