Activity coefficients of ions in sodium halide solutions: Critical remarks

The fundamentals of an experimental method proposed by Zhuo et al. [1], to determine activity coefficients of sodium and halide ions in sodium halide solutions, are critically examined. It is shown that this method relies on a key hypothesis, which proves to be incorrect, about the liquid junction potential, whose value is assumed not to change when the concentration of the sample solution is changed. The direct consequence of this assumption is that results that are interpreted as the activity coefficients of sodium and halide ions are, instead, conventional values, which only depend on the mean activity coefficients and transport numbers, and have no connection with the activity coefficients of the respective ions.Considerations valid for all papers dealing with the experimental determination of ionic activity coefficients are derived. Such papers are to be regarded as incorrect.