Electrolysis : Theory Of Acids And Bases

In aqueous solutions of acids, the hydrogen ion is probably attached to a molecule of water, forming the hydroxonium ion, H₃O˙; H˙ + H₂O = H₃O˙. Regarded as the result of the removal of an electron from a hydrogen atom, the ion H' is the free proton, and this could scarcely be expected to exist in the free state in solution to any appreciable extent. The hydrogen ion is probably also solvated in other media besides water.

In the theory of acids and bases developed by Arrhenius, the hydroxide ion of bases occupies as unique a position as the hydrogen ion of acids. Another theory, first developed by Lapworth (1908), defines a base as a substance which unites with hydrogen ion, or, in the more hypothetical terminology of modern theory, a base is an acceptor of protons, whilst an acid is a donor of protons. Whereas all acids must have a common property, this is not necessary in the case of bases. A neutral ammonia molecule may accept a proton to form the ammonium ion, and thus functions as a base:

NH₃ + H˙ = NH₄˙,

and this is just as much a neutralisation reaction as is the acceptance of a proton by the hydroxide ion:

OH' + H˙ = H₂O.

The NH₃ molecule may abstract protons from water:

NH₃ + H₂O <=> NH₄˙ + OH', and in this case the greater strength of ammonia as compared with water as a base is manifested by the appearance of OH' ions, i.e., by an alkaline reaction.

Lapworth's definition of bases shows that the acetate ion, CH₃COO^-, is a strong base, since when a strong acid such as hydrochloric, which is practically completely ionised, is added to solution of sodium acetate, largely ionised into Na⁺ and CH₃COO^-, the acetate ions unite with practically all the hydrogen ions, forming the weakly ionised acetic acid molecules, CH₃COOH.

The anion of any weak acid is, in fact, able to function as a strong base. When a strong acid is neutralised with sodium carbonate the reaction is, e.g.,

The weak carbonic acid is also unstable, and decomposes almost completely, carbon dioxide escaping with effervescence. In aqueous solutions of sodium carbonate, the CO₃^-- ions withdraw hydrogen ions from the water, forming the very weak carbonic acid and leaving hydroxide ions of the water, which make the solution alkaline and "basic":

CO₃^-- + 2H₂O <=> H₂CO₃ + 2OH^-.

The real base is, however, not Na₂CO₃, but the carbonate ion, CO₃^--, a powerful hydrogen ion acceptor. Reactions of this type are called hydrolysis.

Protein crystallography
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Electrolysis
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