1. Describe how an ion selective electrode works. Provide the Nernst equation to
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Question
1. Describe how an ion selective electrode works. Provide the Nernst equation to show the relationship between Ecell and Concertation of K for the K ion selective electrode shown below: (a on-selective electrode Internal reference Extemal reference Filling solution (b) Filling solution inside electrode B C B- B C Inner surface of membrane R LC Example: LC R L R LC L valinomycin LC R- LC LC' R Outer surface of membrane Excess negative charge Excess positive charge Analyte solution outside electrode AtExplanation / Answer
The ion-selective electrode (Potassium) consists of an internal reference electrode, ion-selective membrane and voltmeter. It works based on the principle of a galvanic cell. It
As the positive K+ ions are transported across the membrane through the selective binding of ions by the diffusion, there is a build-up of positive charge on the inside of the membrane and a corresponding increase in negative charge outside. The electrical potential difference established across the membrane causes the K+ ion migration to slow down and finally stop when the diffusion pressure due to the difference in concentration is exactly balanced by the electric field due to repulsion of similarly charged particles. The potential difference at equilibrium is called the membrane potential.
At equilibrium, this potential is measured with respect to a stable internal reference electrode having a constant potential, and a net charge is determined. The difference in potential between the electrode and the membrane depends on the logarithm of activity (effective concentration) of the of K+ ions in the solution given by Nernst equation.
K+ + e- à K(s); E0cell = -2.93 V
Nernst equation:
Ecell = E0cell + RT/(nF) ln([K+])
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