The alkaline effect is the phenomenon where H+ ions in the gel layer of the pH-sensitive membrane are partly and completely replaced by alkali ions.

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

Answer: (A)

The key idea is ion-exchange at the gel layer of a pH-sensitive membrane. In alkaline conditions, alkali metal ions from the solution (such as Na+ or K+) compete for the exchange sites on the gel layer and replace protons (H+) that are normally present there. This partial or complete replacement by alkali ions changes the surface charge and the membrane potential, producing the alkaline effect observed in pH measurements. Hydrogen ions are the ones being replaced, not the agents causing the change, and calcium ions belong to a different category (alkaline earth metals) and aren’t the drivers of this effect. So the term that best describes what replaces the H+ ions is alkali ions.