Glass-electrode pH measurement principle: when two aqueous solutions with different hydrogen ion concentrations are separated by a thin specialized glass membrane, a millivolt-level electromotive force (mV) is generated across the membrane. This phenomenon forms the working principle of which analytical instrument?

Introduction / Context:
The glass electrode is the most widely used electrochemical sensor for measuring acidity or alkalinity of solutions. When it separates two solutions of different hydrogen ion activity (typically the test solution and an internal reference solution), a small electromotive force (on the order of millivolts) develops due to selective H+ exchange at the hydrated glass surface. This EMF is interpreted by a high-impedance meter to report pH directly.

Given Data / Assumptions:

• Two solutions with different hydrogen ion concentrations are separated by a thin glass wall.
• An EMF of the order of millivolts is produced.
• Electrochemical measurement uses a reference electrode plus a glass indicator electrode.

Concept / Approach:
The Nernst equation predicts that the potential across a selective membrane varies linearly with the logarithm of ion activity. For H+ at 25°C, the theoretical slope is about 59.16 mV per pH unit. A pH meter measures the potential difference between the glass electrode and a reference electrode (e.g., Ag/AgCl), converts that voltage to pH, and displays the result after temperature compensation and calibration with buffer standards.

Step-by-Step Solution:

Verification / Alternative check:
Practical pH meters show approximately 59 mV change per pH at 25°C; multi-point buffer calibration aligns the meter to real electrode behavior.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing pH glass electrode (ion selective) with general redox electrodes; pH response is specific to H+ activity and requires temperature compensation.

Final Answer:
pH meter