Hanna Instruments HI 2550 - Manual (Page 55)

55
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’s glass
formulations have the indicated characteristics.
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.
TEMPERATURE CORRELATION TEMPERATURE CORRELATION TEMPERATURE CORRELATION TEMPERATURE CORRELATION TEMPERATURE CORRELATION
FOR FOR FOR FOR FOR pppppH SENSITIVE GLASS H SENSITIVE GLASS H SENSITIVE GLASS H SENSITIVE GLASS H SENSITIVE GLASS
1.0 Mol L -1 Na +
0.1 Mol L -1 Na +
Sodium Ion Correction for the Glass at 20-25 ºC
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