IM 12B03D02-01E
9-7
9
.
Technical Information
9.6 The Acid Error
The acid error also varies depending on the glass membrane compositions and
the types of acids. It increases gradually with immersion time and finally reaches
equilibrium. Once a glass electrode has the acid error, it cannot recover soon even by
being soaked in a neutral solution and needs considerable time. Practically, the acid
error is small compared to the alkaline error so that it is negligible.
pH
-1
Acid error (mV)
0
10
30
50
1
3
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2
Solution temperature
at 25°C
H
2
SO
4
H
3
PO
4
HCl
Figure 9.8 Acid Type vs Acid Error
9.7 Calibration Calculation
The PH71 meter is calibrated at 2 points using 2 standard solutions. The first
calibration is done so a certain line is drawn through the calibration point (Figure
9.9). The second calibration is done so a line is drawn through the first and second
calibration points (Figure 9.10). One-point calibration is a simplified method where
only the first calibration of 2-point calibration is performed.
Note:
A calibration point is the emf corresponding to the pH value of a standard solution used
(refer to Table 4.1).
pH0
pH14
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pH7, 0mV
+mV
-mV
Calibration line
Theoretical line
Calibration point
Figure 9.9 Calibration at First Point
