Which of the following is false about ion-selective electrode analysis of sodium?

• A. Uses a glass membrane
• B. Errors occur from protein buildup on the membrane
• C. Membrane coated with valinomycin
• D. Principle based on potentiometry

Answer: (C)

Sodium ion-selective electrode analysis relies on a membrane that is selective for Na+ and produces a potential proportional to Na+ activity, read by potentiometry. The classic Na+ electrode uses a glass membrane whose surface interactions with Na+ set up that voltage without drawing current. Proteins or other fouling on the membrane can disrupt these surface interactions, causing drift or errors in the measurement. Valinomycin is a potassium-selective ionophore; it makes membranes highly selective for K+, not Na+. Using valinomycin on a membrane would bias the sensor toward potassium and not provide a correct Na+ measurement. That’s why the statement about coating the membrane with valinomycin is false.