ALIZARIN, a vegetable dyestuff prepared from the root of the madder (Rubin tinctorum), was known to the ancients, and was prepared entirely from this source until 1868. The pure sub stance crystallizes in red prisms melting at 29o° C. It is insoluble in water, and not very soluble in alcohol. It dissolves readily in caustic alkalis on account of its phenolic character, and it forms a yellow-coloured di-acetate. Its value as a dyestuff depends on its power of forming insoluble compounds (lakes) with metallic oxides. It is in the form of these lakes that it produces coloured effects on textile fibres. Alizarin red on wool is a complex lake of alizarin with the oxides of calcium and aluminium (see DYEING).
Madder contains a glucoside (q.v.), ruberythric acid, which is readily broken up (hydrolysed) by acids or ferments into alizarin and glucose: Alizarin has the chemical constitution 1.2 Dihydroxyanthraquinone and the structural formula It owes its phenolic properties to the two —OH (hydroxy) groups attached to a benzene nucleus.
The first step in the synthetical production of alizarin was the discovery in 1868 of C. Graebe and C. Liebermann that on heat ing with zinc dust, alizarin was converted into anthracene, an aromatic hydrocarbon obtained from coal tar. They then oxidized anthracene to anthraquinone and brominated the quinone. The dibrominated product so obtained was fused with caustic potash, the melt dissolved in water, and on the addition of hydrochloric acid to the solution, alizarin was precipitated. This process, owing to its expensive nature, was soon superseded by another, dis covered simultaneously by the above-named chemists and by Sir W. H. Perkin, the method being to sulphonate anthraquinone, and then to convert the sulphonic acid into its sodium salt and fuse this with caustic soda. Alizarin is also synthesised by heating catechol with phthalic anhydride and sulphuric acid at 150° C. ALKAHEST (a pseudo-Arabic word believed to have been invented by Paracelsus), a liquid having the power of dissolving gold and every other substance, much sought after by the alchem ists, who supposed it would possess invaluable medicinal qualities. ALKALI, an Arabic term originally applied to the ashes of plants, from which by lixiviation carbonate of soda was obtained in the case of sea-plants and carbonate of potash in that of land plants. The method of making these "mild" alkalis into "caustic" alkalis by treatment with lime was practised in the time of Pliny in connection with the manufacture of soap, and it was also known that the ashes of shore-plants yielded a hard soap and those of land-plants a soft one. But the two substances were generally confounded as "fixed alkali" (carbonate of ammonia being "vola tile alkali"), till Duhamel du Monceau in 1736 established the fact that common salt and the ashes of sea-plants contain the same base as is found in natural deposits of soda salts ("mineral alkali"), and that this body is different from the "vegetable alkali" obtained by incinerating land-plants or wood (pot-ashes). Later, Martin Heinrich Klaproth, finding vegetable alkali in cer tain minerals, such as leucite, proposed to distinguish it as potash, and at the same time assigned to the mineral alkali the name natron, which survives in the symbol, Na, now used for sodium. The word alkali supplied the symbol for potassium, K (kalium). In modern chemistry alkali is a general term used for compounds which have the property of neutralizing acids, and is applied more particularly to the highly soluble hydroxides of sodium and potas sium, and of the three rarer "alkali metals," caesium, rubidium and lithium, also to aqueous ammonia. In a smaller degree these alkaline properties are shared by the less soluble hydroxides of the "metals of the alkaline earths," calcium, barium and stron tium, and by thallous hydroxide. An alkali is distinguished from an acid or neutral substance by its action on litmus, turmeric and other indicators. (See ALKALI MANUFACTURE.)