A "Primer" on Making one's own Black and White Chemistry?

Discussion in 'B&W: Film, Paper, Chemistry' started by MattKing, Jul 12, 2012.

  1. MattKing

    MattKing Subscriber

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    I found this thread on Photo.Net and found it quite clear and therefore potentially useful.

    http://photo.net/learn/darkroom/making-your-own-black-and-white-chemistry/

    Any thoughts about its accuracy from those here who know far more about this than I do?

    I don't often look to Photo.Net any more for information, but it was the resource that led me to APUG, so I tend to cut it at least a little slack :smile:.
     
  2. Gerald C Koch

    Gerald C Koch Member

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    My only comment is that the author really doesn't understand what reduction or a reducing agent is. Consider a silver chloride molecule, the silver is said to be reduced because its oxidation state is reduced from silver[+1] to metallic silver[0]. Reduction is the gain of electrons and oxidation is the loss of electrons. He also defines the activity of a reducing agent with how fast it reduces silver halide. This is not correct since he confuses kinetics with the redox potential.

    His description of what happens to a silver halide crystal when it is exposed to light is also not completely correct. He says that the grain becomes excited which is only partially true. The entropy of the grain does increase. What really happens is that a defect in the crystal lattice is created. A silver grain is a three dimensional matrix of alternating silver and halide ions. There are several types of defects that can be created. An easy way to visualize a defect in two dimensions is to think of a three dozen size box of eggs. The box contains six rows each containing six eggs. Half the eggs are brown and half are white. If we ignore eggs in the outside rows, each white egg is surrounded by four brown eggs and each brown egg is surrounded by four white eggs. One defect would be a missing egg leaving a hole. Another defect would be two brown eggs or two white eggs in adjacent sites. There are other defects which will not be discussed here. The defect creates what is known as an activation site. When the grain is exposed to the reducing agent the activation sites are attacked first. A useful developing agent is capable of discriminating between an activation site and the rest of the silver grain. This allows an image to be formed rather than the entire emulsion being reduced to metallic silver.

    The site is useful but not for its theory.
     
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