Which statement about ion-selective electrodes (ISEs) is most accurate?

Which statement about ion-selective electrodes (ISEs) is most accurate?

• A. Gas-specific membranes are necessary for oxygen and carbon dioxide electrodes
• B. The pH electrode uses a solid-state membrane
• C. The calcium electrode does not require a reference electrode
• D. The sodium electrode uses an ion-selective carrier (valinomycin)

Answer: (A)

Gas-diffusion control is essential for measuring certain gases electrochemically. For oxygen and carbon dioxide electrodes, the sensing mechanism relies on the gas diffusing through a membrane to reach the sensing interface, where the signal is generated by the gas’s interaction with the electrode rather than by an ion in solution. This requires a membrane that is specifically permeable to the target gas and resistant to the liquid-phase, so the output truly reflects the gas concentration. In other words, gas-specific membranes are a fundamental part of how these gas-sensing electrodes operate, making the statement accurately describing their design. The other points don’t fit as well. A pH electrode uses a glass membrane to respond to hydrogen ions, not a solid-state membrane in the classic sense. Calcium electrodes still rely on a reference electrode to provide a stable potential, since the measured value is a difference from a stable reference. And sodium-selective electrodes do not use valinomycin, which is a potassium-selective ionophore.

Gas-diffusion control is essential for measuring certain gases electrochemically. For oxygen and carbon dioxide electrodes, the sensing mechanism relies on the gas diffusing through a membrane to reach the sensing interface, where the signal is generated by the gas’s interaction with the electrode rather than by an ion in solution. This requires a membrane that is specifically permeable to the target gas and resistant to the liquid-phase, so the output truly reflects the gas concentration. In other words, gas-specific membranes are a fundamental part of how these gas-sensing electrodes operate, making the statement accurately describing their design.

The other points don’t fit as well. A pH electrode uses a glass membrane to respond to hydrogen ions, not a solid-state membrane in the classic sense. Calcium electrodes still rely on a reference electrode to provide a stable potential, since the measured value is a difference from a stable reference. And sodium-selective electrodes do not use valinomycin, which is a potassium-selective ionophore.