Calculation of liquid junction potentials

Equations are given and procedures described for calculating liquid junction potentials over wide ranges of concentration and composition. Use is made of the Henderson continuous mixtures assumption, avoiding treatment of the codiffusion problem. Ionic mobilities and activities are incorporated realistically, making use of standard data on binary electrolyte solutions. For ionic strengths up to about 0.1, an equation is derived in terms of ionic properties specified for the two end solutions; typical accuracy is 0.1 mV for simple junctions between two concentrations of the same electrolyte, and 1-2 mV for more complex junctions. Higher concentrations require specification of ionic properties at points intermediate between the end solutions; approximations are developed which prove accurate to about 0.5 mV in simple cases at ionic strengths as high as 5, while in complex cases an accuracy of 1 to a few mV is found at ionic strengths as high as 4.5. Ionic strength ratios across the junctions examined range from 1:1 to 94:1, typically being about 20:1. A rule is given for determining mobilities and activities of ions in mixtures, using standard data for binary solutions. An expression is obtained for the activity coefficients of single ion constitutents in terms of mean activity coefficients, and an estimate of the associated error is derived. This is the error in dividing total cell potentials into a component due to electrode potentials and a component arising at the liquid junction; it appears to be of comparable magnitude to the errors, stated above, due to other approximations in the calculations described.