Hanna Instruments HI 221 - Manual (Page 35)

35 4
1.0 Mol L -1 Na +
0.1 Mol L -1 Na +
Sodium Ion Correction for the Glass at 20-25 °C
Concentration pH Error
13.00
13.50
14.00
12.50
13.00
13.50
14.00
0.10
0.14
0.20
0.10
0.18
0.29
0.40
Since the resistance of the pH electrode is in the range of 50-200 Mohm
depending on the composition of the glass, the current across the membrane
is in the pico Ampere range. Large currents can disturb the calibration of the
electrode for many hours.
For these reasons high humidity environments, short circuits and static dis-
charges are detrimental to a stable pH reading.
The pH electrode’s life also depends on the temperature. If constantly used at
high temperatures, the electrode life is drastically reduced.
Typical Electrode Life
Ambient Temperature 1- 3 years
90 °C Less than 4 months
120 °C Less than 1 month
Alkaline Error
High concentrations of sodium ions interfere with readings in alkaline solu-
tions. The pH at which the interference starts to be significant depends upon
the composition of the glass. This interference is called alkaline error and causes
the pH to be underestimated. Hanna’s glass formulations have the indicated
characteristics.