The glass pH electrode membrane is primarily composed of silicon dioxide and is selectively sensitive to which ion?

• A. Hydrogen ions
• B. Sodium ions
• C. Potassium ions
• D. Calcium ions

Answer: (A)

The glass pH electrode works because its silicon dioxide–based membrane acts as a hydrogen ion–selective sensor. When the glass is hydrated, its surface has silanol groups that readily exchange protons with the solution. This proton exchange creates a potential at the glass–solution interface that depends on the activity of hydrogen ions (the pH). The electrode then converts that potential into a pH reading, making it primarily sensitive to hydrogen ions. Other ions like sodium, potassium, or calcium interact with the glass only weakly in comparison, so they don’t drive the measured potential as the hydrogen ion activity does.