NAVI F-52, Инструкция по эксплуатации

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Chapter 16 Reference
16.1 pH measurement
216 HORIBA
16.1 pH measurement
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pH measurement and temperature
The temperature of the solution being inspected is an important parameter in the accurate
measurement of pH. There are many possible sources of errors during measurement, such as
the state of the solution junction potential, asymmetric potential, and standard solution pH
concentration, but all of these items contain factors that change with the temperature. The best
way to minimize these potential causes of errors is to keep the temperature of the pH standard
solution uniform at the time of calibration.
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Liquid junction potential
Liquid junction potential is the electric potential that occurs to a greater or lesser degree at
the liquid junction. The size of the electric potential differs depending on the type of solution,
temperature of the solution, and the structure of the liquid junction.
When solutions of different compositions come in contact, ion diffusion occurs on the contact
surface between the two solutions. The ions are of various sizes, so a difference occurs in the
diffusion transfer speed.
As diffusion proceeds, a difference in charges occurs on the contact surface of the two solu-
tions, giving rise to a difference in potential. This potential works to reduce the transfer speed
of fast ions and increase the speed of slow ions, ultimately achieving a state of equilibrium
when the transfer speed of the positive and negative ions on the contact surface of the two
solutions is equal. In this state of equilibrium, the potential at the contact surface between the
two solutions is called the
liquid junction potential. A large liquid junction potential
means very inaccurate measurement.
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Asymmetric potential
The glass electrode is immersed in a pH 7 standard solution. When the electrode is immersed
in the pH 7 solution, both the internal and external sides of the electrode membrane are sup-
posed to take on a pH of 7, making the potential 0. In actuality, however, a potential does
occur. This potential is called an
asymmetric potential. The size of an asymmetric poten-
tial differs depending on any stress that may have occurred during the processing of the glass
and the shape and compositions of the glass. Asymmetric potential also changes depending on
the degree of contamination of the standard solution and the state of the glass membrane.
Also, if the electrode membrane dries out, a large asymmetric potential will occur, giving rise
to measurement errors.
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Temperature compensation
The electromotive force generated by the glass electrode changes depending on the tempera-
ture of the solution.
Temperature compensation is what is used to compensate for the
change in electromotive forces caused by temperature. There is absolutely no relation
between the change in pH caused by the temperature of the solution and temperature compen-
sation. This is often misunderstood. When pH is to be measured, the temperature of the solu-
tion when the pH is measured must be recorded along with that pH value, even if a meter that
has automatic temperature compensation is used. If the solution temperature is not recorded,
the results of the pH measurement are relatively meaningless.