pH meter · Calibration procedure
Buffer calibration & slope
Calibrates the meter with two or three certified buffers, then checks the electrode slope, offset and the reading of a check buffer.
PURPOSE
Why this is checked
The electrode's response changes from day to day. Calibration sets the meter's offset and slope so that it converts mV to pH correctly, and the slope tells you whether the electrode is still healthy.
Frequency
Every day before use (and whenever the sample pH is outside the calibrated range), as defined in your SOP.
Learn how the pH meter worksYOU WILL NEED
Standards & tools
- Certified pH buffers traceable to NIST or equivalent, e.g. pH 4.01, 7.00 and 10.01 at 25 °C
- A separate check buffer, e.g. pH 9.18 (borate), within the calibrated range
BEFORE YOU START
Prerequisites
- Electrode filled with electrolyte, refill hole open
- Buffers in date and at 25 ± 2 °C
- Fresh portions of buffer in clean beakers
PROCEDURE
Step by step
- 1Rinse the electrode with purified water and blot dry.
- 2Immerse in pH 7.00 buffer with gentle stirring. When stable, confirm the calibration point. Record mV and temperature.
- 3Rinse, then calibrate with pH 4.01 buffer. Record mV.
- 4Rinse, then calibrate with pH 10.01 buffer if the samples are alkaline. Record mV.
- 5Note the slope (%) and offset (mV) reported by the meter.
- 6Rinse and measure the check buffer as a sample. Record the pH reading.
RECORD
Example observation table
| Buffer (pH at 25 °C) | Temperature (°C) | Electrode response (mV) |
|---|---|---|
| 7.00 | 25.0 | −2.1 |
| 4.01 | 25.0 | +171.5 |
| 10.01 | 25.0 | −175.0 |
| Check buffer 9.18 | 25.0 | Reads pH 9.20 |
CALCULATION
How to calculate the result
Slope (mV/pH) = (E1 − E2) ÷ (pH1 − pH2); Slope % = Slope ÷ (−59.16) × 100; Offset = mV in pH 7.00 buffer
- Between pH 4.01 and 7.00: (171.5 − (−2.1)) ÷ (4.01 − 7.00) = 173.6 ÷ (−2.99) = −58.06 mV/pH
- Slope % = −58.06 ÷ −59.16 × 100 = 98.1%
- Between pH 7.00 and 10.01: (−2.1 − (−175.0)) ÷ (7.00 − 10.01) = 172.9 ÷ (−3.01) = −57.44 mV/pH = 97.1%
- Offset = −2.1 mV; check buffer error = 9.20 − 9.18 = +0.02 pH
ACCEPTANCE CRITERIA
Pass if
- Slope typically 95–105% of the theoretical value.
- Offset typically within ±30 mV (some SOPs use ±15 mV).
- Check buffer reading typically within ±0.05 pH of its certified value at the measured temperature.
IF IT FAILS
What to do
- Repeat with fresh buffers from new bottles.
- Clean and rehydrate the electrode, replace the electrolyte, and recalibrate.
- If it still fails, replace the electrode and record the change. Do not measure samples until it passes.
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.