On acid mixtures containing hydrochloric and succinic acids in aqueous solution

Abstract

The PH of hydrochloric-succinic acid solutions (in the range 0.002–0.05N HCl) decreases by the progressive addition of succinic acid to a given hydrochloric acid solution. The apparent increase in molality of the strong acid was explained on the basis of a “dehydration effect” caused by the suppressed weak acid present in solution as a non-electrolyte, when the concentration of HCl is above 0.02N. The water molecules attracted by the weak acid molecule were made at the expense of the water molecules present in the hydration sheathes of the ions. The results were readily explained by the equation: $$p_H = p_{H_0 } - M\left( {\alpha + {{0.072} \mathord{\left/ {\vphantom {{0.072} {2.303}}} \right. \kern-\nulldelimiterspace} {2.303}}} \right)$$ , whereα is an interaction coefficient whose value is equal to 0.39±0.02. The optical density of the mixed acid solutions tinted with thymol blue increases linearly with the increase in succinic acid concentration. The value ofα from optical density measurements was not constant due to the assumption that the activity coefficient of the indicator ion remained constant.