Chemical changes can also cause activity change, and changes in D.Min or D.Max, or if some steps have risen and others have fallen, these are characteristically due to changes in chemistry. A good knowledge of photographic processes and the action of each bath may help to track down the cause. However this is not normally the technique used to correct chemical problems.
Chemical control can be basic or advanced according to the equipment available as well as the skill and knowledge of the operator, large laboratories have elaborate systems primarily because they have more to lose if something goes wrong. Small laboratories may appear to take more risks by doing less but errors and drifts are simply more easily corrected. This book is not the place to look at the complexities of full-blooded chemical control in which each solution is analysed, replenisher formulae are subtly altered and tight control exercised. In the smaller laboratory this degree of detail is out of place and is replaced by two measurements that, properly evaluated can tell an experienced operator a great deal. These two simple values are Specific Gravity and pH, and they make it is possible to tell whether a solution is in or out tolerance, but not what is incorrect. The formula for each solution will give an aim figure for pH and Specific Gravity and a tolerance.
The SG reading indicates if the solution has the correct quantity of chemicals dissolved. If one chemical is left out of the solution a lower than aim SG reading is obtained.
The pH value of a solution is an indication of the solution's chemical activity, especially in the case of a developer, but details on these measurements are not appropriate here.
Both SG. and pH are also widely used as checks immediately after mixing a fresh process solution or replenisher to establish in a crude but effective manner whether all the components were remembered! The general principles of chemical control are not that of identifying a photographic problem and knowing how to adjust the chemistry to correct it. The variations possible are so great that it is rare for even the best chemist to be able to control a process that way. The practice is that each solution has a standard analysis formula; one that has been found produces the optimum result. The manufacturer defines most of these, and, in the case of colour processes, almost all are defined by Kodak. In the case of most developer solutions and developer replenishers the standard manufacturers formula needs a little adjustment to keep the correct sensitometry.
The job of the process chemist is then to carry out a regular routine analysis of the solutions and, when they depart from the standard concentrations, to adjust them back to those standards.