In a pH electrode, what is the typical material used for the measuring electrode's covering membrane?

• A. Glass
• B. Plastic
• C. Silicone
• D. Teflon

Answer: (A)

The key idea is that the measuring membrane in a pH electrode must be hydrogen-ion selective and provide a stable, repeatable potential over a wide pH range. Glass fits this role because its surface contains silanol groups that readily exchange protons with the sample solution, creating a potential proportional to hydrogen ion activity (a near-Nernstian response). This glass membrane is durable, chemically resistant, and exhibits minimal drift, which is essential for accurate, long-term pH measurements.

Plastic, silicone, and Teflon membranes don’t provide the same reliable hydrogen-ion exchange and can introduce drift, gas permeability, or interference, making them unsuitable as the covering membrane for the measuring electrode.