Which molecules form the ammonium-selective membrane in ion-selective electrodes?

• A. Nonactin and monactin
• B. Valinomycin
• C. TNOPAC
• D. Ion exchanger

Answer: (A)

Ammonium-selective membranes rely on an ionophore that forms a stable, selective complex with NH4+ inside the membrane, so the electrode responds specifically to ammonium activity. Nonactin and monactin are macrocyclic ionophores with cavities that fit ammonium ions well, allowing NH4+ to bind, be carried through the membrane, and produce a consistent potential proportional to its concentration. This specific binding gives high selectivity for ammonium even in the presence of other cations, which is exactly what the electrode needs for accurate measurement.

Valinomycin binds potassium far more readily, so it would bias the electrode toward K+ rather than NH4+. An ion exchanger membrane relies on fixed charges and does not provide the same selective, carrier-based transport for ammonium. TNOPAC is not a standard ammonium-selective ionophore, so it wouldn’t deliver the same targeted NH4+ selectivity.

Therefore, the best choice is nonactin and monactin.