VEGETABLE ALKALOIDS. 
§§ 3 IO- 3 I 2 -] 
253 
120°, it still contains 2 molecules of water ; the strychnine salt is 
similar, but retains only 1 mol. of water on drying. These compounds 
are insoluble in acids or cold water ; for the most part they do not 
interfere with colour tests, and on treatment with dilute alkalies, silicium 
and tungsten pass into solution, and the alkaloid can either be filtered 
off or shaken out with appropriate solvents. 
3. Scheibler’s Method for Alkaloids: Phospho - Tungstic Acid.— 
Ordinary commercial sodium tungstate is digested with half its weight 
of phosphoric acid, specific gravity 1*13, and the whole allowed to stand 
for some days, when the acid separates in crystals. A solution of these 
crystals will give a distinct precipitate with the most minute quantities 
of alkaloids, ^oo 1 ^00 strychnine, and tooumj 0 quinine. The alkaloid 
is liberated by digestion with barium hydrate (or calcium hydrate) ; and 
if volatile, may be distilled off ; if fixed, dissolved out by chloroform. 
In complex mixtures, colouring-matter may be removed by plumbic 
acetate, the lead thrown out by SH 2 , and concentrated, so as to remove 
the excess of SH 2 . 
§ 310. Schulze’s reagent is phospho-antimonic acid. It is prepared 
by dropping a strong solution of antimony trichloride into a saturated 
solution of sodic phosphate. The precipitation of the alkaloids is 
effected by this reagent in a sulphuric acid solution. 
§ 311. Dragendorff’s reagent is a solution of potassium-bismuth iodide 
—80 grms. bismuth subnitrate dissolved in 200 c.c. dilute nitric acid 
(sp. gr. 1-18) and added to a strong solution of potassic iodide, 227 
grms. On strongly cooling the solution much of the potassic nitrate 
crystallises out; this salt is removed and the solution made up to 
1 litre. It must be kept in the dark. On the addition of an acid 
solution of an alkaloid, a kermes-red precipitate falls down, which 
is in many cases crystalline. 
Marine’s reagent is a solution of potassium-cadmium iodide. It is 
made on similar principles. 
Potassium-zinc iodide in solution is also made similarly. The pre¬ 
cipitates produced in solutions of narceine and codeine are crystalline 
and very characteristic. 
§ 312. Colour Tests, — Frohde’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 in their identification. It is specially useful to 
supplement and correct other tests. (See p. 64 for the spectroscopic 
appearances of certain of the colour tests.) 
