The properties of the precipitate are as follows:—It is crystalline, and under the microscope is seen to consist of fine stars of needles; its smell is peculiar; it is insoluble in water and acid liquids, but soluble in alkalies, ether, and absolute alcohol; a very minute quantity of water suffices to precipitate it from an alcoholic solution; it is therefore essential to the success of the test that the watery liquid to be examined is either neutral or acid in reaction.
§ 231. Tri-bromo-phenol may be used for the quantitative estimation of carbolic acid, 100 parts of tri-bromo-phenol are equal to 29·8 of carbolic acid; by the action of sodium amalgam, tri-bromo-phenol is changed back into carbolic acid.
That bromine-water precipitates several volatile and fixed alkaloids from their solutions is no objection to the bromine test, for it may be applied to a distillation product, the bases having been previously fixed by sulphuric acid. Besides, the properties of tri-bromo-phenol are distinct enough, and therefore there is no valid objection to the test. It is the best hitherto discovered. There are also other reactions, such as that Millon’s reagent strikes a red—molybdic acid, in concentrated sulphuric acid, a blue—and potassic dichromate, with sulphuric acid, a brown colour—but to these there are objections. Again, we have the Euchlorine test, in which the procedure is as follows:—A test-tube is taken, and concentrated hydrochloric acid is allowed to act therein upon potassic chlorate. After the gas has been evolved for from 30 to 40 seconds, the liquid is diluted with 11⁄2 volume of water, the gas removed by blowing through a tube, and solution of strong ammonia poured in so as to form a layer on the top; after blowing out the white fumes of ammonium chloride, a few drops of the sample to be tested are added. In the presence of carbolic acid, a rose-red, blood-red, or red-brown tint is produced, according to the quantity present. Carbolic acid may be confounded with cresol or with creasote, but the distinction between pure carbolic acid, pure cresol, and creasote is plain.
§ 232. Cresol (Cresylic Acid, Methyl-phenol),
—There are three cresols—ortho-, meta-, and para-. Ordinary commercial cresol is a mixture of the three, but contains but little ortho-cresol; the more important properties of the pure cresols are set out in the following table:—
| Melting-point. | Boiling-point. | Converted by fusion with Potash into— | |
|---|---|---|---|
| Ortho-, | 31-31·5° C. | 188·0° | Salicylic Acid (Ortho-oxybenzoic acid). |
| Meta-, | Fluid at ordinary temperature. | 201·0° | Meta-oxybenzoic acid. |
| Para-, | 36° | 198° | Para-oxybenzoic acid. |
Pure ortho-, meta-, or para-cresol have been obtained by synthetical methods; they cannot be said to be in ordinary commerce.
Commercial cresol is at ordinary temperatures a liquid, and cannot be obtained in a crystalline state by freezing. Its boiling-point is from 198° to 203°; it is almost insoluble in strong ammonia, and, when 16 volumes are added, it then forms crystalline scales. On the other hand, carbolic acid is soluble in an equal volume of ammonia, and is then precipitated by the addition of 11⁄2 volume of water. Cresol is insoluble in small quantities of pure 6 per cent. soda solution; with a large excess, it forms crystalline scales; while carbolic acid is freely soluble in small or large quantities of alkaline solutions.