Chapter 7 Reference
7.1 pH measurement
F-51
89
Chapter 7
Reference
This chapter provides a simple compilation of information for those who
would like to know about the functions of the main unit of the pH meter
and other measurement principles in greater detail.
It also serves as a reference for spare and optional parts.
7.1
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 reference solution pH
concentration, but all of these items contain factors that change with
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, the 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
measurements will be very inaccurate.
●
Asymmetric potential
The glass electrode is immersed in a pH 7 reference 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
asymmetric potential. The size of the
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
