In the alkaline effect observed in pH-sensitive membranes, which ions partially replace the hydrogen ions in the gel layer?

• A. Alkali Ions
• B. Hydrogen Ions
• C. Calcium Ions
• D. Sodium Ions

Answer: (A)

The change being tested is ion-exchange at the surface of a pH-sensitive membrane. In an alkaline environment, alkali metal ions from the solution partially take the place of hydrogen ions in the gel layer. This swap alters the surface charge and the electrical potential the membrane develops, which is what you observe as the alkaline effect. That’s why alkali ions are the right answer: they are the species that participate in exchanging with the protons in the gel layer. Hydrogen ions are the ones ordinarily involved in keeping the surface protonated, so they aren’t the exchanging species here. Calcium ions, being divalent, interact differently and aren’t the primary players in this specific effect. Sodium ions are a type of alkali ion, but the broader category of alkali ions best captures the mechanism across alkali metals.