USP <791> pH (Potentiometric Determination of pH)

USP General Chapter <791> pH establishes the mandatory potentiometric method for measuring the pH of pharmaceutical articles. pH is a critical quality attribute (CQA) that governs the solubility, stability, and biological activity of drug substances and drug products.

The method relies on measuring the potential difference (voltage) between a hydrogen ion-selective electrode (glass electrode) and a reference electrode immersed in the solution. This potential difference is related to the activity of hydrogen ions via the Nernst equation.

Harmonization Status: This chapter is harmonized under the Pharmacopoeial Discussion Group (PDG) and is considered interchangeable with Ph. Eur. 2.2.3 and JP 2.54  pH Determination (per ICH Q4B Annex 4C), provided the calibration and temperature requirements are met.

Scope USP <791> applies to all unbuffered and buffered aqueous solutions, including:

• Drug Substances (DS) and Drug Products (DP)
• Buffers, media, and process intermediates
• Note: This chapter generally does not apply to Water for Injection (WFI) or Purified Water, which are governed by USP <645> because pH measurement is not reliable in very low ionic strength water.

Calibration (The Critical Requirement) Unlike simple "dip and read" approaches, USP <791> strictly mandates:

• Multipoint Calibration: The instrument must be standardized using at least two standard buffer solutions.
• Bracketing: The standard buffers chosen must bracket the expected pH value of the sample. (e.g., if measuring pH 7.4, you must calibrate with pH 7.0 and pH 10.0).
• Multi-buffer calibration (e.g., pH 4.0, 7.0, 10.0) may be used to verify electrode linearity, but USP <791> only requires at least two buffers that bracket the sample.

Temperature Compensation pH is temperature-dependent. The measurement is only valid if:

• The calibration buffers and the sample are within ± 2°C of the same temperature (typically 25°C), OR
• The pH meter utilizes Automatic Temperature Compensation (ATC) to mathematically correct the reading based on a temperature probe.

Procedure

• Rinse the electrode with distilled/deionized water and blot dry (do not wipe, to avoid static charge).
• Immerse in the sample and swirl gently.
• Allow the reading to stabilize.
• Report the value to the precision required by the specification (e.g., if spec is 7.0–7.6, report to 0.1; if 7.40 ± 0.05, report to 0.01).

Common industry mistakes

• Using non-bracketing buffers
• Not equilibrating buffers and sample to same temperature
• Storing electrodes dry
• Using outdated or single-point calibration