Hydrochloric acid, activity coefficient ratios

Activity coefficient ratios in the second term were approximated by known values of the square of the mean activity coefficient of HC1, and it was shown that the rate increase in hydrochloric acid (in comparison to perchloric acid at the same h0) depends on the first power of the chloride ion concentration. 

A plot of some typical activity coefficient ratios as a function of the square root of the concentration is shown in Fig. 1. The emf data for hydrochloric acid are from the work of Harned and Ehlers10 on cells of the type represented by equation (21). The mean activity coefficient ratios were computed by arbitrarily assuming that the most dilute solution, 0.003205 normal (which is the most nearly ideal) has the activity coefficient of unity. It will be observed that as the concentration increases the mean activity coefficient, on this arbitrary basis, steadily decreases whereas, if the ions were perfect solutes the activity coefficient ratios would be unity throughout the concentration range. The shift of activity coefficient ratios in the range of concentration included in this plot is roughly typical of the behavior of uni-univalent electrolytes. Fig. 1 also shows the activity coefficient ratios of zinc sulphate from the work of Cowperthwaite and LaMer,11 based on emf measurements at 25°, of the cell... 

Fig. 1. Concentration Dependence of the Activity Coefficient Ratios of Hydrochloric Acid and Zinc Sulphate.

For the purpose of exact calculation of the liquid junction potential according to the last mentioned equation the activity coefficient of the hydrogen or chloride ions would have to be known. As their value is unknown it is assumed that their ratio is equal to the ratio of the mean activity coefficients of hydrochloric acid, or in other words ( +)2/( +)i — (a+)2l(a )v Then the equation (VI-28) will be written in this form ... 

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