pH calibration
pH is not measured directly: a glass electrode produces a voltage proportional to pH, and a buffer set fixes the slope and offset. Fresh buffers, temperature and a clean electrode make most of the accuracy.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Glass combination electrode with meter | 0 to 14 pH | ±0.01 to ±0.02 pH after calibration | Laboratory work |
| Process pH sensor and transmitter | 0 to 14 pH | ±0.05 to ±0.1 pH | Water treatment, food, chemical plants |
| ISFET sensor | 0 to 14 pH | ±0.05 to ±0.1 pH | Rugged field and food use |
| Reference electrode and potentiometer | pH 1 to 13 | ±0.003 to ±0.01 pH | Standards laboratories |
| Indicator paper or strip | 0 to 14 pH | ±0.5 to ±1 pH | Quick screening |
The meter resolution (0.001 to 0.01 pH) is finer than the real accuracy. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
Ideal electrode slope by temperature
| Temperature | Slope (mV per pH) |
|---|---|
| 0 °C | 54.20 |
| 10 °C | 56.18 |
| 20 °C | 58.17 |
| 25 °C | 59.16 |
| 30 °C | 60.15 |
| 37 °C | 61.54 |
| 50 °C | 64.12 |
Nernst slope 2.303 R T / F. Real electrodes reach 95 to 100 % of it when new.
Reference standards and traceability
- Primary buffers. NIST and IUPAC standard buffers: potassium hydrogen phthalate 4.005, phosphate 6.865, borax 9.180 at 25 °C, with tetroxalate 1.679, carbonate 10.012 and calcium hydroxide 12.45 for the ends of the scale.
- Technical buffers. Commercial buffers at 4.01, 7.00 and 10.01 (25 °C) with a certificate, traceable to the primary buffers.
- Method. IUPAC recommendations, ASTM D1293 and ISO 10523 describe measurement in water.
- Quality system. ISO/IEC 17025, scope listing pH with range and uncertainty.
How it is calibrated
Typical procedure
- Use fresh buffers at the same temperature as the sample, and never pour them back into the bottle.
- Rinse the electrode with deionised water and blot it dry (do not rub).
- Calibrate with two or three buffers that bracket the sample pH, starting at 7.
- Wait for a stable reading (drift criterion) and stir at a constant rate.
- Read the slope and the offset: slope 95 to 105 % and offset within about ±30 mV are common limits.
- Check with a third buffer or a control standard and record the result.
Where it goes wrong
- Old or contaminated buffers. Alkaline buffers absorb CO₂ from the air and drift.
- Dry or aged glass. A bulb that has dried out responds slowly and with a lower slope.
- Clogged junction. A blocked reference junction causes noise and drift.
- Temperature. Both the slope and the sample pH change with temperature.
- Low ionic strength samples. Pure water gives unstable pH readings.
What goes in the uncertainty budget
- Buffer value uncertainty and its temperature dependence.
- Electrode drift, response time and junction potential.
- Temperature measurement and compensation.
- Stirring, carryover and sample effects.
- Meter resolution and repeatability.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
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