100 § 69. BASES AND ACIDS WITH EE AGENTS. 



When an oxalate is heated with plumbic binoxide and 

 concentrated^ sulphuric acid, a brisk effervescence ensues, 

 carbonic acid being set free ; if a drop of lime-water, on 

 the end of a glass rod, is held in the tube above the 

 liquid, it is made turbid by precipitation of calcic car- 

 bonate. 



A solution of an oxalate in which no free acid except 

 acetic is present, gives a fine white precipitate, CaC^O^, 

 with calcic sulphate, insoluble in acetic acid. 



Quantitative estimation. — a. Oxalic acid may be ac- 

 curately determined by a volumetric process with potassic 

 permanganate. 



First make a '^\^^ standard solution of oxalic acid, by 

 m.ixing together 10 c.c. of the standard acid already 

 made and 90 c.c. of distilled water. Put 50 c.c. of this 

 new standard solution, containing 0.315 grm. of the acid, 

 in a beaker, add about 100 c.c. of water and 6 to 8 c.c. of 

 concentrated sulphuric acid, and heat to about 60° C. ; 

 put the beaker on a sheet of white paper and add the 

 standard solution of permanganate with constant stirring, 

 in the same manner as directed for the determination of 

 iron (§ 52). When the reaction is completed, make another 

 trial with the other 50 c.c. of the 'l^^ standard acid. 



The standard of the permanganic solution with refer- 

 ence to oxalic acid being determined, to estimate the acid 

 in any substance, whether free or combined, the substance 

 must be freed from all other compounds that act in the 

 same manner on the permanganate, such as ferrous oxide, 

 or organic matter ; dissolve it in water or hydrochloric 

 or sulphuric acid, add 400 to 500 c.c. of water for every 

 gramme of oxalic acid supposed to be present, and 6 to 8 

 c.c. of concentrated sulphuric acid, and proceed to titrate 

 with the permanganic solution in the usual way. Let 

 m = the amount of permanganate used to oxidize 0.315' 

 grm. of crystallized oxalic acid, or 0.18 grm. of oxalic an- 

 hydride, and m' the amount required to oxidize the acid 



