"As more and more K⁺ ions move out the charge will increase until the separation of charge develops an electrical potential that opposes the concentration gradient causing the outward movement of K⁺.

Then the potential is large enough to prevent any net movement of K⁺ and the system is at equilibrium.

This potential difference is referred to as the K⁺ equilibrium potential and can be determined by using the Nernst Equation.

Inside mammalian cells the K⁺ concentration is normally 140 mM/L while the K⁺ concentration in the solution surrounding the cells is only 4 mM/L.

The equilibrium potential required to prevent the K+ from leaving the cells is around -90 mV (millivolts) The specific potential depends upon the exact cell type."

courtesy of Robert H. Parsons, Ph.D., Rensselaer Polytechnic Institute, used with permission