Yokogawa PH72 Personal pH/ORP Meter - Manual (Page 73)

IM 12B03D02-01E 9-6
9. Technical Information
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.
Alkaline error
Acid error
EMF at pH 7
Theoretical value
Slope difference
0
2
E
(mV)
1 4 3 5
7
9 8 11 6 10 12 pH
pHx
F0905.EPS
Figure 9.5 EMF Characteristics of Glass Electrode
Test solution pH
9
0
20
40
60
A
lka
line e
rr
o
r
(mV)
10 11 13 14
F0906.EPS
12
Solution temperature
at 25
؇
C
3mol/l-Na
+
3mol/l-K
+
3mol/l-NH
4
+
Figure 9.6 Ion Type vs Alkaline Error