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A base which has more affinity for protons than the limiting base cannot exist in solution, as it will react with the solvent. The addition of pure acetic acid and the addition of ammonium acetate have exactly the same effect on a liquid ammonia solution: The pKa value of ammonia is estimated to be approximately 34 c.
Gas phase acidities normally known as proton affinities can be measured, and their relative order is often quite different from that of the aqueous acidities of the corresponding acids.
This is the case for acetic acid, which is completely dissociated in liquid ammonia solution. This is a stronger base than the hydroxide ion and so cannot exist in aqueous solution.
Comparisons of acidity and basicity between solvents[ edit ] There exists a large corpus of data concerning acid strengths in aqueous solution pKa valuesand it is tempting to transfer this to other solvents. Many carbon anions can be formed in liquid ammonia solution by the action of the amide ion on organic molecules see sodium amide for examples.
However, the hydroxide ion is often a much stronger base in nonaqueous solvents e. Few quantitative studies on acidities in nonaqueous solvents have been carried out, although some qualitative data are available.
Any acid which is a stronger acid than the ammonium ion will be a strong acid in liquid ammonia. The other extreme is a superacida medium in which the hydrogen ion is only very weakly solvated.
It appears that most acids which have a pKa value of less than 9 in water are indeed strong acids in liquid ammonia. An acid which has more of a tendency to donate a hydrogen ion than the limiting acid will be a strong acid in the solvent considered, and will exist mostly or entirely in its dissociated form.
For example, the limiting acid in liquid ammonia is the ammonium ion, which has a pKa value in water of 9.
Bases can exist in solution in liquid ammonia which cannot exist in aqueous solution: Such comparisons are, however, fraught with danger, as they only consider the effect of solvation on the stability of the hydrogen ion, while neglecting its effects on the stability of the other species involved in the equilibrium.
The classic example is a mixture of antimony pentafluoride and liquid hydrogen fluoride: