Calcium and lithium ions in ion-selective electrodes use which type of membrane?

• A. Ion exchanger
• B. PVC
• C. Glass aluminum silicate
• D. Valinomycin

Answer: (A)

Ion-selective electrodes for calcium and lithium use membranes that function by ion exchange. These membranes have fixed charged sites that selectively exchange the target ion (Li+ or Ca2+) with counter-ions in the membrane. This exchange creates a controlled activity gradient across the membrane, producing a potential according to the Nernst equation that is directly related to the activity of the ion of interest. Other membrane types listed, like a glass membrane, are typically tuned for hydrogen or other ions, and valinomycin is an ionophore specialized for potassium, not Li+ or Ca2+. So the ion-exchanger membrane best fits how these electrodes achieve selectivity for Li+ and Ca2+.