Nernst equation

noun

ˈnernst- 

: an equation used to find the potential difference across a semipermeable membrane through which an ion diffuses between solutions where its concentration differs and that for a temperature of 37°C (98.6°F) gives a result in millivolts equal to plus or minus 61.5 times the logarithm of the ratio of the concentration of the ion on one side of the membrane to its concentration on the other