Liquid junction potential
Liquid junction potential occurs when two solutions of different concentrations are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated one. The rate of diffusion of each ion will be roughly proportional to its speed in an electric field. If the anion diffuses more rapidly than the cation, it will diffuse ahead into the dilute solution, leaving the latter negatively charged and the concentrated solution positively charged. So an electrical double layer of positive and negative charges will be produced at the junction of the two solutions. So at the point of junction, a potential difference will develop because of the ionic transfer. This potential is called liquid junction potential or diffusion potential. The magnitude of the potential depends on the relative speeds of the ions.
- Ewithout transference = RT/F . ln(a2/a1)
- Ewith transference = tM RT/F . ln (a2/a1)
where a2 and a1 are activities of HCl solutions of right and left hand electrodes respectively and tM be transport number of Cl–
- Liquid Junction potential = Ewith transference– Ewithout transference = (tM – 1) RT/F . ln (a2/a1)
Elimination of liquid junction potential 
The liquid junction potential interferes with the exact measurement of EMF of a chemical cell. So it should be eliminated or at least its effect should be minimized. The general practice to eliminate the liquid junction potential is to place a salt bridge consisting of usually a saturated solution of Potassium chloride (KCl) and Ammonium nitrate (NH4NO3) with Lithium acetate (CH3COOLi) between the two solutions constituting the junction. When such a bridge is used, the ions in the bridge are present in large excess at the junction and they carry almost the whole of the current across the boundary. The efficiency of KCl/NH4NO3 is connected with the fact that in these salts, the transport numbers of anion and cation are the same.
See also 
- Advanced Physical Chemistry by Gurtu & Snehi
- Principles of Physical Chemistry by Puri, Sharma, Pathania