LESSON 3
standard referenced solution (commonly called a buffer) to standardize the
pH meter and the other to obtain the pH value of the test sample, relative
to the standard.
(1) Buffer solution. A buffer solution is used to standardize the pH
meter to a known value.
Normally, three different pH solutions, having
values of 4, 7, and 9 are used for this purpose. The exact pH value of a
buffer is listed on its label. If a buffer is used at temperatures other
than those recommended by the manufacturer, a temperature compensation table
should be consulted. To assure accuracy, always select a buffer which most
closely approximates the pH value of the solution being tested.
For
example, when testing a developer solution, use a buffer with a pH value of
9, while for a fixing bath use a buffer with a pH value of 4. When testing
a solution with an unknown value, use a buffer with a value of 7 (neutral).
(2) Litmus paper. Litmus paper is used to determine the acidity or
alkalinity of an unknown solution.
If the solution contains acid, the
litmus paper turns red.
Blue litmus paper indicates that a solution is
alkaline. Certain types of litmus paper respond in various shades of red or
blue, depending on the amount of acid or alkali contained in the solution.
This paper is matched to a reference scale to determine the actual pH value
of the solution. Once this test has been completed and the identity of the
solution known, an appropriate buffer is used to standardize the pH meter.
5. The pH
meter.
Measurement of pH by a meter is accomplished by
determining
the potential developed by an electrical cell.
This cell
consists of
two electrodes, a glass electrode and a reference electrode,
immersed in
a test solution.
a. The purpose of the reference electrode is to provide a constant
reference voltage to permit measurement of the potential of the glass
electrode. The reference electrode is filled with a saturated solution of
potassium chloride (KC1). The constant voltage is supplied by this KC1. A
small, but constant flow of KC1 solution is maintained through a liquid
junction in the tip of the reference electrode. The KC1 solution forms a
conductive salt bridge to the sample solution, between the two electrodes.
b. The basic purpose of the glass electrode is to measure the hydrogen
ion concentration of the sample. The electrical potential developed at the
glass electrode is proportional to the pH of the solution. The measurement
of the electrical potential, developed at the glass electrode, is
accomplished with the pH meter. The potential may be read directly either
in pH units or millivolts.
c. Some pH meters are equipped with a combination electrode which
incorporates both a pH-sensing element (glass measuring
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