Hanna Instruments HI 120 - Manual (Page 51)

51
Typical Electrode Life
Ambient Temperature 1 – 3 years
90 ºC (194 ºF) Less than 4 months
120 ºC (248 ºF) 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’s glass formulations have
the indicated characteristics.
1.0 Mol L -1 Na +
0.1 Mol L -1 Na +
Sodium Ion Correction for the Glass at 20-25 ºC (68-77 ºF)
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
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 (77 ºF).
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.
TEMPERATURE CORRELATION
FOR pH SENSITIVE GLASS