To obtain the free acid, the potassium salt is dissolved in a little water and acidulated with hydrochloric acid; the liquid is then shaken out in a separating tube, with a mixture of 8 vols. of ether and 1 of alcohol. The ether-alcohol is distilled off, the residue treated with moist silver oxide until no farther separation of silver chloride occurs, the silver chloride is separated by filtration, the soluble silver salt decomposed by SH₂, and the filtrate carefully evaporated to a syrup; after a few hours, the acid crystallises in stars of needles. Urobutylchloral acid can be obtained in quite a similar way.[190] [190] V. Mering u. Musculus, _Ber._, viii. 662; v. Mering, _ibid._, xv. 1019; E. Kulz, _Ber._, xv., 1538. § 207. =Separation of Chloral from Organic Matters.=--It will be most convenient to place the organic fluid or pulped-up solid, mixed with water, in a retort, to acidify with tartaric acid, and to distil. Chloral hydrate distils over from a liquid acidified with tartaric acid; to obtain the whole of the chloral requires distillation in a vacuum almost to dryness. The distillation will, unless there is also some partly decomposed chloral, not smell of chloroform, and yet give chloroform reactions. To identify it, to the distillate should be added a little burnt magnesia, and the distillate thus treated boiled for half an hour in a flask connected with an inverted condenser; in this way the chloral hydrate is changed into chloroform and magnesium formate-- 2CCl₃CH(OH)₂ + MgO = 2CHCl₃ + (HCOO)₂Mg + H₂O. The fluid may now be tested for formic acid: it will give a black precipitate with solution of silver nitrate-- (HCOO)₂Mg + 4AgNO₃ = 4Ag + Mg(NO₃)₂ + 2CO₂ + 2HNO₃. It will give a white precipitate of calomel when treated with mercuric chloride solution-- (HCOO)₂Mg + 4HgCl₂ = 2Hg₂Cl₂ + MgCl₂ + 2HCl + 2CO₂. Chloral (or chloroform), when boiled with resorcinol and the liquid made strongly alkaline with NaHO, gives a red colour, which disappears on acidifying and is restored by alkalies. If, on the other hand, there is an excess of resorcinol and only a very small quantity of NaHO used, the product shows a yellowish-green fluorescence; 1/10 of a milligramme of chloral hydrate gives this reaction distinctly when boiled with 50 mgrms. of resorcinol and 5 drops of a normal solution of sodium hydrate.[191] [191] C. Schwarz, _Pharm. Zeit._, xxxiii. 419.