ACID, the name loosely applied to any sour substance; in chemistry it has a more precise meaning, denoting a substance containing hydrogen which may be replaced by metals with the formation of salts. An acid may therefore be regarded as a salt of hydrogen. A still more recent and broader conception is that of an acid as a "proton donator" (T. M. Lowry) i.e., a substance which is capable of yielding a hydrogen atom stripped of its elec tron to a "proton acceptor" or base. Of the general characters of acids we may here notice that they dissolve alkaline substances, certain metals, etc., neutralize alkalis and redden many blue and violet vegetable colouring matters such as litmus.
The ancients probably possessed very little knowledge of acids. Vinegar (or impure acetic acid), which is produced when wine is allowed to stand, was known to both the Greeks and Ro mans, who considered it to be typical of acid substances; this is philologically illustrated by the words Oin, acidus, sour, and 6Eos acetus, vinegar. Other acids became known during the alchemistic period ; and the first attempt at a generalized conception of these substances was made by Paracelsus, who supposed them to contain a principle which conferred the properties of sourness and solu bility. - Somewhat similar views were promoted by Becher, who named the principle acidum primogenium, and held that it was composed of the Paracelsian elements "earth" and "water." At about the same time Boyle investigated several acids and estab lished their general characteristics.
The phlogistic theory of the processes of calcination and com bustion necessitated the view that many acids, such as those pro duced by combustion, e.g., sulphurous, phosphoric, carbonic, etc., should be regarded as elementary substances. This principle more or less prevailed until it was overthrown by Lavoisier's doctrine that oxygen was the acid-producing element ; Lavoisier was led to this conclusion by the almost general observation that acids were produced when non-metallic elements were burnt. This doctrine, in turn, was overthrown by the researches of Davy, Gay-Lussac and Thenard on hydrochloric acid and chlorine, and of Gay-Lussac on hydrocyanic acid, which established beyond all cavil that oxygen was not essential to acidic properties. Davy and, almost simultaneously, Dulong suggested that hydrogen and not oxygen was the acidifying principle.
Dalton's theory was strongly supported when J. Liebig pro moted his doctrine of polybasic acids. Dalton's idea that elements preferentially combined in equiatomic proportions had as an imme diate inference that metallic oxides contained one atom of the metal to one atom of oxygen, and a simple expansion of this con ception was that one atom of oxide combined with one atom of acid to form one atom of a neutral salt. This view, which was specially supported by Gay-Lussac and Leopold Gmelin and accepted by Berzelius, necessitated that all acids were monobasic. The untenability of this theory was proved by Thomas Graham's investigation of the phosphoric acids ; for he then showed that the ortho- (ordinary), pyro- and meta-phosphoric acids contained respectively 3, 2 and I molecules of "basic water" (which were replaceable by metallic oxides) and one molecule of phosphoric oxide, Graham's work was developed by Liebig, whose researches on organic acids—citric, tartaric, cyanuric, comenic and meconic—led him to formulate as the criterion of polybasicity the existence of compound salts with different metallic oxides, and ultimately to adopt the tenet that "acids are particular compounds of hydrogen, in which the latter can be replaced by metals." Fur ther, he held that "neutral salts are those compounds of the same class in which the hydrogen is replaced by its equivalent in metal." The hydrogen theory and the doctrine of polybasicity as enun ciated by Liebig is the fundamental characteristic of the modern theory. A polybasic acid contains more than one atom of hydro gen which is replaceable by metals ; moreover, in such an acid the replacement may be entire with the formation of normal salts, partial with the formation of acid salts, or by two or more differ ent metals with the formation of compound salts (see SALTS). These facts may be illustrated with the aid of orthophosphoric acid, which is tribasic: Normal salt. Acid salts. Compound salt.
Ag3PO4. Na2HPO4 NaH2PO4. Na (NH4) HPO4.
Silver phosphate. Acid sodium Microcosmic phosphates. salt.
This conception of acids was extended along the lines of the electrolytic dissociation theory of S. A. Arrhenius (1887). Pure hydrogen chloride is practically a non-conductor of the electric current, and so also is pure water ; when the two are mixed, how ever, highly conducting solutions of hydrochloric acid result. Ac cording to the Arrhenius hypothesis, the hydrogen chloride mole cule was split into two "ions"—a positive hydrogen ion, written as H± or IF and a negative chlorine ion, or Cl'—each of which contributed towards the carrying of the current ; all the funda mental properties of acids were attributed to the existence of the hydrogen ion in their aqueous solutions. Objections often urged against this view were (I) that the chlorine ion shows none of the peculiar properties of the element, and (2) that, since hydro gen and chlorine combine so strongly with such a large evolution of heat, it was difficult to understand how the mere process of dissolution in water could suffice to separate them as ions. To a large extent these objections are met by the current views on the electronic structure of atoms (q.v.), according to which the chlorine atom, holding seven valency electrons, achieves its maxi mum stability by acquiring the single electron of the hydrogen atom to form an octet and to become negatively charged, whereas the hydrogen atom, having lost its electron and become positively charged is now unique in that it holds no electrons at all (i.e., it is what is called a proton) ; these ions differ f undamentally, therefore, from their corresponding elements.
