Liquid Junction DC Potential, Salt Bridge

Between two dissimilar electrolyte solutions a potential difference is created, just like between a metal and an electrolytic solution. By Brownian motion, the ions randomly walk with a velocity proportional to the Boltzmann factor kT. The corresponding E-field will have a direction to slow down the rapid ions and accelerate the slow ones in the interface zone. The resulting potential difference is called the liquid junction potential and follows a variant of the Nemst equation called the Henderson equation  

The liquid junction potential is usually less than 100 mV. For instance, for a junction of different concentrations of NaCl and with cj = 10c2, the dilute side is 12.2 mV negative with respect to the other side. 

The salt bridge usually represents a low resistance bridge to the tissue and makes an effective contact with only a small liquid junction potential. The solution may be in the form of a liquid, a paste, a gel, or a hydrogel. The ionic mobilities are less the higher the viscosity of the medium, and the liquid junction potential thus changes according to Eq. (7.11). 

The Cl ions have about the same mobilities, and therefore KCl creates a lower liquid junction potential than, for example, NaCl. The liquid junction DC potential can be kept small by inserting a salt bridge between the solutions, so that there will be two junctions 

In electrophysiology requiring DC stability, AgCl electrodes are widely used. They may be used with a salt bridge filled with saturated KCl. Even if the salt solution is immobilized with agar gel at the tissue side, potassium is known to influence excitable cells, and it may be preferable to use NaCl 0.9% instead. A liquid junction DC potential must then be accounted for. 

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