Liquid Junction Potential

What Liquid Junction Potential (LJP) is and how it can be corrected for in patch-clamp experiments

What is LJP?

When two different ionic solutions meet, a small voltage can form at the junction. In patch-clamp experiments, this occurs at the tip of the pipette where the internal solution meets the bath solution.
This voltage difference occurs because smaller ions (like Cl^-) move faster than large ions (like gluconate), so a small standing voltage is present across the liquid junction.
When patch-clamping cells the amplifier offset voltage is adjusted to zero when the pipette is in open-tip configuration with the bath, but this reading is not really zero because LJP is present. This is the cause of LJP error in patch-clamp experiments.
When a patch-clamp recording shows a voltage of -70 mV, the voltage isn’t actually -70 mV because it is off by the size of the LJP.
Scientists can measure or calculate the LJP, then use its known value to correct patch-clamp recordings.
Changing the extracellular solution or intracellular solution will change LJP.

Consider the case when you zero your meter as the pipette is in the bath:

V_measured = V_actual + LJP

Rearranging this formula, the actual voltage of the cell is what the meter says minus the LJP:

V_actual = V_measured - LJP