POLYDA'SIC ACIDS. Most of the inorganic acids combine with bases in such a man ner that one atom of the acid is united with one atom of a metallic oxide to form a neu tral salt. Nitric acid may be taken as an illustration of the acids possessing this prop erty, and which may therefore be called monobasic. In other cases, as, for example, that of pyrophosphoric acid (see Pnosrnonus), one atom of acid possesses the property of combining with two atoms of base; such acids are termed bibasic or dibasic. There are strong grounds for believing that sulphuric acid is bibasic, in which case its formula would require to be doubled, and to be written Common phosphoric and arsenic acids are examples of a third class of acids in which one atom combines with three atoms of base, and Mitch are therefore termed tribasic. Whether any polybasic acids beyond tribasic acids exist, is uncertain, but it is probable that sinew acid is a tetrabasic acid.
Amongst the organic acids a similar relation takes place, acetic, succinie, and citric acids affording examples of the moqobasic, dibasic, and tribasic class.
The following are the most important general differences shown by acids of different degrees of basicity: 1. Each monobasie acid can form but one ether, which is neutral. 2. A monobas.'e
acid cannot form a stable, well-defined acid salt or in salt with two or more metallic bases.
1. Each dibasic acid can form tiro ethers, one neutral. and the other acid. 2. Dibattie acids can form with each metallic base a neutral salt and an acid salt. They can also form double salts containing two metallic bases.
1. Each trthaeie acid can form three ethers, one neutral, and two acid. 2. Tribasie acids can form three salts with the same metallic base, two of them acid and one neutral.
Many attempts have been made to account for the polvbasie or monobasic character of an acid, from its composition. According to Keltul6 (Lehrbuch der Organisch. Wand& vol. i. p. 210-210), the basicity depends cot, as was formerly supposed. on the molecular constitution of the acid, but upon the amount of oxygen contained in its radical. For further details on this subject, the reader is referred to the article ACIDS in Watt's Dktionary of Chentistry,-crol. i. 1863.