94 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



FORMIC ACID. H . COOH. 



Preparation. 



Formic acid was first prepared by distilling crushed ants with 

 water hence its name. The stings of some insects and plants also 

 probably contain it. It occurs together with acetic and other 

 lower fatty acids in urine. It can be obtained by oxidising methyl 

 alcohol with potassium permanganate. It is formed in the decom- 

 position of chloroform by alkali (p. 60), by the action of water upon 

 hydrogen cyanide '(p. 154), and its alkaline salts are obtained by the 

 reaction of carbon monoxide with alkalies : 



CO + KOH = HCOOK. 



It is manufactured by heating glycerol with oxalic acid. It has 

 been shown by Chattaway that in this reaction glyceryl acid oxalate 

 is formed ; on raising the temperature carbon dioxide is evolved and 

 glyceryl monoformin is produced On hydrolysing this ester with a 

 further quantity of oxalic acid, formic acid is produced and the acid 

 .oxalate again formed. There is thus a continuous reaction : 



CH 2 OH CH 2 O OC . COOH 



HOOC | 



CH.H + I = CHOH + HoO 



HOOC I 

 CH 2 OH CH 2 OH. 



CH 2 O OC . COOH CH 2 O OC . H 



CHOH = CO 2 + CHOH 



CH 2 OH CH./)H. 



CH,O OC . H 



COOH 

 CHOH + | = HCOOH + CHOH 



COOH 

 CH 2 OH CH 2 OH. 



Properties. 



Formic acid is a colourless volatile liquid with pungent odour. 

 It has a sp. gr. of 1-221 at 20, freezes at 8-3, and boils at 100. 

 It is a very strong acid, about 12 times as strong as acetic acid, and 

 produces blisters on the skin and intense irritation. 



It dissolves in water, alcohol and ether, and in general properties 

 resembles acetic acid. 



The formates crystallise well and are prepared in the same way 

 as acetates (p. 97). The lead and magnesium salts are insoluble in 

 alcohol ; the corresponding acetates are soluble. The acids may 

 therefore be separated by treating a concentrated solution of these 

 salts with alcohol; the formate is then precipitated. Potassium 

 formate is almost insoluble in alcohol and may thus also be separated 

 from the acetate, .which is soluble. 



