VI] THE VEGETABLE ACIDS 83 



The dibasic acids of the methane series contain several important 

 members : 



Dibasic acids Corresponding amino-acids 



Oxalic acid (C00H)2 

 Malonicacid CH2-(COOH)2 



Succinic acid CH2 * CH2 * (C00H)2 amino-succinic or aspartic acid 



Glutaric acid CH2 ' CH2 * CH2 * (C00H)2 amino-glutaric or glutaminic acid 



Adipic acid CH2 • CH2 • CHg ' CH2 • (C00H)2 



Oxalic acid occurs very frequently and widely distributed in plants, 

 usually as the calcium salt, and apparently less frequently as the sodium 

 and potassium salts. It has rarely been detected as the free acid. It is 

 especially abundant in spp. of Oxalis, in the Rhubarb {Rheum Rhaponticum) 

 and Sorrel (Rumeoo Acetosa). The calcium salt is precipitated on adding 

 calcium acetate to a solution of the acid. Calcium oxalate is insoluble 

 in acetic acid, but soluble in dilute mineral acids. 



Ea;pt. 81. Tests for oxalic acid. Take a 2% solution of oxalic acid, neutralize 

 with caustic soda (or use a soluble oxalate) and make the following tests : 



(a) To 5 c.c. add a few drops of 5 % calcium chloride solution. A white pre- 

 cipitate of calcium oxalate is formed. Divide the precipitate into two portions. To 

 one add an equal quantity of strong acetic acid : the precipitate is insoluble even 

 on heating. To the other add strong hydrochloric acid drop by drop : the precipitate 

 is soluble. Hence the free acid can be precipitated with calcium acetate but not with 

 calcium chloride. 



(6) To 5 c.c. add a few drops of 5 % lead acetate solution. A white precipitate of 

 lead oxalate is formed. Add an equal quantity of strong acetic acid and warm ; the 

 precipitate is insoluble. 



Ba:pt. 82. Preparation of calcium oxalate from leaves of the Sorrel (Rumex Acetosa). 

 Take 100 gms. of fresh leaves of the Sorrel. Boil them in an evaporating dish with 

 200 c.c. of water and squeeze the boiled mass through linen. Boil the filtrate again 

 and filter on a pump. Acidify the filtrate with acetic acid, and add a concentrated 

 solution of calcium acetate until no more precipitate is formed. The precipitate 

 cannot readily be filtered off" so that it should be allowed to settle for 12 hours. Then 

 decant ofl:' the liquid and boil up the precipitate in the minimum amount of 10 ^/^ 

 hydrochloric acid. On cooling, calcium oxalate will separate out in characteristic 

 crystals. On examining under the microscope, these will be seen to be octahedra, 

 giving the appearance of a square with a diagonal cross (envelope form). Leaves of 

 Rhubarb {Rheum Rhaponticum) can also be used, taking about 250 gms. in 500 c.c. 

 of water. 



It is stated that there is an enzyme widely distributed in plants 

 (Staehelin, 3) which has the power of decomposing oxalic acid with the 

 production of carbon dioxide. 



6—2 



