Activity of Hydrochloric Acid in the Presence of Added Salts

Citation

Urmston, Joseph Winchester (1930) Activity of Hydrochloric Acid in the Presence of Added Salts. Master's thesis, California Institute of Technology. doi:10.7907/3609-3p39. https://resolver.caltech.edu/CaltechTHESIS:02272024-214831724

Abstract

[Summary] The effect of added salts without a common ion upon the activity of hydrochloric acid has been determined by measurements of the electromotive force of cells of the type

H₂, HCl c₁m + NaClO₄ c₂m, AgCl+Ag,

where e₁ = 0.25 m, 0.1 m, 0.05 m and 0.005 m, and for values of c₁ + c₂ up to 1 m.

Similar cells substituting KClO₄ for NaClO₄ were investigated over a concentration range which, however, was rather restricted due to the limited solubility of potassium perchlorate.

The results show that the activity coefficient of hydrochloric acid in the presence of the added salts without a common ion is, at similar ionic strengths, somewhat less than that of the pure acid, that is at a fixed concentration of acid the activity goes thru a minimum with increasing ionic strength and then increases in a manner which is roughly similar to that observed for pure acid solutions.

The ion attraction theory is discussed qualitatively. In dilute solutions it undoubtedly accounts for the facts upon a relatively simple physical basis, but its extension to more concentrated solutions resulting in a two constant equation and the application of this to mixtures of electrolytes is probably largely empirical in nature.

The fact that the activity coefficient of hydrochloric acid in its own pure solution, and in solution in the presence of a salt without a common ion including solutions which contain no acid, and whose properties therefore depend entirely upon those of the added salt goes thru a minimum. with increasing ionic strength shows that this electrolyte is very readily "salted out". This behaviour is probably associated with the unique structure of the hydrogen ion in that it has no electron orbits to be deformed in the strong electrical field prevailing in concentrated solutions.

The principle of linear variation of logo α with the concentration of acid at constant total molality applies to mixtures of the acid and sodium perchlorate in dilute solutions, but does not hold at concentrations from 0.25 m. to 1.0 m. In this last respect the behaviour is different from that observed when a chloride is the added salt.

The results of this investigation are discussed in terms of the effect of added salt upon the solubility of hydrochloric acid and its change in solubility with added salt is compared at similar initial solubilities with those of thallous chloride and potassium perchlorate. For hydrochloric acid in the presence of an added salt without a common ion, a solubility curve of a new type is disclosed, in that its solubility increases to a maximum and then decreases as the concentration of added salt is increased.

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