(2) Importance of checking alkalinity. The silver reduction ability
of a color developer depends on its alkaline state.
This state of
alkalinity decreases during development of the emulsion as bromide ions
diffuse into the developing solution.
(a) To clarify this further, silver bromide is the light-sensitive
ingredient in the photographic emulsion. During development, silver bromide
diffuses into the developing solution and couples with potassium bromide in
This coupling action causes a build-up of bromide in the
developer, therefore, lowering the alkalinity and decreasing the developing
(b) Monitor the pH of a solution closely to ensure that the
solution is within the set standards, since pH will remain within standards
only if proper replenishment of the solutions is maintained.
(3) Monitor the processing solutions. All other processing solutions
must be maintained at their proper pH to properly react with the emulsion.
For example, the developing stopping action of a stop bath depends upon its
Without replenishment, the pH of the stop bath changes to
neutral and thereby loses its ability to stop development.
(4) Methods of measuring pH.
measuring pH: a pH meter or litmus paper.
(a) When litmus paper is used and placed in an acid solution it
will change color from white to red. When placed in an alkaline solution it
will change to blue.
The degree of red or blue indicates the acidity or
alkalinity of the solution.
(b) A pH meter is a much more accurate method of measuring pH.
The meter's scale indicates a pH of 0 through 14 with 0 being the most acid
state and 14 being the most alkaline state. There are various types of pH
meters used in photographic laboratories. Different procedures are used in
obtaining the readings but the end result is the same. We will not discuss
the exact procedures in this course.
Figure 2-8 indicates a sample of pH
readings and a pH scale.
c. Sensitometry. Sensitometry is a way of putting photographic theory
to work to improve photographs while saving time, effort, and materials.
Being closely related, the two terms are often used together.
Sensitometry is the science of determining the photographic