HYDROXY-, KETO- AND DIBASIC ACIDS 109 



The gases may be passed into lime or baryta water; barium 

 carbonate is precipitated and the carbon monoxide may be ignited. 



(3) On warming a solution of oxalic acid with dilute sulphuric 

 acid and potassium permanganate, it is oxidised with liberation of 

 carbon dioxide and the permanganate is decolorised : 



COOH . COOH + O = 2C0 2 + H 2 0. 



(4) Calcium oxalate is precipitated when calcium chloride is added 

 to a solution of a neutral oxalate. It is insoluble in acetic acid, but 

 soluble in mineral acids. 



(5) Mercurous nitrate gives a precipitate of mercurous oxalate even 

 in very dilute solutions of neutral oxalates. 



Detection. 



Oxalic acid is precipitated from a neutral solution, or a solution 

 acidified with acetic acid as calcium oxalate. The crystalline form of 

 the precipitate (p. 565) is very characteristic. The calcium oxalate is 

 filtered off and can be identified by heating it or by the action of 

 permanganate in sulphuric acid solution. 



Estimation. 



The solution, e.g. urine, is treated with calcium chloride and ammonia 

 and evaporated to a small bulk. The precipitate which consists of calcium 

 phosphate, sulphate and oxalate is separated by filtration and washed with 

 water. It is dissolved in a small quantity of dilute hydiochloric acid (30 c.c.) 

 and extracted from aqueous solution by extracting in an extractor (see p. 600) 

 with ether. The ether is evaporated off, the residue dissolved in water, 

 neutralised with ammonia, acidified with acetic acid, and precipitated with 

 calcium chloride. The calcium oxalate is filtered off, ignited, and weighed 

 as CaO, or dissolved in dilute sulphuric acid and titrated with standard per- 

 manganate. 



