AN′THOTYPE. See Photography.
ANTHRACENE. C14H10. Anthracene is one of the last products passing over in the dry distillation of coal-tar. Dr Calvert says it is “found most abundantly in the ten or fifteen per cent. which comes over between the temperature at which soft pitch is produced and that at which hard pitch is formed.”
Coal-tar contains very variable quantities of anthracene, those tars procured from coals which are richest in naphtha yielding it most abundantly. The coals of South Staffordshire give the largest yield, whilst the Newcastle coals give very little. In consequence of the solubility of anthracene in the oily hydrocarbons which accompany it, owing to “slight elevation of temperature, its extraction can only be carried on advantageously in cold weather.”
Gessert prepares anthracene from coal-tar as follows: He places the last pasty portions (the ‘green grease’) of the coal-tar distillation (which must not be carried beyond the point at which white pitch is formed) first in a centrifugal machine, and then in a hydraulic press at 40°, or subjects the mass heated to 30°-40° directly to pressure in a filter press. The pressed mass consists of about 60% of anthracene; for further purification it is boiled with light tar-oil or petroleum naphtha, and finally heated till it melts. The residue contains 95% of anthracene.
The following method for the purification of crude anthracene contaminated with oily matters is by Schuller:—The crude anthracene is carefully heated to commencing ebullition in a capacious retort connected with a tubulated receiver of glass or earthenware, the lower aperture of which is closed with a fine wire sieve. A strong current of air is
then blown into the retort with a pair of bellows, whereby the anthracene is driven over in a very short time nearly pure and dry, and condenses in the receiver as a faintly yellowish showy mass. By this method a quantity of anthracene, the purification of which by re-crystallisation or sublimation would take several days, may be purified in as many hours; moreover it is obtained in a pulverulent form, in which it is very readily acted on by oxidising agents. Anthraquinone prepared from crude anthracene may also be obtained by this method in the form of a light yellow powder, resembling flowers of sulphur.
Fritzsche obtained anthracene in crystals exhibiting a beautiful violet colour by exposing a solution of anthracene in coal-tar naphtha to sunshine, until the solution became colourless.
Pure anthracene assumes the form of fluorescent transparent crystals, consisting of four- or six-sided plates, which when seen by transmitted light are of a very pale blue colour, but of a pale violet by reflected light.
The process for obtaining pure anthracene is a very troublesome one. Mr Crookes says:—“A trustworthy method for determining the amount of pure anthracene either in commercial anthracene or in crude green grease is the following:—The melting-point of the sample in question is first determined. 5 to 10 grammes are sufficient for the operation. It is put between thick folds of blotting paper, and placed under a press, between plates which have been previously warmed. The anthracene remaining upon the paper after pressure is weighed. The residue after it has been boiled with a certain quantity of alcohol, filtered, washed with cold alcohol and dried, is weighed as pure anthracene. It is now advisable to determine the melting-point of the purified product, which will generally be 210° C.” Anthracene is only slightly soluble in alcohol, but rather more so in ether and bisulphide of carbon. It is more soluble in hot, but less so in cold benzene. Petroleum boiling between 160° and 195° F. dissolves less than benzene.
“Anthracene dissolves in concentrated sulphuric acid with a green colour, and forms conjugated monsulpho or bisulpho-anthracene acid, according to the temperature employed. Chlorine and bromine give rise to substitution products. Nitric acid acts on it with great violence, with formation of anthraquinone, nitro-anthraquinone, and other compounds according to the temperature and proportion of the substances taken. With picric acid anthracene forms a compound crystallising in very bright ruby-red needles, which by the aid of the microscope are seen to be prisms. To prepare it a saturated solution of picric acid in water at 80° F. is mixed with a saturated solution of anthracene in boiling alcohol; on cooling the compound is deposited in the crystalline state. It is rapidly decomposed by an excess of alcohol into picric acid and anthracene, the solution assuming a yellow tint. This reaction can be employed to distinguish anthracene from naphthalene and other hydrocarbons, naphthalin under similar circumstances forming a compound which crystallises in fine golden yellow needles, whilst chrysene gives rise to clusters of very small yellow needles.” (Calvert’s ‘Dyeing and Calico Printing,’ edited by Stenhouse and Groves). Another characteristic of anthracene, noticed by Fritzsche, is its deportment under the microscope with a solution of binitro-anthraquinone in benzene. In this reaction fine rhomboidal scales of a beautiful pink colour are formed, the purity and brilliancy of the colour depending on the purity of the anthracene.