What is the equilibrium potential for potassium (EK)?

• A. -96 mV
• B. +52 mV
• C. +134 mV
• D. 0 mV

Answer: (A)

Potassium equilibrium potential is the voltage where the chemical gradient of K+ is exactly balanced by the electrical gradient, so there’s no net K+ flux. This is given by the Nernst equation, which for monovalent ions simplifies to E_K ≈ 61.5 mV × log10([K+]out/[K+]in) at body temperature.

Using typical intracellular and extracellular potassium levels (inside about 140 mM, outside about 4 mM) gives [out]/[in] ≈ 0.0286; log10 ≈ -1.54, so E_K ≈ -95 to -97 mV. The commonly cited value around -96 mV falls right in this range, reflecting why the resting membrane potential in cells with high K+ permeability sits near this level.

A positive value for E_K would require a reversal of the K+ concentration gradient, which isn’t the case physiologically. Zero millivolts would imply no gradient for K+, which also isn’t true with the normal intracellular and extracellular K+ concentrations.