46 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



IDENTIFICATION OF AN ORGANIC COMPOUND. 



Knowing the formula of a pure organic compound from its analysis 

 and molecular weight, it ha^ to be identified. The compound may be 

 a known or an unknown one. 



To find out if the compound is known reference is made to 

 Richter's " Lexicon of Carbon Compounds " 1 in which the melting- 

 points and other constants of the various compounds are given. Cor- 

 responding properties identify the substance. 



If the compound be unknown, further analysis is necessary ; it 

 must be ascertained to what group of carbon compounds the unknown 

 body belongs, whether it is an alcohol, an ester, an acid, a carbo- 

 hydrate, an amide, an amine, a protein, etc. With the complex 

 natural substances this is a matter of great difficulty, and it may take 

 many years before a question is settled ; e.g. tyrosine was discovered 

 in 1846 and its constitution only definitely proved in 1882. 



The identification of an unknown substance, or the rapid identifica- 

 tion of a known substance, is greatly facilitated by a few preliminary 

 tests. If in solution a portion of it should be evaporated to see if there 

 is a residue and whether it is solid or liquid. The residue can be 

 tested for the elements present, especially nitrogen. If there is no 

 residue the solution must be distilled and the boiling-point observed. 



1. Colour. Vegetable colouring matters : if blue, they are changed 

 to red by acid and the blue colour is restored, or changed to a green, 

 by ammonia ; if yellow, they are changed to brown by alkali and the 

 colour is restored by acid. 



Ferric salts and copper salts are reddish-brown and blue or green 

 respectively. 



Many coloured compounds show absorption spectra, such as 

 haemoglobin and its derivatives. 



2. Taste. Tasting must be done carefully on account of the 

 extremely poisonous nature of some organic compounds. A drop of 

 a weak solution in water or alcohol may be used. Acids have a sour 

 and astringent taste. Alkaloids and glucosides are bitter. Sugars 

 and glycerol are sweet. 



1 The most recent compounds are given in the yearly volumes of the Journals of the 

 English and Foreign Chemical Societies. 



