§ 306. =Colour Tests.=--=Fröhde’s reagent= is made by dissolving 1 part of sodic molybdate in 10 parts of strong sulphuric acid; it strikes distinctive colours with many alkaloids. =Mandelin’s reagent= is a solution of meta-vanadate of ammonia in mono- or dihydrated sulphuric acid. The strength should be 1 part of the salt to 200 of the acid. This reagent strikes a colour with many alkaloids, and aids to their identification. It is specially useful to supplement and correct other tests. The following table gives the chief colour reactions, with the alkaloids. (See also p. 55 for the spectroscopic appearances of certain of the colour tests.) METHODS OF SEPARATION. § 307. =Stas’s Process.=--The original method of Stas[325] (afterwards modified by Otto)[326] consisted in extraction of the organic matters by strong alcohol, with the addition of tartaric acid; the filtered solution was then carefully neutralised with soda, and shaken up with ether, the ethereal solution being separated by a pipette. Subsequent chemists proposed chloroform instead of ether,[327] the additional use of amyl-alcohol,[328] and the substitution of acetic, hydrochloric, and sulphuric for tartaric acid. [325] _Annal d. Chem. u. Pharm._, 84, 379. [326] _Ib._, 100, 44. _Anleitung zur Ausmittel. d. Gifte._ [327] Rodgers and Girwood, _Pharm. Journ. and Trans._, xvi. 497; Prollin’s _Chem. Centralb._, 1857, 231; Thomas, _Zeitschr. für analyt. Chem._, i. 517, &c. [328] Erdmann and v. Ushlar, _Ann. Chem. Pharm._, cxx. pp. 121-360. COLOUR REACTIONS[329] OF CERTAIN ALKALOIDS. [329] Caustic potash also gives characteristic colours with certain alkaloids. Out of seventy-two alkaloids (using 0·5 mgrm.), the following alone gave characteristic colours when fused with KHO:--Quinine, grass-green, and peculiar odour; quinidine, becoming yellower and finally brown; cinchonine, at first brownish-red to violet, with green edges, later, bluish-green; cinchonidine, blue passing into grey; cocaine, greenish-yellow, turning to blue, and then dirty red on strong heating.--W. Lenz, _Zeit. f. anal. Chem._, 25, 29-32.