Mixing Chemicals
When preparing solutions, the components must be dissolved in the correct order to avoid undesirable reactions and to allow complete mixing.
When making solutions from packaged preparations, the instructions provided with the package should be followed. For preparations supplied in powder form, the entire contents of a container should be used to prepare a solution, and no attempt should be made to prepare a small amount of solution using only a portion of the chemicals. Otherwise, there may be considerable difficulty in obtaining solutions with uniform characteristics due to possible segregation of the granular components during transportation and handling.
Proper agitation during mixing is important, not only to increase the rate of solution of the chemicals, but also to avoid undesirable effects. Developers are particularly prone to oxidation; a few minutes of vigorous agitation can noticeably weaken the developer and produce staining compounds. On the other hand, insufficient agitation can cause the chemicals to settle to the bottom of the mixing tank and form a hard cake that does not dissolve easily.
Measuring small quantities
The accuracy required in measuring small quantities of chemicals for photographic formulas may exceed the accuracy of the equipment found in some photographic laboratories. So, if you intend to mix your own photographic chemicals from based on formulas, be sure that your balance can weigh quantities up to 100 grams with an accuracy of 0.5 grams and that you have a small syringe or pipette capable of measuring liquid to within 0.5 milliliter.
When tiny amounts of a dry chemical are needed, it is often more practical to mix a dilute stock solution of the chemical and use an appropriate amount of that solution instead. To prepare a 10 percent stock solution, mix 100 grams of the dry chemical with a small amount of water and then add water to make 1 liter. Each milliliter of this solution will then contain 0.1 gram of the solid chemical.
Cleanliness
Contamination of solutions during mixing is a common cause of unusable negatives and prints. All mixing containers, pipettes, spoons, etc., should be thoroughly cleaned immediately after use to prevent the formation of incrustations that can dissolve when a new solution is mixed.
It is best to use a separate mixing container for each solution. If several solutions are mixed consecutively in the same container, they should be prepared in the order in which they are processed. Traces of developer in a fixer have little or no effect, but small amounts of hypo in a developer can cause serious gray haze or image tone changes.
Chemicals, especially those in the form of fine powder, should not be mixed in the darkroom itself or in places where people eat. Chemical dust can become airborne and settle on surfaces. As a result, stains and spots may appear on prints or negatives. Chemical dust can also settle on the surfaces of other process solutions and cause contamination. For this reason, solution tanks should be equipped with dust covers. This is a clear processing advantage of liquid products.