io8 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Oxalic Acid. 



Oxalic acid occurs naturally in many plants e.g. sorrel, rhubarb 

 deposited in the cells as calcium oxalate. Small quantities of oxalic 

 acid are present in normal urine, from -O2--I2 gm. in 24 hours. 

 It arises most probably from the carbohydrate of the diet. An in- 

 creased output follows the consumption of rhubarb and other vege- 

 tables which contain oxalic acid, and occasionally an increased output 

 occurs in certain diseases, e.g. in diabetes. Calculi of calcium oxalate 

 are sometimes found in the bladder and kidneys. 



Preparation. 



Oxalic acid is formed by the oxidation of numerous organic com- 

 pounds, acetic acid and sugar. It is made commercially by the oxidation 

 of the cellulose of sawdust with air and caustic alkali. A mixture of 

 caustic potash and caustic soda is made into a paste with sawdust 

 and heated in open vessels to about 240. The mass is extracted 

 with cold water ; the potash dissolves leaving sodium oxalate which 

 is only slightly soluble. By boiling the sodium oxalate with milk of 

 lime, insoluble calcium oxalate is formed. This is washed and de- 

 composed with sulphuric acid, and the oxalic acid isolated from the 

 solution by crystallisation. 



The alkali salts of oxalic acid are made commercially by heating 

 alkali formates. The reaction proceeds most easily in the presence 

 of small amounts of alkali at 280 under diminished pressure, or at 

 400 in absence of air : 



2HCOONa = H 2 + COONa 

 COONa. 



Properties. 



Oxalic acid crystallises from water in colourless prisms containing 

 2 molecules of water of crystallisation (m.p. 101*5). On heating to 

 100, it loses the water, becomes opaque and forms a white powder 

 which melts at 189 



It is easily soluble in alcohol, but only slightly soluble in ether. 

 It is insoluble in chloroform, benzene and petroleum ether. 



Reactions. 



(1) On heating on platinum, nickel, or a crucible lid, oxalic acid 

 is volatilised without charring. 



(2) No charring occurs on heating oxalic acid with concentrated 

 sulphuric acid, but it is decomposed yielding carbon monoxide and 

 carbon dioxide : 



COOH . COOH - CO, + CO + H 2 O. 



