288 CONSIDERATION OF CARBON COMPOUNDS. 



the form of potassium, sodium, or calcium salts. It may be ob- 

 tained from these vegetables, or by the oxidation of many or- 

 ganic substances, chiefly fats, sugars, starch, etc., by nitric acid 

 or other strong oxidizing agents. 



Oxalic acid is manufactured on a large scale by heating saw- 

 dust with potassium or sodium hydrate, when the oxalate of 

 these metals is formed; by the addition of calcium hydrate to 

 the dissolved alkaline oxalate, insoluble calcium oxalate is 

 formed which is decomposed by sulphuric acid. 



Oxalic acid crystallizes in large, transparent, colorless prisms, 

 containing two molecules of water; it is soluble in water and 

 alcohol, and has poisonous properties. When heated slowly it 

 sublimes; when heated rapidly by itself or with sulphuric acid 

 it is decomposed into water, carbonic oxide, and carbon di- 

 oxide : 



H 2 C 2 4 = H 2 + CO + CO 2 . 



Oxalic acid acts as a reducing agent, decolorizing solutions of 

 the permanganates, and precipitating gold and platinum from 

 their solutions: 



PtCl 4 + 2H 2 C 2 O 4 = Pt + 4CO 2 + 4HC1. 



Analytical reactions. 



1. Oxalic acid or oxalates when heated with strong sulphuric 

 acid evolve carbonic oxide and carbon dioxide (see above). 



2. Neutral solutions of oxalic acid give with calcium chloride 

 a white precipitate of calcium oxalate, CaC 2 4 , which is in- 

 soluble in acetic, soluble in hydrochloric acid. 



3. Silver nitrate produces a white precipitate of silver oxalate, 

 Ag 2 2 0, 



4. A dry oxalate (containing a non-volatile metal) heated in 

 a test-tube evolves carbonic oxide, whilst a carbonate is left 

 which shows effervescence with acids. 



Antidotes to ' oxalic acid. Calcium carbonate or lime-water 

 should be administered, but no alkalies as in cases of poisoning 

 by mineral acids, because the alkaline oxalates are soluble. 



Oxalates. The acid potassium oxalate, KHC 2 O 4 , or its combina- 

 tion with oxalic acid, is known under the name of salt of sorrel 



