Laboratory Instrumentation and Analytical Principles Flashcards

Ion-selective electrodes for sodium and potassium operate on which analytical principle?

Answer: Potentiometry

ISEs measure the voltage difference between an indicator and reference electrode at zero current.

Which ionophore is incorporated into the membrane of a potassium ion-selective electrode?

Answer: Valinomycin

Valinomycin binds K+ with high selectivity over Na+.

The Clark electrode used to measure pO2 operates by which principle?

Answer: Amperometry

Oxygen is reduced at a platinum cathode under a fixed voltage, and the resulting current is proportional to pO2.

In the Severinghaus pCO2 electrode, CO2 is detected by measuring a change in what?

Answer: pH of a bicarbonate solution

CO2 diffuses through a membrane into a bicarbonate buffer, changing the pH measured by a glass electrode.

A patient with severe hyperlipidemia shows a falsely low sodium on one analyzer but a normal sodium on another. Which method explains the falsely low result?

Answer: Indirect ISE

Indirect ISE dilutes the sample, so excess lipid volume causes pseudohyponatremia, while direct ISE is unaffected.

According to the Nernst equation, what is the theoretical slope of an electrode for a monovalent ion at 25°C?

Answer: 59.2 mV per decade

The Nernstian slope is about 59.16 mV per tenfold change in activity for a +1 or −1 ion at 25°C.

A chloridometer measures chloride by which technique?

Answer: Coulometric-amperometric titration with silver ions

Silver ions generated at a constant rate precipitate chloride, and the time to endpoint is proportional to chloride concentration.