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Example SOP: pH Meter Operation and Calibration (QC Lab)

Last reviewed 27 Oct 2026 · 4 min read

Contents
  1. 1. Objective
  2. 2. Scope
  3. 3. Responsibility
  4. 4. Precautions
  5. 5. Procedure
  6. 5.1 Preliminary checks
  7. 5.2 Basic operation
  8. 5.3 Calibration
  9. 5.4 Measurement
  10. 5.5 Cleaning and maintenance
  11. 6. Acceptance criteria (example in-house criteria)
  12. 7. Records and retention
  13. 8. Training, distribution and annexures
  14. See also
  15. References

In short: This short example SOP covers day-to-day operation, calibration and care of a pH meter in a Quality Control laboratory. It is a template for QC analysts and must be adapted to your instrument manual, pharmacopoeial requirements and quality system.

1. Objective

To describe the procedure for operation and calibration of the pH meter used in the QC laboratory (Make: ______, Model: ______, ID No.: ______).

2. Scope

This example SOP applies to pH meters in the Quality Control department of a pharmaceutical manufacturing facility.

3. Responsibility

  • Analyst: operates and calibrates the meter, performs the test and records the data.
  • Section head or designee: makes sure this SOP is followed and reviews the records.
  • Head of QC or designee: implements the SOP and decides on action for failures.

4. Precautions

  • Follow the manufacturer’s user manual for the key sequence of your model.
  • Keep the meter on a vibration-free bench, away from strong magnetic or electrical fields.
  • Switch the meter off before unplugging it.
  • Do not leave the electrode dry or in purified water; store it in the solution the electrode manufacturer specifies (often a storage solution or pH 4 buffer, as per the electrode manual).

5. Procedure

5.1 Preliminary checks

  1. Check that the meter is clean and dust-free; wipe with a soft lint-free cloth.
  2. Inspect the electrode for cracks, scratches, salt crust on the junction or a low filling-solution level.
  3. Check the calibration status label and buffer expiry dates.

5.2 Basic operation

  1. Connect the meter to the mains and switch it on; check the display and that the keys respond.
  2. Fit the electrode (and temperature probe) in the stand, rinse the electrode with purified water and blot the tip gently.

5.3 Calibration

Calibrate as per the user manual and the pharmacopoeial chapter that applies to your tests (for example USP general chapter <791> pH, or Ph. Eur. 2.2.3).

  1. Use certified buffer solutions within expiry, at the temperature for which their pH is stated.
  2. Use at least two buffers that bracket the expected sample pH (three, for example pH 4.0, 7.0 and 10.0, for a wide range).
  3. Rinse the electrode between buffers and let the reading stabilise.
  4. Record buffer values, temperature and slope in the calibration data sheet (Annexure I).

Frequency (example in-house criterion): daily before use, and whenever the electrode is changed or a result seems doubtful.

5.4 Measurement

  1. Rinse the electrode with purified water, blot it and immerse it in the sample.
  2. Wait for a stable reading, then record the pH and sample temperature.
  3. Rinse the electrode between samples and record use in the instrument usage logbook (Annexure II).

5.5 Cleaning and maintenance

  1. After use, rinse the electrode and return it to its storage solution; wipe the meter daily with a dry lint-free cloth.
  2. For a sluggish electrode, follow the electrode maker’s cleaning advice (a brief soak in dilute hydrochloric acid, for example 0.1 M, and thorough rinsing is common; check your electrode allows it).
  3. If calibration fails, do not use the meter: clean or replace the electrode, use fresh buffers and repeat. If it still fails, label the meter “Out of use”, inform the section head and raise a deviation or maintenance request (see handling of incidents and deviations).

6. Acceptance criteria (example in-house criteria)

ParameterExample in-house criterion
Measured pH of each buffer after calibrationWithin ±0.05 pH units of the stated value at the buffer temperature
Electrode slope95% to 105% of the theoretical slope (some sites accept 90% to 105%; follow the electrode manufacturer and your own justification)

7. Records and retention

Calibration data sheets and usage logbooks are GMP records. Keep them at least 1 year after the expiry date of the respective product batch tested (EU GMP Part I Chapter 4 §4.11; 21 CFR 211.180(a)), or longer if local law or the marketing authorisation requires.

8. Training, distribution and annexures

  • Training: as per the site training SOP. Distribution: controlled copies to QC; master copy with QA.
  • Annexure I: Calibration data sheet. Annexure II: Instrument usage logbook.

See also

References

  • USP–NF general chapter <791> pH, United States Pharmacopeia (current official version).
  • European Pharmacopoeia general chapter 2.2.3 Potentiometric determination of pH, EDQM (current edition).
  • EudraLex Volume 4, EU GMP Part I Chapter 4 Documentation, §4.11 (record retention).
  • 21 CFR 211.180(a), US FDA (record retention).
  • Manufacturer’s pH meter and electrode user manuals.

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