Measuriing the pH , what
is pH ? , the pH scale , pH Meter , electrodes , buffer and
reagents , electrode care .
MEASURING
THE pH OF PHOTOGRAPHIC PROCESSING SOLUTIONS
A correct pH is important in
photographic processing, because the solutions must remain
within a specified pH range to produce good results. To use
some of the options in this information , you must be able
to make accurate pH readings.
What
is pH ?
pH is a measurement of the
acidity or alkalinity of a substance. It is an accepted and
common unit of measure just as a "meter" is a measure
of length, and a "liter" 'is a measure of fluid
volume. It is necessary because, just as it is not enough
to say that water is hot, it is also not enough to say that
lemon juice is an acid. We need to be specific. By saying
that water is at 91° C or 196 °F we convey exactly how hot
it is. The same is true with lemon juice. To say that it is
acidic is often not enough- to know that its pH is 2.3 tells
us the degree of acidity. So measuring acidity and alkalinity
is important, but how is pH related to these measurements-
The
pH Scale
There are hundreds of acids
-strong acids like sulfuric acid, which can dissolve steel
nails- and weak acids like boric acid, which is safe enough
to use as an eyewash. There are also many alkaline solutions,
called "bases" mild alkaline solutions like Milk-Of-Magnesia,
that soothe upset stomachs-and strong alkaline solutions like
lye and sodium hydroxide which dissolve human hair .
Acids and bases have a characteristic
that lets us tell them apart and that we can measure -the
concentration of hydrogen ions. Strong acids have high concentrations
of hydrogen ions- weak acids have low concentrations. pH then
is a numerical value that expresses the hydrogen ion concentration.
The real numerical values for
these hydrogen ion concentrations is typically a very small
fraction 1/10,000,000 , for example. Because that's an inconvenient
number to work with, a unique scale was devised. The scale
created uses the negative logarithm of the hydrogen ion concentration
(or activity) for acidic and basic solutions. Values read
from this scale are called "pH" measurements.
Numbers from 0 to 7 on the
scale indicate acid solutions , 7 to 14 indicate alkaline
solutions. The more acid a substance is, the closer its pH
will be to 0; the more alkaline a substance, the closer its
pH will be to 14. Some photographic solutions are neither
highly acidic or highly alkaline but are closer to the neutral
point, pH 7- the pH of tap water .Developer solutions measure
on the alkaline portion of the pH scale, typically ranging
from pH 9 to 12. Stop baths measure on the opposite end of
the scale because they contain large amounts of acid; they
typically have pH values of 1 to 3.
How
is pH Measured ?
A simple way to determine whether
a material is an acid or base is to use litmus paper .Litmus
paper is a treated paper strip that turns pink when dipped
in an acid solution, and blue when dipped into an alkaline
solution. Although other pH papers can now provide a more
accurate pH estimate, they are not accurate enough for measuring
photographic solutions, and they are not very useful for measuring
the pH of colored or turbid liquids.
The most accurate and commonly
used method for measuring pH is by using a pH meter and a
pair of electrodes. A pH meter is basically a very sensitive
voltmeter The electrodes connected to it will generate an
electrical current when they are immersed in solutions.
Two types of electrodes are
used to measure pH, and each electrode has a specific purpose.
The "glass" electrode has a bulb made of special
composition glass that is very selective and sensitive to
hydrogen ions. When this glass bulb is immersed in a solution,
the voltage generated at the bulbs surface is related to the
pH of the solution.
The other electrode is called
the "reference electrode," it provides a stable
and reproducible voltage when it is immersed in a solution.
When the two electrodes are connected to a pH meter, the voltage
difference is amplified and displayed on an analog or digital
meter. An electrode that combines both the pH-sensitive glass
bulb and a reference cell in one electrode body is called
a "combination electrode," it is used in the same
way as an electrode pair
For accuracy and consistency,
you must standardize the meter with solutions with known pH
values called "buffers." A buffer is a specially
prepared solution with two important qualities; it resists
changes in pH, and it has a specific pH value at a specific
temperature. For accurate and reliable pH readings, you must
also maintain and calibrate the pH meter and the electrodes
often. You must also measure the solutions at the correct
temperature and use the proper technique.
The
pH Meter
The pH meter must be capable
of two-point calibrations with either an adjustable slope
control or a readout of slope values. Readability to 0.01
pH unit and accuracy to 0.02 are required.
The Corning 250 pH meter, or
equivalent, is a satisfactory single channel meter This meter
accommodates one pair of electrodes. However, you can upgrade
it with accessories to accommodate multiple pairs of electrodes.
The Orion EA 940 pH meter, or equivalent, is a satisfactory
dual-channel meter Two pairs of electrodes can be attached
simultaneously.
Electrodes
For increased accuracy, use
a separate pair of electrodes or at least a separate glass
electrode for high and low pH measurements. Store electrodes
in the recommended solutions . Rinse and fill reference electrodes
with 3.5 M rather than saturated potassium chloride solution.
The lower salt concentration produces less crystallization
inside the electrodes and in the reference junction. The complex
composition of photoprocessing solutions can produce unwanted
effects on the glass membranes of pH electrodes. Use the following
reference and glass electrodes to avoid pH differences that
occur with reference elements and the pH-sensitive glass of
some manufacturers. Although electrodes that combine reference
and glass electrodes in a single probe can perform satisfactorily,
it is easier to maintain and troubleshoot a reference and
glass pH-electrode pair
- Recommended Reference Electrodes
- Beckman 38423 (formerly
39417), reference, ceramic junction, calomel
- Corning 476002, reference,
ceramic junction, calomel
- Corning 476029*, reference,
ceramic junction, Ag/AgCl
- Fisher 13-620-52, reference,
ceramic junction, calomel
- Fisher 13-620-258, reference,
ceramic junction, calomel
- Beckman 39421, reference,
reverse sleeve, Ag/AgCl
- Beckman 39420, reference,
reverse sleeve, calomel
- Corning 476360 (formerly
476171), reference, reverse sleeve
- Fisher 13-620-61, reference,
reverse sleeve, calomel
- Recommended Glass pH Electrodes
- Corning 476281, glass, rugged
bulb (US Standard Connector)
- Corning 476280, glass, rugged
bulb (BNC Connector)
- This electrode is suitable,
but has been discontinued by the manufacturer
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