LIQUID-JUNCTION POTENTIAL

"A potential difference between two solutions of different compositions separated by a membrane type separator. The simplest example is the case of two solutions containing the same salt in different concentrations. The salt will diffuse from the higher concentration side to the lower concentration side. However, the diffusion rate of the cation and the anion of the salt will very seldom be exactly the same (see mobility). Let us assume for this example that the cations move faster; consequently, an excess positive charge will accumulate on the low concentration side, while an excess negative charge will accumulate on the high concentration side of the junction due to the slow moving anions. This sets up a potential difference that will start an electromigration of the ions that will increase the net flux of the anions and decrease the net flux of the cations. In steady-sate conditions, the two ions will move at the same speed and a potential difference will be created between the two solutions. This ""steady-sate"" potential difference seems constant, but this is misleading because it slowly changes as the concentrations between the two solutions equalize. The diffusion process will ""eventually"" result in equal concentrations of the salt in the two solutions separated by the membrane, and the liquid-junction potential will vanish. For a simple case, the value of the liquid junction potential can be calculated by the so called ""Henderson"" equation. "