Chemistry used in Photo Development

<p>Hello! I am doing a project for my summer chemistry class on the chemicals used in photo development. However, I know absolutely nothing about the subject! If anyone has any information on the names of chemicals used to develop black and white photos, PLEASE let me know asap on this thread! Thanks so so so so much!</p>

<p>google and the search function on this site are your friends;</p>

<p>Or if you're really adventurous, click on the right side on the green plus sign under the "Older posts by category" heading after the list unfolds, choose developers.<br>

With the the names you find there you can search on for the chemicals used in them</p>

<p>Rebecca, buy this book: <a href="http://www.amazon.com/gp/product/0240810554?ie=UTF8&tag=credify&linkCode=as2&camp=1789&creative=390957&creativeASIN=0240810554">The Darkroom Cookbook, Third Edition</a> . Your project is almost done for you. It will tell you all the chemicals that can be used and why you would want to use them. It's like the Cliff-note version of photographic chemistry.</p>

<p>It's a complicated topic, but either The Darkroom Cookbook or The Film Developing Cookbook are usable summaries. Try your local library before buying.</p>

<p>Check out the Ilford and Kodak sights they have it all and in Englsh and other languages you may need.</p>

<p >The black-and-white film is a light-sensitive mixture coated on a base material that is clear and flexible. In the past the film base was cellulous nitrate, a highly flammable material often called “gun cotton” because it was an ingredient used in munitions. Fires were frequent. Next film base was made from cellulous acetate. This material was called “safety film” because of its low flammability. Mylar plastic and cellulous acetate dominate today’s films. </p>

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<p >The light sensitive substances are silver combined with one of three halogens. Halogens are “salt makers”. Silver iodide has the lowest sensitivity followed by silver chlorine with silver bromine having the highest. Films and papers are made using one or more silver salts. Very pure silver salts have low sensitivity to light. Inducing an impurity, a process called doping, yields a remarkable increase in sensitivity. Doping agents are gold and cadmium and other metals. </p>

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<p >Silver salts are held onto the film base using glue made from purified gelatin. Gelatin holds the sliver salts in a colloidal suspension that was misidentified as an emulsion. The wrongly applied name emulsion is too engrained in our vernacular to correct. Gelatin is used because it is flexible, transparent, has low solubility and swells when wet. This swelling allows fluids to percolate in and through the structure allowing chemicals to contact the silver salts. Gelatin shrinks when dry thus it has dimensional stability. </p>

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<p >When light plays on silver salts the surface of the crystal alters slightly. These surface altered spots are called fog centers. A developer is a solution that has the ability to recognize the difference between exposed and unexposed silver salts. The developer, having identified exposed silver salts reduces them to their two component parts. The silver component is reduced to metallic silver and remains imbedded in the gelatin. The halogen component is soluble in water and dissolves away into the waters of the developer solution.</p>

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<p >The developer solution has 5 major ingredients.</p>

<ol type="1">

<li >Solvent</li>

<li >Developing agent</li>

<li >Alkali accelerator</li>

<li >Preservatives to reduce oxidation of the developing agent</li>

<li >Retarder </li>

</ol>

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<p >The solvent is water. The developing agent is a derivative of benzene. The accelerator sets the pH of the developing solution. Development requires an alkaline environment pH 10 ~ pH 12. The accelerating agents are borax, potassium carbonate, sodium hydroxide. The preservatives reduce aerial oxidation as these are staining agents. The preservative also neutralizes staining agents. The most common one is sodium sulfite. The retarder is potassium bromide.</p>

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<p >After the developing step the film contains patches of metallic silver, this is the image. Also imbedded in the film, unexposed and thus undeveloped silver salts.</p>

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<p >At some point in the process the developing action of the developing solution must be arrested. To stop development the film is immersed in a stop bath solution. The stop bath is a mild acid usually derived from the over fermentation of grapes. This is acetic acid or common vinegar. </p>

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<p >These silver salts must be removed or they will self reduce in time and darken. This effect will ruin the film causing the image to fade. Next the film is immersed in a fixing bath. This bath is a solvent for silver salts but not a solvent for metallic silver. Two fixing solutions are in common usage, sodium thiosulfate and a rapid acting ammonium thiosulfate. Often called hypo a name derived from hyposulfite of soda. The fixing bath renders the image permanent. </p>

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<p >After fixing, the material must be copiously washed with running water to remove residual chemicals. Primarily because the fixer contains sulfur which, if allowed to remain in contact with the photo material, will attack the metallic silver and tarnish it. The result is a yellow and brown stain. </p>

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<p >This is a thumbnail synopsis. Need more? Ask.</p>

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