sampling point, so successive analyses will indicate
variations in the chemical composition.
Before sampling large batches of newly mixed
photographic solutions, allow sufficient time for all
the chemicals to dissolve properly, generally about 30
minutes to an hour.
You should draw a sample 1 inch below the
surface of the solution with a pipet. In general, a
sample bottle should not be shaken, and it should be
allowed to stand for 10 minutes after the sample is
taken from the processing solution. This wait allows
large particles to settle or turbidity (caused by
aeration) to clear.
Chemical Certification
The chemical certification of processing solutions
seldom requires a complete chemical analysis.
Ordinarily, determining the pH and the specific
gravity of the solution is enough, particularly when
these tests are followed up with valid sensitometric
tests.
CERTIFYING THE pH OF SOLUTIONS.--
pH
is one of the first tests made of a photographic
solution. The pH of a solution is the negative
logarithm of the hydrogen ion concentration. The
electrical potential between the glass electrode of a pH
meter and the solution surrounding it indicates the
hydrogen ion concentration.
The pH value of a solution changes with
temperature; therefore, to obtain a reproducible pH
value, you must standardize the temperature.
Generally, you make pH measurements at the same
temperature that the photographic process is operated.
A change of 10F produces a pH change of
approximately 0.10 in a processing solution at a pH of
10.5 and a change of approximately 0.20 at a pH of
13.0. Thus temperature control is more important at
higher pH values. To properly use, calibrate, and
adjust the meter consult the instruction manual or the
particular pH meter you are using.
When certifying the pH of a solution, you can
take the pH reading of only one sample or you can
read several samples. Of course, the more samples
you read, the more reliable your certification.
Usually, you must determine the pH value of more
than one sample. After four samples have been read,
the meter should be cross-checked, then standardized.
Also, remember that no more than 15 minutes should
elapse between any pH meter standardization or
cross-check.
When using the multiple sample procedure, take
the pH for each sample and average the values.
Check the average value against the standard pH for
that solution. When the pH is within limits, enter the
value in the appropriate location on the certification
sheet. When the pH is not within limits, further
investigation is necessary.
CERTIFYING THE SPECIFIC GRAVITY OF
SOLUTIONS.--
Many applications of specific gravity
are used in certifying a solution; for example, the
actual dilution of a developer is obtained by
comparing a fresh developer to a used one. Another
application may be to compare a fresh fixing bath to
a used one. Since the silver content of a fixer
increases with use, it is logical to assume that a
change in specific gravity will occur. Through
experience, you will be able to establish upper- and
lower-control limits for specific gravity of the various
solutions needing such a check.
Specific gravity tolerances are provided by the
manufacturer of photographic processing solutions.
The specific gravity must be taken at the temperature
recommended by the manufacturer, because
temperature affects the specific gravity of a solution.
Specific gravity standards for black-and-white
processing solutions are set at 70F, because many of
these solutions are used in 70F surroundings.
By themselves, the specific gravity readings you
take are not enough to tell what is wrong. They
simply indicate a change. So, further testing is
required. When a developer is being replenished
properly, specific gravity remains constant. If the
flow of replenisher stops, a change of specific gravity
will become evident. Replenisher flowmeters should
indicate such a problem, but a backup check of
specific gravity and pH, along with control strips, are
recommended as well.
SENSITOMETRIC CERTIFICATION OF
SOLUTIONS.--
Once the developer has been mixed,
a sensitometric strip should be processed to check the
developer solution. Assuming the pH and specific
gravity measurements of the developer are within
tolerance, this test can certainly validate the activity of
the developer.
2-22

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