Which molecule is used as the membrane for the potassium-selective ion electrode?

• A. Valinomycin
• B. Nonactin
• C. Monactin
• D. TNOPAC

Answer: (A)

Potassium-selective electrodes use a carrier molecule in the membrane that binds K+ much more strongly than other ions. Valinomycin does this exceptionally well because it is a cyclic peptide with a hydrophobic pocket lined by carbonyl oxygens that perfectly accommodates the size and coordination preferences of K+. This snug fit lets valinomycin shuttle K+ across the membrane, creating a stable potential that directly tracks the activity of potassium in the solution and remains highly selective against Na+ and other ions. Other ionophores like nonactin, monactin, or TNOPAC have different ion preferences and don’t provide the same level of K+-specific binding, so they’re not as suitable for a potassium-selective electrode. In practice, valinomycin is embedded in a plasticized membrane to deliver the characteristic, reliable potassium response.