62 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



IODOFORM. 



Preparation. 



4 gm. of crystallised sodium carbonate are dissolved in 20 c.c. 

 of water. 2 c.c. of absolute alcohol and 2 gm. of iodine are added, 

 and the solution* warmed to about 70 on the water-bath until it 

 is decolorised. lodoform separates as a lemon-yellow powder. It 

 is filtered off, washed with cold water and dried on an unglazed 

 plate. 



The melting-point of the preparation serves to prove its identity. 



Properties. 



lodoform is a light yellow, shining crystalline solid with a per- 

 sistent unpleasant odour. It has a characteristic microscopic appear- 

 ance hexagonal plates, stars, or rosettes, and melts at 119. On 

 gently heating it sublimes without change, but on heating strongly it 

 is decomposed : violet vapours of iodine are formed and a deposit of 

 carbon is left. 



lodoform is nearly insoluble in water (i part in 10,000) and in 

 dilute acids and alkalies. It is slightly soluble in alcohol (i part in 

 50) but more easily soluble in absolute alcohol (i part in 23). It is 

 easily soluble in ether, chloroform and carbon disulphide, but very 

 slightly soluble in glycerol, benzene and petroleum ether. In its 

 chemical properties iodoform closely resembles chloroform. 



Tests for Impurities in lodoform. 



1. No residue should remain when it is heated in the air. 



2. It should be completely soluble in boiling alcohol, but insoluble in 

 brine. 



3. On shaking up with water and filtering, the filtrate should give no re- 

 action with barium chloride or silver nitrate. 



4. If picric acid be present as adulterant, it may be detected (a) By testing 

 the aqueous extract with potassium cyanide when a reddish-brown coloration 

 is produced, (b) By treating with caustic soda solution and shaking this 

 solution with chloroform. Picric acid remains in the aqueous solution. 

 (c) By extracting the acid with dilute sodium carbonate solution, neutralising 

 exactly with acetic acid and adding potassium nitrate ; potassium picrate 

 is precipitated. 



