Which material is used as the membrane for the sodium-selective ion electrode?

• A. Glass aluminum silicate
• B. Valinomycin
• C. Nonactin and monactin
• D. Ion exchanger

Answer: (A)

Ion-selective electrodes respond to a specific ion through the membrane that separates the sample from the internal reference solution. For sodium, the classic and most reliable membrane is a glass membrane made of sodium-aluminosilicate glass. This type of glass has surface properties that allow Na+ ions from the solution to exchange with ions in the glass, creating a potential that changes predictably with the activity of Na+ in the sample. The result is a stable, near-Nernstian response over the relevant concentration range.

Valinomycin is a potassium-selective ionophore, so it’s used to make potassium-selective membranes, not sodium. Nonactin and monactin are ionophores that can transport alkali metals and can be used in some Na+-selective membranes, but they are not the standard, widely used membrane material for sodium electrodes. Ion-exchanger membranes exist too, but the well-established sodium-selective electrode relies on the glass membrane.

Thus, the membrane material used for the sodium-selective ion electrode is the glass aluminum silicate type glass.