Chemistry in Photography
A photograph is an image made by a photo-chemical reaction which records the impression of light on a surface coated with silver atoms. The reaction is possible due to the light-sensitive properties of silver halide crystals. Equation form for silver halides: Ag + + e - Ag
Species produced include: Ag2+, Ag2o, Ag3+, Ag3o, Ag4+, Ag4o In 1556, the alchemist Fabricius was the first to discover that light can photochemically react with these crystals to change the silver ions (Ag+) to elemental silver (AgO). As the reaction proceeds, the silver atoms grow into clusters, which are large enough to scatter light and produce colors in a pattern identical to that of the original light source. Photography utilizes this chemical principle to record color or black and white images. One of the first researchers to produce photographic images using silver halide chemistry was Schultze. As early as 1727, he formed metallic silver images by first reacting solutions of silver nitrate and white chalk and then exposing these solutions to light through stencils. Schultze's work was improved upon through the efforts of Louis Jacques Mandé Daguerre who, in 1837, developed a process for printing images on a silver coated copper plate. This type of printed image is called a daguerreotype, and is made by polishing and cleaning a silver-coated copper plate and then reacting the silver coating with iodine vapors to form light-sensitive silver iodide. The silver iodide coated plate is then exposed to light through the optics of a camera that projects and focuses an image on the plate. In the ensuing reaction, the silver ions are reduced to silver metal. Finally, the plate is treated with mercury to produce an amalgam. In this type of print, the areas of the plate exposed to light appear white and the unexposed areas remain dark. The problem with this method was that it required long exposure times because the intensity of the image depends solely on the strength of the light forming the image.
An example of a daguerreotype photograph
In 1841, William Henry Fox Talbot overcame the problem encountered by Daguerre by developing a quicker method that did not depend entirely on reflected light to produce the image. He found that silver halide could be exposed in such a way so as to produce a preliminary latent image which required only a small amount of light. This latent image could then be subsequently reacted, without additional light, to produce a final image. Using this technique, known as calotyping, Talbot was one of the first to produce continuous tone images. Unfortunately, these early images were not stable and darkened over time. Fortunately, around the same time Talbot did his work, John Frederick William Herschel discovered a way to stabilize images. His process, known as fixation, chemically converts unexposed silver halide to silver thiosulfate, which can easily be washed off of the image. Equation Form: 2AgBr + 3 Na2S2O3 = Ag2Na4(S2O3)3 + 2 NaBr
The next major advance in photography came with the discovery that certain materials could enhance the sensitivity with which latent images are formed. This enhancement is achieved by coating the silver halide crystals with chemical agents, such as sulfur and gold, which increase the light sensitivity of crystals. Gelatin, which for years had been used as a photographic coating agent, was found to be an effective medium for these light-sensitive materials. Equation form: AgNO3 + KBr = AgBr + KNO3 in gelatin * AgBr precipitates due to light sensitivity and remain in the gelatin to form minute grains. The formed latent image can then be developed further. In 1888, George Eastman, who pioneered modern film development, coated gelatin-dispersed silver halide crystals onto celluloid sheets. By the next year, Eastman had commercially sold rolls of films prepared by dissolving nitrocellulose with camphor and amyl acetate in a solution...
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