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9 REFERENCE
Electrode cleaning
This system adopts bead cleaning to maintain the electrode surface and measurement stable.
9.6
pH Measurement
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.
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
solutions, 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.
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
supposed 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 potential
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.
Temperature compensation
The electromotive force generated by the glass electrode changes depending on the
temperature 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
compensation. This is often misunderstood. When pH is to be measured, the temperature of
the solution 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.
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