46 PROXIMATE CONSTITUENTS OF PLANTS 



The presence of COg can be demonstrated by its 

 action on lime water, while the CO will burn at the 

 mouth of the test-tube with its characteristic blue flame. 



(iii.) A neutral solution of ammonium oxalate is 

 prepared by adding a slight excess of NHg to an 

 aqueous solution of the acid, and boiling for a few 

 seconds. 



On adding a solution of CaClg to this neutral 

 solution, a white crystalline precipitate of calcium 

 oxalate is produced, insoluble in acetic acid, but soluble 

 in HCl or HNO3. 



(iv.) On adding a solution of an oxalate or oxalic 

 acid to an acidified solution of potassium permanganate, 

 the colour of the permanganate is discharged, owing to 

 reduction by the oxalic acid — 



2KMnO,+ 5 H,C A+ 3H2SO4 



= K2S04+2MnS04+ioC02+8H20. 



The presence of oxalic acid in plants may be conveniently 

 shown in the case of sorrel {oxalis acefosella), in which it exists in 

 the form of potassium hydrogen oxalate and as calcium oxalate. 



About 25 grams of sorrel are extracted by grinding 

 up with warm water, and the solution filtered off. The 

 liquid is boiled down to a small volume, and tests 

 applied as in (iii.) and (iv.) above. 



27. Qualitative Reactions of Tartaric Acid and 

 Tartrates. 



Tartaric acid, COOH . CHOH . CHOH. COOH, exists in the 

 form of large colourless transparent crystals readily soluble in 

 water. It is a dibasic acid, and therefore forms acid and normal 

 salts. It exists in four stereo-isomeric forms, two of which are 

 optically active (dextro and Icevo) and two optically inactive. 



(i.) On heating in a dry tube, tartaric acid and 

 tartrates char readily, with production of inflammable 

 vapours and a smell of burnt sugar. A residue of 



