Page:EB1911 - Volume 01.djvu/716

 can also be synthesized by heating catechol with phthalic anhydride and sulphuric acid at 150° C.



Pure alizarin crystallizes in red prisms melting at 290° 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 has no affinity for vegetable fibres, and consequently cotton goods must be mordanted before dyeing with it (see .)

Numerous derivatives of alizarin are known. On solution in glacial acetic acid and addition of nitric acid, -nitroalizarin (alizarin orange) is produced, and this on heating with sulphuric acid and glycerin is converted into alizarin blue.

The trioxyanthraquinones—purpurin, anthrapurpurin, anthragallol and flavopurpurin—are also very valuable dyestuffs. These compounds may be represented by the following formulae:



Purpurin (1⋅2⋅4 trioxyanthraquinone) is found with alizarin in madder root; it is now prepared synthetically by oxidizing alizarin with manganese dioxide and sulphuric acid. After the separation of the silver salt (see above) obtained on sulphonating anthraquinone, the remaining acid liquid gives on treatment with calcium carbonate the calcium salt of anthraquinone 2⋅6 disulphonic acid (anthraquinone--disulphonic acid). This is converted into the sodium salt by means of sodium carbonate, and on alkali fusion yields flavopurpurin. In a similar manner anthrapurpurin is prepared by alkali fusion of anthraquinone 2⋅8 disulphonic acid. Anthragallol is synthetically prepared by the condensation of benzoic and gallic acids with sulphuric acid or from pyrogallol and phthalic anhydride in the presence of sulphuric acid or zinc chloride.

A. Baeyer in 1890, by heating alizarin with fuming sulphuric acid for 24-48 hours at 35-40° C., obtained a product, which after treatment with caustic soda gave a sulphuric acid ester of quinalizarin, and this after acidification and boiling was converted into quinalizarin (Alizarin Bordeaux) or 1⋅2⋅6⋅9 tetra-oxyanthraquinone. Penta-oxyanthraquinones have been obtained from purpurin and anthrapurpurin, while a hexa- oxyanthraquinone has been obtained from 1⋅5 dinitro-anthraquinone.

ALKAHEST (a pseudo-Arabic word believed to have been invented by Paracelsus), a liquid, much sought after by the alchemists, having the power of dissolving gold and every other substance, which it was supposed 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 connexion 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 “volatile 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 certain 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 hydrates of sodium and potassium 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 hydrates of the “metals of the alkaline earths,” calcium, barium and strontium, and by thallium hydrate. An alkali is distinguished from an acid or neutral substance by its action on litmus, turmeric and other indicators.

ALKALI MANUFACTURE. The word “alkali” denotes both soda and potash, but by “alkali manufacture” we understand merely the manufacture of sodium sulphate, carbonate and hydrate. The corresponding potash compounds are not manufactured in the United Kingdom, but exclusively in Germany (from potassium chloride and from the mother-liquor of the strontia process in the manufacture of beetroot sugar) and in France (from vinasse). The term alkali is employed in a technical sense for the carbonate and hydrate (of sodium), but since in the Leblanc process the manufacture of sodium sulphate necessarily precedes that of the carbonate, we include this as well as the manufacture of hydrochloric acid which is inseparable from it. We also treat of the utilization of hydrochloric acid for the manufacture of chlorine and its derivatives, which are usually comprised within the meaning of the term “alkali manufacture.” A great many processes have been proposed for the manufacture of alkali from various materials, but none of these has become of any practical importance except those which start from sodium chloride (common salt); and among the latter again only three classes of processes are actually employed for manufacturing purposes, viz. the Leblanc, the ammonia-soda, and the electrolytic processes.

The Leblanc process, which was invented by (q.v.) about 1790, begins with the decomposition of sodium chloride by sulphuric acid, by which sodium sulphate and hydrochloric acid are produced. The sodium sulphate is afterwards fluxed with calcium carbonate and coal, and a mixture is thus obtained from which sodium carbonate can be extracted by exhausting it with water.

Leblanc himself for a time carried out his process on a manufacturing scale, but he was ruined in the political troubles of the time and died by his own hand in 1806. His invention was, however, at once utilized by others in France; and in Great Britain, after a few previous attempts on a small scale, it was definitely introduced by (q.v.) in 1823. From that time onward the Leblanc process spread more and more, and for a considerable period nearly all the alkali of commerce was made by it. The rise of the ammonia-soda process (since 1870) gradually told upon the Leblanc process, which in consequence has been greatly restricted in Great Britain and Germany, and has become practically extinct in all other countries, except as far as its first part, the manufacture of sodium sulphate and hydrochloric acid, is concerned.

The production of alkali in Great Britain, soon after the introduction of the Leblanc process, became the most extensive in the world, and outstripped that of all other countries put together. With the rise of the ammonia-soda process, for which the economic conditions are nearly as favourable in other countries, the predominance of Great Britain in that domain has become less, but even now that country produces more alkali than any other single country. Most of the British alkali works are situated in South Lancashire and the adjoining part of Cheshire, near the mouth of the Tyne and in the West of Scotland.