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TEMPERATURE CORRELATION
FOR pH SENSITIVE GLASS
The resistance of glass electrodes partially depends on the temperature.
The lower the temperature, the higher the resistance. It takes more time
for the reading to stabilize if the resistance is higher. In addition, the
response time will suffer to a greater degree at temperatures below 25 ºC.
Since the resistance of the pH electrode is in the range of 50 – 200
Mohm, 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
discharges 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 ºC Less than 4 months
120 ºC
Less than 1 month
Alkaline Error
High concentrations of sodium ions interfere with readings in alkaline
solutions. 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 Instruments’ glass
formulations have the indicated characteristics.
Sodium Ion Correction for the Glass at 20-25 °C
Concentration
pH
Error
0.1 Mol L
-1
Na
+
13.00
0.10
13.50
0.14
14.00
0.20
1.0 Mol L
-1
Na
+
12.50
0.10
13.00
0.18
13.50
0.29
14.00
0.40