How does a glass electrode measure pH and what is its slope at 25°C?

• A. It measures pH by detecting a hydrogen gas evolution potential; slope about -60 mV per pH unit.
• B. It uses a metal-sulfide membrane; slope about -58 mV per pH unit.
• C. It measures H+ activity via a glass membrane; slope ≈ -59.16 mV per pH unit at 25°C.
• D. It relies on optical absorbance changes with pH.

Answer: (C)