Salt Bridge. Solutions of different concentrations, or containing different substances, when in contact develop a potential across the interface called a junction potential. This ‘potential’ s the result of different ions having different migrating rates due to size, weight and shape. Such potentials develop across all solution interfaces and depend on the mobility of the ions and the concentration gradient. Junction potentials cannot be eliminated, but can be minimized by employing a concentrated electrolyte solution, or salt bridge, between the two electrolytes. For example, a saturated potassium chloride solution (>4M) will have only a 4% difference between the mobility of the potassium ion and that of the chloride ion. Most liquid junction potentials can be reduced in this manner to less than 4mv and sometimes less than 1 mv.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links