When measuring K+ with an ion-selective electrode by means of a liquid ion-exchange membrane, what antibiotic will be incorporated into the membrane?

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

Answer: (D)

In a potassium-selective liquid ion-exchange membrane, the membrane includes a specific ionophore that binds K+ tightly and selectively, producing a potential proportional to the activity of potassium. Valinomycin is the potassium-selective ionophore used for this purpose because its cavity and coordinating groups match the size and chemistry of K+, allowing it to bind K+ very strongly while discriminating against Na+. This strong, selective binding creates the membrane potential the ion-selective electrode measures. Monactin and Nonactin are other ionophores with broader cation affinity and less strong selectivity for K+ (not as ideal for a dedicated K+ sensor), and Streptomycin is a general antibiotic with no role as an ionophore in these membranes. Therefore, Valinomycin is the correct choice for incorporating into the membrane to measure potassium.