The operating principle behind a pH meter is based on the electrochemical measurement of the hydrogen ion concentration (H⁺) in a solution. The pH meter works by measuring the voltage (electromotive force) generated between two electrodes—a glass electrode and a reference electrode—immersed in the solution being tested.

1. Understanding pH

• pH is a measure of the hydrogen ion concentration in a solution.

• It is defined as:

• 

• The pH scale ranges from 0 to 14:

  • pH < 7 → Acidic

  • pH = 7 → Neutral

  • pH > 7 → Basic (alkaline)

• 2. Main Components of a pH Meter

  • Glass Electrode (Measuring Electrode):

    • Sensitive to hydrogen ion activity.

    • Has a special pH-sensitive glass membrane at the tip.

    • Generates a potential (voltage) depending on the H⁺ concentration in the solution.

  • Reference Electrode:

    • Usually contains a stable solution of potassium chloride (KCl).

    • Provides a constant reference voltage against which the glass electrode’s voltage is compared.

  • High-Impedance Voltmeter / pH Meter:

    • Measures the potential difference (voltage) between the glass electrode and the reference electrode.

    • Converts this voltage into a pH value based on the Nernst equation.

  3. Working Principle

  • When both electrodes are dipped into the test solution:

    • The glass electrode interacts with H⁺ ions in the solution.

    • A potential difference is generated between the inner solution (fixed) and the external solution (variable).

    • This potential varies with the pH of the solution.

  • The reference electrode maintains a constant potential, providing a stable baseline for measurement.

  • The pH meter detects the voltage difference and translates it into a pH value using a calibration curve derived from standard buffer solutions.

  4. Nernst Equation (Simplified)

  

  Where:

  • $E$ is the measured potential,

  • E0E_0E0​ is a constant,

  • $R$ is the gas constant,

  • $T$ is temperature in Kelvin,

  • $F$ is Faraday’s constant,

  • $n$ is the charge of the ion (for H⁺, it’s 1).

  This shows the potential is directly related to the logarithm of hydrogen ion concentration, i.e., pH.

   Summary

  A pH meter determines pH by:

  • Using a glass electrode that develops a voltage depending on the H⁺ ions in the solution.

  • Comparing this voltage with a reference electrode.

  • Converting the resulting voltage difference into a pH value using the Nernst equation.

Working Principle of pH Meter :-

A pH meter measures the acidity or alkalinity of a solution by detecting the hydrogen ion concentration (H⁺). It uses two electrodes: a glass electrode (sensitive to H⁺ ions) and a reference electrode (provides a stable voltage). When both are immersed in a solution, they generate a voltage difference that depends on the solution’s pH. The meter then converts this voltage into a pH value using the Nernst equation.

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