366 NITROGEN BASES 



aqueous solutions of the salts usually have a strongly acid 

 reaction due to hydrolytic dissociation. 



The majority of alkaloids are optically active, rotating the 

 plane of polarized light to the left, though a few, such as 

 coniine, laudanosine, pelletierine and pilocarpine, are dextro- 

 rotatory. 



GENERAL REACTIONS OF ALKALOIDS. 



The alkaloids are precipitated from solution by a large 

 number of different reagents with formation of amorphous or 

 sometimes crystalline precipitates. 



The commonest of these reagents are the following : — 

 I. A solution of iodine in potassium iodide, sometimes 

 known as potassium ter-iodide, gives a chocolate- 

 brown precipitate. 



2. Mercuric iodide in potassium " 



iodide, 



3. Tannic acid, 



4. Phosphotungstic acid. 



all of which give colour- 

 less amorphous precipi- 

 tates. 



5. Auric chloride, 



6. Platinic chloride, 



which give crystalline 

 precipitates often having 

 characteristic melting- 

 points. 



The alkaloids are, however, not the only substances which 

 are thrown out of solution by these reagents, since most 

 nitrogen bases behave in a similar way, and the formation of 

 a precipitate is therefore not conclusive proof of the presence 

 of alkaloids. On the other hand, if none of the above re- 

 agents produce precipitates, it is tolerably certain that there 

 are no alkaloids present. 



In examining plant tissues for alkaloids, Errera recom- 

 mends testing the fresh sections with alkaloidal reagents and 

 also sections which have been soaked in a 5 per cent alcoholic 

 solution of tartaric acid. In the second case no precipitate 

 should be obtained, owing to the extraction of the alkaloid. 



The final identification of the various alkaloids is usually 

 effected by means of colour reactions. 



Thus, if a section of the endosperm of Strychnos nux 



