Working Principle of a pH Meter

Definition

A pH meter is an analytical instrument used to measure the hydrogen ion concentration ([H⁺]) in a solution, expressing it as pH (potential of hydrogen).

Principle

• A pH meter works on the electrochemical principle of measuring the electromotive force (EMF) between two electrodes:

  1. Glass Electrode (measures H⁺ activity in the test solution)

  2. Reference Electrode (provides a stable reference potential, e.g., silver/silver chloride electrode)

• The EMF generated depends on the difference in hydrogen ion activity between the test solution and the reference electrode.

• The meter uses the Nernst equation to convert this voltage into pH:

pH=Eref−Emeasured2.303×R×T/F\text{pH} = \frac{E_{\text{ref}} – E_{\text{measured}}}{2.303 \times R \times T / F}pH=2.303×R×T/FEref​−Emeasured​​

Where:

• R = Gas constant (8.314 J/mol·K)

• T = Absolute temperature (K)

• F = Faraday’s constant (96485 C/mol)

At 25°C, the slope is approximately 59.16 mV per pH unit.

Working Steps

1. Calibration using standard buffer solutions (e.g., pH 4.00, 7.00, 10.00).

2. Immersion of the electrode system into the sample solution.

3. Measurement of EMF generated due to H⁺ concentration difference.

4. Display of pH value on the meter after electronic conversion.

Key Points for Accurate pH Measurement

• Keep the glass electrode hydrated (store in KCl solution, not distilled water).

• Avoid contamination of buffers and samples.

• Calibrate regularly (especially before critical measurements).

• Compensate for temperature effects (automatic or manual).