It will be noted that, although the solvent (water, in the case discussed) was stated to be essential for the development of the acidic properties of hydrogen chloride, the various theories described attach no significance to the role of this solvent. H. E. Armstrong has consistently pointed out this anomaly, and he and many others (A. Lapworth, H. Geldschmidt, and T. M. Lowry, for example) have attempted to remove it ; for Armstrong's views his writings should be consulted (Proc. Roy. Soc., 1923, A, 103, p. 610), but it is possible to generalize as to the trend of other workers' views. These are, briefly, that the hydrogen ion (proton) becomes associated with one or more molecules of solvent to give positive ions, of which the simplest would be OH:. Here again there is a divergence of opinion, because some attribute the pecu liar properties of acids (especially that of catalysis [q.v.]) to this complex ion, whereas others ascribe them to a small or even a very minute portion of hydrogen ions which remain as such and escape this association with the solvent.
Studies of the absorption spectra and other properties of acids, their esters, their dilute solutions, and their saline solutions have led to the belief, more especially in the case of oxygen-containing acids, that the pure or concentrated acid has a structure resem bling that of the esters (A. Hantzsch, K. Schafer), whereas in dilute aqueous solution its structure is analogous to that of the salts. In this respect, the pure acid is what Hantzsch terms a pseudo-acid—that is, it only functions as a true acid after having undergone a change of structure. Such a change is indicated by Schafer's scheme below, which, however, is merely diagrammatic and has no other significance.
It is remarkable that very pure concentrated acids are inert towards metals in many cases. V. H. Veley has made a study of the properties of pure nitric acid, which, according to Hantzsch, has the same absorption spectrum as its vapour or its esters, but quite different from that of dilute solutions. Hantzsch (1925) found similar relationships to hold for hydrobromic and hydri odic acids, and he therefore suggests that aqueous solutions of acids are salts of the hypothetical "hydroxonium" radical, thus: hydroxonium bromide; hydroxonium ni trate. His views receive support from the isolation of "nitronium" salts from very concentrated solutions of acids; thus, nitronium diperchlorate, [N(OH)3] and nitronium monoperchlc rate, are stable crystalline salt-like compounds which can be recrystallized from concentrated perchloric or nitric acid, respectively. Moreover, mixtures of concentrated nitric and sulphuric acids are highly conducting, whereas the pure acids alone are only feebly so ; this is regarded as evidence in favour of the existence of salt-like compounds in the solution, and analogous to the perchlorates, and the isolation (by Weber, 1871) of nitronium pyrosulphate, [N(OH)3]S207, affords similar evidence. By analogy, therefore, concentrated nitric acid contains a certain amount of nitronium nitrate, (NO3)3.
Reference should be made to the articles CHEMICAL ACTION and SOLUTIONS for the theory of the strength or avidity of acids. Organic Acids.—Purely organic acids are characterized by the presence of the univalent group—CO.OH, termed the carboxyl group, in which the hydrogen atom is replaceable by metals with the formation of salts, and by alkyl radicals with the formation of esters. The basicity of an organic acid, as above defined, is deter mined by the number of carboxyl groups present. Hydroxy-acids are carboxylic acids which also contain a hydroxyl group; simi larly we may have aldehyde-acids, ketone-acids, etc. Since the more important acids are treated under their own headings, or under substances closely allied to them, we shall here confine our selves to general relations.
It is convenient to distinguish between aliphatic and aromatic acids ; the first named being derived from open-chain hydrocar bons, the second from ringed hydrocarbon nuclei. Aliphatic mono basic acids are further divided according to the nature of the parent hydrocarbon. Methane and its homologues give origin to the "paraffin" or "fatty series" of the general formula ethylene gives origin to the acrylic acid series, and so on. Dibasic acids of the paraffin series of hydrocarbons have the general formula C,1127., (COOH)2; malonic and succinic acids are important members. It is evident from the foregoing general formulae that the complications due to iso merism will increase with the length of the chain of carbon atoms, and for further information on the higher or more complicated acids the reader is referred to any standard work on organic chemistry (see CHEMISTRY : Organic). A list of certain of the acids present in fats and oils is given in the article GLYCERIDES.
Phenylopropionio acid, is eliminated in urine as hippuric acid, Mention should also be made of syntheses of organic acids through the Grignard reaction (q.v.) and through malonic ester and acetoacetic ester (q.v., and CHEMISTRY : Organic). By elec trolysing a solution of potassium ethyl succinate, the groups are split off and the two residues • combine to form the ester (CH2)4(CO2C2/15)2. In the same way, by electrolysing a mixture of a metallic salt and an ester, other nuclei may be condensed ; thus potassium acetate and potassium ethyl succinate yield (A. D. M.; G. T. M.) Lunge and Cummings, Manufacture of Acids and Alkalis (1923) .