Spring 2001 12 Chemistry of Photography Pre-lab assignment: Collect objects suitable as “subjects” for your photography experiments. The subjects must lie flat on a 3x 5-light…

Chemical Reactions Involved in Photographic Processes. A. Silver-based photographic processes . Capturing light to produce an image utilizes two properties of the silver cation: (1) Ag+ is reduced to silver metal in the presence of a halide which can be oxidized photochemically (i.e., a photon ejects an electron from the halide). (2) Although the halide salts of silver, AgX, have very low aqueous solubility, many complex ions of Ag+ (such as that formed with hyposulfite) do dissolve in water. Modern silver-based photography relies on oxidation-reduction chemistry to capture the image. The media-specific solubility of silver halide salts make the initial image permanent. The key reactions are outlined below: 1. Forming the image by exposure to light (h ?) _: A very small number of X- ions in the AgX crystals in the film are oxidized to X. The electrons released from this oxidation reduce the Ag+ to silver metal in the surrounding AgX crystal. 2. Development. The small number of Ag metal atoms formed (the latent image ) act as a catalyst and sensitizes the surrounding halide salt so that, in the presence of a developer–a reducing agent–the sensitized AgX is reduced, to produce black silver metal in the area exposed to light. Modern developers contain one of many reducing agents for this process. The most common is hydroquinone, which reacts with Ag+ (in AgX), as shown in equation 2a:… Note that the reaction of (2a) occurs in basic medium (OH-). The development can be stopped, therefore, by dipping the photographic film in acid. The most common “stopper” contains acetic acid. In this experiment, we will remove excess reagents by washing the exposed “film” in water before fixing the image. B. Cyanotypes. The blue photographs on formation of insoluble Prussian Blue through photoreduction of Fe(III) to Fe(II) are called cyanotypes. Toning reactions may alter the color of the initial image. In this lab, one or more toning treatments as well as the wavelength dependence of the iron photoreduction will be explored. Photoreaction and formation of insoluble Prussian Blue. Experimental Procedures. General Directions . All the experiments involve coating papers or cloth with photosensitive reagents or using prepared photosensitive paper. Consequently, these materials should not be exposed to strong light except when the desired photographs are made. Taking a photograph” in this lab consists of placing the “subject, ” one of the objects selected by the student, on top of the photosensitive paper or cloth, covering the object with glass to hold it in place, and exposing this package to light. In some cases, a filter will be placed between the light source and the paper to evaluate the wavelength dependence of the photographic process. Development of the photographs consists of dipping the exposed papers in aqueous solutions and washing with water…

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