Which material is used as the chloride-selective membrane in ion-selective electrodes?

• A. TNOPAC (Tri-n-octylpropylammonium chloride)
• B. Valinomycin
• C. Nonactin
• D. Glass aluminum silicate

Answer: (A)

Chloride-selective membranes in ion-selective electrodes rely on a lipophilic ion exchanger that carries a fixed positive charge and exchanges chloride with the sample. The membrane is usually a PVC/plasticizer matrix containing a quaternary ammonium salt with chloride as the counterion, which binds Cl- and translates its activity into a measurable potential via the Nernst equation. TNOPAC, or tri-n-octylpropylammonium chloride, is a classic example of such a material: a lipophilic quaternary ammonium cation paired with chloride that acts as the primary ion exchanger in the membrane, providing selective response to chloride over other anions. In contrast, valinomycin is a potassium ionophore, nonactin prefers ammonium and certain cations, and glass aluminum silicate is not used as the chloride-selective membrane, so they do not provide the desired chloride selectivity.