pH meter · Calibration procedure
Temperature compensation
Checks that the temperature probe reads correctly and that the meter's automatic temperature compensation gives the right pH at a temperature other than 25 °C.
PURPOSE
Why this is checked
The electrode slope changes with temperature (59.16 mV/pH at 25 °C, about 60.15 mV/pH at 30 °C). If the temperature reading is wrong, the meter uses the wrong slope and reports the wrong pH, especially far from pH 7.
Frequency
Typically every 6 months or yearly, and when a new electrode or temperature probe is fitted, as defined in your calibration SOP.
Learn how the pH meter worksYOU WILL NEED
Standards & tools
- Calibrated reference thermometer (traceable certificate)
- Certified pH buffer with a temperature table on its certificate, e.g. pH 10.01 buffer
- Thermostatic water bath
BEFORE YOU START
Prerequisites
- Meter calibrated with buffers at 25 °C and ATC switched on
- Water bath set to two temperatures, e.g. about 20 °C and 30 °C
PROCEDURE
Step by step
- 1Place the ATC probe and the reference thermometer side by side in the water bath at about 20 °C. Record both readings when stable.
- 2Repeat at about 30 °C (or another temperature used by your methods).
- 3Place a beaker of pH 10.01 buffer in the bath at 20 °C, measure its pH with the electrode, and compare with the certificate value at 20 °C.
- 4Repeat with the buffer at 30 °C.
RECORD
Example observation table
| Bath (°C, reference) | ATC probe (°C) | Difference (°C) | Buffer certificate pH | Observed pH | Difference (pH) |
|---|---|---|---|---|---|
| 20.1 | 20.3 | +0.2 | 10.06 | 10.07 | +0.01 |
| 30.0 | 29.8 | −0.2 | 9.97 | 9.96 | −0.01 |
CALCULATION
How to calculate the result
Theoretical slope (mV/pH) = 0.19841 × (t + 273.15), where t is in °C
- At 25 °C: 0.19841 × 298.15 = 59.16 mV/pH
- At 30 °C: 0.19841 × 303.15 = 60.15 mV/pH
- Temperature difference at 30 °C = 29.8 − 30.0 = −0.2 °C
ACCEPTANCE CRITERIA
Pass if
- ATC probe within ±0.5 °C of the reference thermometer (or the limit in your SOP).
- Buffer reading within ±0.05 pH of the certificate value at that temperature.
IF IT FAILS
What to do
- Check the probe connection and that ATC is switched on; repeat.
- Apply a temperature offset only if the meter software allows it and your SOP permits.
- Otherwise replace the temperature probe or electrode and repeat the check.
MORE
Other pH meter calibrations
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.