47.134

On the Affinities of Polybasic Acids. By Bevan Lean, D.Sc., B.A., Assistant Lecturer, and late Bishop Berkeley Fellow of Owens College. Communicated by W. H. Perkin, jun., F.R.S.

Received October 30th, 1894.

Introduction.

The author has described elsewhere1 some tetracarboxylic acids which possess some very remarkable properties. They are derivatives of butane, of the general formula

in which R represents the alkyl group methyl, ethyl, cetyl or benzyl, and, although they contain four carboxyl groups, these acids do not in all cases behave as tetrabasic acids.

On determining their basicity by titration with standard solution of potassium hydrate, some of these acids show dibasicity notably is this the case with dibenzylbutanetetracarboxylic acid, the result being the same whether phenol phthalein or litmus solution is used as the indicator. On the other hand, dimethyl-, diethyl-, and dicetyl- butanetetracarboxylic acids, on being titrated with potassium hydrate, give different results, according as phenol phthaleïn or litmus is employed as an indicator. They behave as tetrabasic acids when phenol phthalein is employed. If, however, one or two drops of litmus solution be added to

1 "On Ethyl Butanetetracarboxylate and its Derivatives,” a Thesis accepted for the degree of Doctor of Science of the University of London.

B

the solution of these acids in potassium hydrate, which, as shown by phenol phthaleïn, has been neutralised by hydrochloric acid, a distinctly blue coloration is produced. On adding more hydrochloric acid, the blue coloration changes gradually to a red tint, and the solution appears to become really neutral only when sufficient hydrochloric acid has been added to neutralise one half of the potassium hydrate, which was equivalent, as shewn by phenol phthaleïn, to the tetracarboxylic acid present.

In view of these results, obtained in determining the basicity of the acids, it appeared of interest to examine some of their salts. While the silver and calcium salts prepared in the usual way from dimethyl and diethylbutanetetracarboxylic acid are found to be tetrabasic, those prepared from dibenzylbutanetetracarboxylic acid are found to have the formulae C22H20O8Ag2, and C22H20O8Ca + 2 H2O respectively; so also the salts prepared from dicetylbutanetetracarboxylic acid are found to be dibasic.

On the different behaviour of Indicators.

By these observations is brought out, in the first place, the fact of the different behaviour of indicators towards one and the same acid. It has been shown that the tetrabasic potassium salts of dimethyl-, diethyl-, and dicetyl- butanetecarboxylic acids are neutral to phenol phthaleïn, but alkaline to litmus, and in titrating these acids with litmus the final colour-changes are very indefinite.

To illustrate further the different behaviour of indicators, reference may be made to a systematic investigation of the use of litmus, methyl-orange, and phenol-phthaleïn as indicators, which was carried out by Smith1 in 1883. The following table, compiled from his results, shows the character of the final colour-changes with several organic acids :

1Smith, Chem. News, 47-136. (1883).

Engel', too, has shewn that when a solution of phosphoric acid is titrated it appears monobasic if methyl-orange is used as the indicator, but dibasic with phenol-phthaleïn, and tribasic with Poirier's soluble blue. He also showed that boric acid is neutral to methyl-orange, feebly acid to litmus or phenol-phthaleïn, and acid to soluble blue.

From these instances it appears that litmus is liable to give lower results than phenol-phthaleïn, and in the case of litmus, the final colour-change is usually very indefinite. It also is evident that any knowledge of the basicity of an acid obtained by titration is only relative to the nature of the indicator employed. And, further, when a polybasic acid has been only partially neutralised its acid character may be enfeebled altogether out of proportion to the amount of a base which has been added to it.

This raises the wider question of the basicity of a poly. basic acid, and of its affinity as a function of the affinity of any one of the replaceable hydrogen atoms which it contains.

On the Affinities of Polybasic Acids.

It will be in place, at the outset, to point out the essential character of an acid. The evolution of an all-embracing yet sufficient definition of an acid has been a gradual one, and it is of no small interest to trace the views of Paracelsus, Boyle, Stahl, and Becker, the oxygen-acid theory of Lavoisier, and Berzelius, and the hydrogen-acid views of

1 Comptes Rendus, 102, 262 (1886).

Davy, Dulong, and Liebig. It may be remarked, too, that after the work of Kolbe and Frankland, Kekulé stated that the basicity of an organic acid was determined solely by the number of carboxyls which it contained-a conclusion which can only be maintained by defining an organic acid as a substance containing carboxyl. The following definition of an acid states as clearly as may be the modern conception of an acid; it is due to Ramsay :

"An acid is a compound of hydrogen, which when mixed with or dissolved in water, is capable of exchanging the whole or a portion of the hydrogen which it contains for a metal, with simultaneous formation of water, by the action on the aqueous solution of the acid, of a metallic oxide or hydroxide."

Substances which come under this definition are found to contain hydrogen in intimate combination with one or more of the following: fluorine, chlorine, bromine, iodine, oxygen, sulphur, selenium, tellurium, or certain groups of elements of which carbon is one. Amongst organic compounds there are many besides carboxyl derivatives which can exchange hydrogen for a metal by the action of an oxide or hydroxide. The chief of these are certain acid ethereal salts, the mercaptans, many nitro-derivatives of the alkyl radicals, and phenols. From a consideration of these compounds Ramsay has drawn the following deductions :

(a) A powerfully electro-negative element such as chlorine bromine, or iodine, confers acid properties on its compound with hydrogen.

(b) In compounds of elements exhibiting less markedly electro-negative properties than the halogens, the presence of an electro-negative element is necessary for the development of acid character. In illustration of this the following series of compounds may be noted:methane-methyl alcohol-prussic acid.

anthracene-anthraquinone-alizarine.

The study of the basicity of acids has done much to enlarge our knowledge of acids. From the dualistic stand