SOP for Operation and Calibration of pH Meter

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STANDARD OPERATING PROCEDURE
Operation and Calibration of pH Meter

1.0 Purpose

To describe the standardized procedure for operation, calibration, and verification of the pH meter using USP buffer solutions to ensure accurate pH measurement in accordance with USP <791>, IP, BP, and EP.

2.0 Scope

This SOP applies to all pH meters used in the Quality Control Laboratory for analysis of raw materials, intermediates, and finished pharmaceutical products.

3.0 Responsibility

DesignationResponsibility
QC AnalystOperation, calibration, verification, slope check, and documentation
QC SupervisorReview and verification of results
QC ManagerOverall compliance and approval
EngineeringMaintenance and servicing

4.0 Accountability

The QC Manager is accountable for implementation and compliance with this SOP.

5.0 References

  • USP <791> pH
  • Indian Pharmacopoeia
  • British Pharmacopoeia
  • European Pharmacopoeia 2.2.3
  • pH Meter Manufacturer’s Instruction Manual

6.0 Definitions and Abbreviations

TermDescription
pHNegative logarithm of hydrogen ion activity
ATCAutomatic Temperature Compensation
USPUnited States Pharmacopeia
SlopeElectrode response expressed as % of theoretical Nernst slope

7.0 Safety and Precautions

  • Handle buffer solutions carefully.
  • Do not allow the electrode to dry.
  • Rinse electrode with purified water between buffers.
  • Perform calibration at 25°C ± 2°C.

8.0 Instrument Description

The pH meter consists of a glass measuring electrode, reference electrode, temperature sensor, and electronic unit for determination of pH of aqueous solutions.

9.0 Operating Procedure

  1. Switch ON the pH meter and allow warm-up for 15–20 minutes.
  2. Rinse the electrode with purified water and blot dry.
  3. Immerse electrode into the sample solution.
  4. Stir gently and allow the reading to stabilize.
  5. Record the pH value.

10.0 USP Buffer Solutions for Calibration

Buffer Solution USP Nominal pH (25°C) Acceptance Criteria
Potassium Chloride 1.68 ± 0.05 pH units
Potassium Hydrogen Phthalate 4.01 ± 0.05 pH units
Phosphate Buffer 6.86 ± 0.05 pH units
Borate Buffer 9.18 ± 0.05 pH units
Sodium Hydroxide 12.45 ± 0.05 pH units

11.0 Calibration Procedure (Full)

  1. Ensure all buffer solutions are at 25°C ± 2°C.
  2. Rinse electrode with purified water and blot dry before each measurement.
  3. Immerse electrode into pH 1.68 buffer, allow reading to stabilize, and adjust meter if necessary.
  4. Rinse electrode and immerse into pH 4.01 buffer, stabilize reading, and adjust slope if required.
  5. Rinse electrode and immerse into pH 6.86 buffer (mid-point), stabilize, and adjust meter.
  6. Rinse electrode and immerse into pH 9.18 buffer, stabilize, and check electrode response.
  7. Rinse electrode and immerse into pH 12.45 buffer for high-end calibration, stabilize, and record value.
  8. Calculate slope using buffer pairs (4.01 & 6.86; 6.86 & 9.18):
    • Slope (%) = (Observed mV difference / Theoretical mV difference) × 100
    • Acceptance: 95–105%
  9. Verify mV difference between each buffer pair according to Nernst equation. Acceptance: ±2 mV.
  10. Calibration is acceptable only if all 5 buffers are within ±0.05 pH units, and slope/mV criteria are met.

12.0 Verification Procedure (Post-Calibration)

  1. Rinse electrode and blot dry.
  2. Measure verification buffers: pH 4.01, 6.86, and 9.18.
  3. Record observed pH and mV readings.
  4. Acceptance: Observed pH within ±0.05 pH units, slope 95–105%, mV difference consistent with Nernst equation.

13.0 Calibration Frequency

  • Daily before sample analysis
  • After electrode replacement
  • After maintenance or abnormal readings

14.0 Deviations and Corrective Actions

  • Do not proceed with analysis if calibration or verification fails.
  • Recalibrate using fresh buffers and recheck slope/mV.
  • Inform QC Supervisor and initiate deviation if required.

15.0 Cleaning and Maintenance

  • Rinse electrode with purified water after use.
  • Store electrode in recommended storage solution.
  • Do not wipe or rub electrode bulb.
  • Perform preventive maintenance as scheduled.

pH METER CALIBRATION & VERIFICATION LOG

Buffer Solution Nominal pH (25°C) Observed pH Observed mV Acceptance (pH ±0.05) Remarks
Potassium Chloride1.68
Potassium Hydrogen Phthalate4.01
Phosphate Buffer6.86
Borate Buffer9.18
Sodium Hydroxide12.45

Electrode Slope Calculation

Buffers Used mV Difference Observed Theoretical mV Difference Slope (%) Acceptance (95–105%) Remarks
4.01 & 6.86
6.86 & 9.18

Verification After Calibration

Buffer Solution Nominal pH Observed pH Observed mV Acceptance (pH ±0.05, Slope 95–105%) Remarks
Potassium Hydrogen Phthalate4.01
Phosphate Buffer6.86
Borate Buffer9.18

Prepared by: _____________________    Date: ___________
Reviewed by: _____________________    Date: ___________
Approved by: _____________________    Date: ___________

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