Pharmacognosy

DEPARTMENT OF PHARMACOGNOSY AND BOTANY

 

General reactions for precipitation of alkaloids

 

Reagent

Reagent constituents

Results of reactions

Buchards or Vagners

iodine in potassium iodine solution

brown precipitate

Mayers

mercuric chloride in potassium iodine solution

white or yellow precipitate

Dragendorffs

bismuth nitrate and potassium iodine in acetic acid

reddish-orange precipitate

Sonnenstains

phosphorous molibdate acid solution

yellow precipitate

Marmaes

cadmium iodine in potassium iodine

white or yellow precipitate

Shablers

phosphorous wolframic acid solution

white amorphous precipitate

Godfruas or Berthranes

siliceous wolframic acid

white precipitate

 

10% tannin solution

white amorphous precipitate

 

1% picric acid solution

yellow precipitate

 

5% mercuric chloride solution

white precipitate

 

5% platinous hydrochloric acid solution

white precipitate

 

 

The general reactions of precipitation are based on the fact that alkaloids form combinations with metals and metalloids: bismuth, mercury and iodine. In practice, what is used is a solution containing iodine and iodide, or a solution containing potassium iodide and mercuric chloride - known as Mayer's reagent - or a reagent containing bismuth nitrate and potassium iodide, better known as Dragendorff s reagent. Proteins, some coumarins, hydroxyflavones, lignans, and other compounds can give false positive reactions with Dragendorff s reagent.