332 CONSIDERATION OF CARBON COMPOUNDS. 



temperature of about 155 C. (311 F.) ; but if heated higher or with 

 sulphuric acid it is decomposed into water, carbonic oxide, and carbon 



dioxide : 



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 solu- 



tions : 



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



Analytical reactions. 

 (Sodium oxalate, Na 2 C 2 O 4 , may be used.) 



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 O 4 , which is insoluble in 

 acetic, soluble in hydrochloric acid. 



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

 Ag 2 C 2 4 . 



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 admin- 

 istered, but no alkalies as in cases of poisoning by mineral acids, because the 

 alkali 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. 

 Calcium oxalate, CaC 2 O 4 , is, in small quantities, a normal constituent 

 of urine. Ferrous oxalate, FeC 2 O 4 .H 2 O, is made by adding potas- 

 sium or ammonium oxalate to ferrous sulphate, when double decom- 

 position takes place, and the ferrous oxalate is precipitated as a pale- 

 yellow, crystalline, nearly insoluble powder. 



Dibasic acids with alcoholic hydroxyl. 



/OH 



Malic acid = C 4 H 6 O 5 or C 2 H 3 CO 2 H 



2 



X C0H 



//OH. 



Tartaric acid == C 4 H 6 O 6 or C 2 H 2 



In the various acids heretofore considered, the hydrogen is derived 

 either from the unsaturated hydrocarbon residue, or from the hydroxyl 



