Ion-selective electrode analysis of sodium typically uses which type of membrane?

• A. Glass membrane
• B. Silver/silver chloride membrane
• C. Parylene membrane
• D. Ceramic membrane

Answer: (A)

In ion-selective electrodes, the membrane must be selective for the target ion and transduce its activity into a measurable potential. For sodium, that role is fulfilled by a glass membrane. The Na+-selective glass is a special silicate composition whose surface exchanges Na+ ions with the solution, creating a potential that changes in proportion to the log of the Na+ activity. This produces a stable, Nernstian response (about 59 mV per tenfold change in Na+ at room temperature) across clinical concentration ranges, while rejecting many other ions. The other membranes listed aren’t used as the sensing layer for sodium: a silver/silver chloride membrane is typically used as a reference electrode, not the ion-selective membrane; parylene is mainly a protective polymer coating, not a sodium-selective membrane; and ceramic membranes aren’t the standard sodium-selective option in common clinical ISEs. Therefore, the glass membrane is the appropriate choice for sodium ISE analysis.