ALKALOIDS AND ALKALOIDAL-TYPE PRF.CIPITA TION 



cipitate the chromate of a strong base but its 

 really sensitive tests are those in which it 

 acts as a basic group reagent for weak com- 

 plex alkaloids. 



Substances soluble in both strong aqueous 

 acids and bases may be insoluble at some 

 intermediate point. The method is obviously 

 general and acidic substances may be pre- 

 cipitated from alkaline solution by ordinary 

 acids. This is one way of obtaining crystal 

 tests for barbitm-ates. While using various 

 ways of separating the free substance for 

 microcrystal tests, the chemical meaning of 

 the reactions should always be noted. 



Tests are also made on an acidic substance 

 (which may be in fine particles, or amor- 

 phous, to start with) by dissolving it in 

 NaOH solution, adding HCl in slight excess, 

 and allowing the drop to evaporate to dry- 

 ness. The reagents leave merely grains of 

 NaCl, which are isotropic, and therefore in- 

 visible with crossed nicols; the acidic sub- 

 stance set free may crystallize, and may 

 often be birefringent and easily seen, and 

 then its refractive indices and other optical 

 properties can be determined. 



Halogen Reagents for Nitrogen Com- 

 pounds in Aqueous Solutions. Bromine 

 in water, or in HBr or NaBr solution, and 

 iodine in KI or HI solution, are the reagents 

 commonly used; iodine may also be used in 

 HBr or NaBr solution. These are very gen- 

 eral precipitants not only for the alkaloids 

 but in general for amine compounds of any 

 complexity. In a somewhat different type of 

 reaction their use extends even to compounds 

 in which the nitrogen no longer has appre- 

 ciable basic properties. 



The alkaloid-type reaction takes place in 

 acid solution, or at least the alkaloid is com- 

 bined with acid in a salt and itself acts as a 

 cation. When an amine-derivative is pre- 

 dominantly acidic and is precipitated as an 

 iodine compound in acid solution, it may 

 not be clear whether the reaction should be 

 attributed to residual basic qualities, or not. 

 The related but anionic reaction is most 



clearly seen as a distinct type in neutral or 

 slightly basic solution; occurs best with a 

 high concentration of iodine. This type of 

 reaction, which has not been explored nearly 

 enough occurs with some barbiturates and 

 with other compounds without appreciable 

 basic properties, and also with some com- 

 pounds that can react either as bases or 

 acids, including the alkaloids caffeine, theo- 

 bromine, theophylline, and colchicine, which 

 give both the alkaloidal-type and the ani- 

 onic-type of iodine precipitation. They all 



have acidic C=0 groups, 



/ 



Oxygen Acids. These may be divided 

 into two kinds, complex and simple. The 

 complex oxygen acids, phosphoritungstic, 

 phosphorimolybdic, arsenimolybdic, sihco- 

 tungstic, etc., are very general, precipitating 

 from aqueous solution all the alkaloids and 

 related compounds, and, generally speaking, 

 potassium and the heavier alkali metals, 

 ammonium and the simple amines, as w^ell. 

 As a rule they give crystals only with the 

 simpler bases; most of their alkaloidal pre- 

 cipitates are amorphous. Phosphorimolybdic 

 acid is, however, very useful for comparative 

 precipitation. Instead of the absolute con- 

 centration of reactive substance, sensitivi- 

 ties may be reported in terms of the relative 

 sensitivity compared with phosphorimolyb- 

 dic acid. This is also important in classifying 

 alkaloids by precipitation. 



The simpler oxygen acids used as alkaloi- 

 dal precipitants are perchloric, chromic, 

 permanganic, etc., used either as free acid 

 or alkali salt. Permanganate especially fre- 

 quently reacts as an oxidizing reagent and is 

 itself reduced, but it gives some excellent 

 crystals with compounds not readily oxi- 

 dized. Unlike the complex oxygen acids 

 these compounds are not very general or 

 sensitive as reagents, and so are chiefly use- 

 ful with fairly complex bases which are easily 

 precipitated. Chlorochromic acid, HCrOsCl, 

 is more sensitive and general than CrOs , 



17 



