YOKOGAWA PH72, Руководство пользователя

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IM 12B03D02-01E
9-5
9 . Technical Information
Table 9.2 Deviations from True Values in Measurement without Temperature
Compensation
Temp.(°C)
pH
0 20 25 40 60 80
1 0.50 0.10 0.00 -0.30 -0.70 -1.11
3 0.34 0.07 0.00 -0.20 -0.47 -0.74
5 0.17 0.03 0.00 -0.10 -0.23 -0.37
7 0.00 0.00 0.00 0.00 0.00 0.00
9 -0.17 -0.03 0.00 0.10 0.23 0.37
11 -0.34 -0.07 0.00 0.20 0.47 0.74
13 -0.50 -0.10 0.00 0.30 0.70 1.11
In addition, the pH value of a solution changes with temperature. The pH value of
a solution at the actual temperature may be converted into the one at a reference
temperature. This is generally called “conversion to reference temperature,” which is
different from the temperature compensation.
9.4 The Asymmetry Potential
Theoretically when identical buffer solutions (pH
i
= pH
S
) are present on both sides
of the membrane of a glass electrode, the emf should be 0 mV. In reality, some
potentials (C
S
– C
i
) develop depending on the thickness of the glass membrane, heat
treatment process, service history, or other factors. This is called the real asymmetry
potential. In addition to this potential, the difference in single electrode potential
between the inner electrodes of the glass electrode and of the reference electrode
and a liquid junction potential* are collectively referred to as the apparent asymmetry
potential or just the asymmetry potential. This asymmetry potential is E
AS
in equation
9.4.
* Liquid junction potential occurs due to dirt or clogging of the liquid junction or other factors.
9.5 The Alkaline Error
As shown in Figure 9.5, the emf of a glass electrode deviates from the linear value on
the alkaline side. This is called the alkaline error. The magnitude of the alkaline error
varies depending on the glass membrane compositions. The alkaline error is likely to
occur with the presence of sodium and lithium and even with the same pH, it varies
depending on the types and concentrations of cations and on the temperature.