liquid junction potential
Picture a doorway between two crowded rooms where one room is full of fast walkers and the other of slow walkers. As people drift through, the quicker ones pull ahead and a slight imbalance of crowding builds up across the doorway. At the boundary where two different solutions meet, fast ions outrun slow ions in just this way, and the tiny voltage that builds up across that boundary is the liquid junction potential.
Formally, the liquid junction potential is the small potential difference that arises at the interface between two electrolyte solutions of different composition, caused by the different mobilities (and concentrations) of the ions diffusing across it. In potentiometry it appears at the porous junction where a reference electrode's filling solution meets the sample, and it adds, uninvited, to the measured cell voltage.
It matters because it is an error you usually cannot eliminate, only minimize — and it limits the ultimate accuracy of pH and ion measurements. The standard trick is to fill the reference with a salt whose cation and anion move at nearly the same speed (such as potassium chloride), making the junction potential small and roughly constant so it largely cancels between calibration and sample. The caveat is that very different or low-ionic-strength samples can shift it enough to matter.
When a pH probe calibrated in dilute buffers is then dipped in a strong acid, part of the small shift in reading comes not from the acidity itself but from a changed liquid junction potential — a quiet source of error that fresh, saturated KCl in the reference helps keep in check.
Unequal ion speeds at a solution boundary create a small, hard-to-remove voltage.
Potassium chloride is the favorite reference filling because potassium and chloride ions happen to diffuse at almost the same speed, which keeps the liquid junction potential small. A clogged or fouled junction makes it unstable — a common cause of drifting pH readings.