ScienceWeek

ACIDS AND BASES

The conceptual division of certain substances into acids and bases was already evident in the Middle Ages, the terms "acid", "base", and "salt" occurring in the writing of medieval alchemists. Acids were probably the first to be recognized, apparently because of their sour taste: The English word "acid", the French word "acide", the German "Sauer", and the Russian "kislota" are all derived from words meaning "sour". Of particular historical significance was the medieval introduction, apparently in the 14th century, of acids obtained from minerals, e.g., sulfuric acid and nitric acid. The discovery of the strong mineral acids has been considered the most important practical advance in chemistry after that of the successful production of iron from its ore approximately 3000 years before. Strong mineral acids made possible many chemical reactions and dissolutions of substances previously impossible: vinegar was the strongest acid known to the Greeks and Arabs. Medieval chemistry began to be transformed during the 17th century. During the 18th, 19th, and 20th centuries, the study of acid-base chemistry involved a series of significant conceptual alterations.

The following points are made by W. de Vos and A. Pilot (J. Chem. Educ. 4 Apr 01 78:494):

1) The authors point out that sulfuric acid, hydrochloric acid, nitric acid, and other acids were already known in the Middle Ages, although by other names. It was also understood that bases (or "alkalis") such as soda (sodium carbonate), potash (potassium carbonate), and lime (calcium carbonate), were the opposites of acids, and that the reaction of an acid and a base results in mutual neutralization. Litmus, obtained from lichens, was used to test for acidity or alkalinity of a solution. A "base" was originally defined as a residue that remained behind after heating or burning. Potash, obtained from plant ashes, was such a base. The association with a residue was lost when a base was redefined as any substance that can neutralize an acid. Strong and weak acids were already distinguished in the 17th century, the adjectives "strong" and "weak" referring to the ability of a strong acid to displace a weaker acid from its salts.

2) Antoine Lavoisier (1743-1794) proposed that combustion of nonmetallic elements such as sulfur, phosphorus, and carbon yields oxides that form acid solutions in water. The name "oxygen" can be translated as "acid generator", and oxygen was believed to be the essential ingredient of all acids. Thus the concept of "acid" became closely linked to the combustion of nonmetallic elements, and the corresponding oxides were called "anhydrides". Lavoisier introduced a systematic nomenclature for inorganic acids, naming each acid after the element from which it was derived: phosphoric acid, sulfuric acid, carbonic acid, etc., and in addition, naming such acids as phosphorous acid and sulfurous acid, with the suffix (-ous) indicating a lower oxygen content. As for bases, Lavoisier's view was that bases were derived from metallic elements via their oxides ("basic anhydrides"), with acids and bases reacting to form salt and water.

3) The ionic theory of acids and bases, as developed in the early 1880s by Svante A. Arrhenius (1859-1927), stated that in aqueous solutions acids and bases are ionized, completely ionized when they are strong and partly ionized when they are weak. Weak acids and bases in solution were considered to obey the laws of reaction-equilibrium theory (which holds that when a reaction system reaches equilibrium, the reactions have not stopped but are proceeding in their respective directions at equal rates). An acid came to be defined in terms of Arrhenius theory as a hydrogen-containing substance that in aqueous solution ionizes to produce hydrogen ions, a base as a substance containing the hydroxyl group (OH), which ionizes as the hydroxide ion. Ionic equations were written and degrees of ionization were calculated. (*Note #1)

4) The 20th century brought to acid-base chemistry the theories of Johannes N. Bronsted (1866-1951), Thomas M. Lowry (1874-1936), and Gilbert N. Lewis (1875-1946), all of whom published their theories in 1923, with Bronsted and Lowry working independently but publishing essentially identical ideas. Both the Bronsted-Lowry theory and the Lewis theory of acids and bases refer to protons and electrons, thus making use of atomic structure and producing an important conceptual change in the minds of chemists. The Bronsted-Lowry approach gave rise to the concept of the hydronium ion, since the theory postulates that water is a proton acceptor as well as a proton donor. An essential difference with the Arrhenius theory is that acids and bases are no longer defined in terms of substances but in terms of particles: not sodium hydroxide, but the hydroxide ion is the base. An acid-base reaction is now defined in terms of its apparent reaction mechanism, namely, the reversible transfer of a proton from the acid to the base. In a general theory of acids and bases, G.N. Lewis added the concept that an acid is a substance that can accept an electron pair, and a base is a substance that can donate a pair of electrons; these are now the definitions of a "Lewis acid" and a "Lewis base". As the authors (de Bos and Pilot) point out, these conceptualizations of acids and bases are not mutually exclusive, but are rather complimentary, involving a "layering" of concepts.

5) The authors point out that in recent years progress in chemistry has not inspired new theoretical layers in the study of acids and bases. Acids and bases are not a spearhead in modern chemical research: no university has a department of acid-base chemistry, and there is no /Journal of Acids and Bases/ in university libraries. One "acid", however, that is undergoing thorough investigation these days is deoxyribonucleic acid (DNA). The authors point out that it is ironic that the first thing that students learn about this acid is that it consists of four bases. The name "nucleic acid" expresses the observation of early biochemists that this material, when isolated from cell nuclei, was soluble in slightly alkaline solutions and could be precipitated by adding dilute acid. The name "base", in this context, refers to specific decomposition products -- purines and pyrimidines, organic nitrogen compounds known to behave as weak bases.

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Notes:

Note #1: During most of the 19th century, atoms were considered structureless and indivisible, and the idea of a stable substance like sodium chloride breaking up into ions in water was inconceivable. Arrhenius developed his ionic theory of acids and bases while he was still a student, and his ideas were dismissed by his teachers. In 1884, when Arrhenius (then 25 years old) presented his theory as part of his PhD dissertation, he experienced a rigorous 4-hour examination and was then awarded the lowest possible passing grade by his examiners.