76 



PHYSIOLOGY OF NUTRITION. 



25. The Decomposition of Nitrates in Plants. 

 It has already been pointed out (see 24) that the Xitric acid 

 necessary for the formation of proteid is liberated from nitrates 

 by the action of vegetable acids, especially Oxalic acid, with 

 simultaneous production of oxalates. This and similar reactions 

 are farther of importance because they serve to free the plant sap 

 from excess of Calcium salts, Calcium oxalate being of course only 

 very slightly soluble, and, in fact, very generally separating out 

 in plants in the crystalline form. 



The interaction between Oxalic acid and lime or potash salts in 

 the plant must take place in very dilute solution, and hence it is 

 interesting to ascertain by experiment whether the reactions in 

 question do proceed in presence of very large quantities of 

 water. 1 



We get ready a number of beakers containing solutions of 

 0'205 gr. of Calcium nitrate in 400 c.c. of water. We further 

 prepare solutions containing 0'090 gr. of Oxalic acid (considered 

 anhydrous) to every 100 c.c. of water. If now we mix these 

 solutions together in pairs, 400 c.c. of Calcium nitrate solution to 

 100 c.c. of Oxalic acid solution, at the ordinary temperature, pre- 

 cipitates of Calcium oxalate are formed and Nitric acid is set free. 

 Time, however, is a factor in determining the extent of the 

 reaction, as we may learn by ascertaining the amount of preci- 

 pitated Calcium oxalate in one case im- 

 mediately, in others after the lapse of 

 one, two, or three days. The longer the 

 Oxalic acid is allowed to act on the 

 Calcium salt, the greater the quantity 

 of Calcium oxalate precipitated. 



Tha/t Nitric acid is actually liberated 

 when Oxalic acid acts on Potassium 

 nitrate we can prove as follows : Five 

 beakers, a, b, c, d, and e, are arranged 

 each containing 500 c.c. of water. To a 

 we add 0'210 gr. of Nitric acid, HN0 3 ; 

 to 6, 3*000 gr. of Oxalic acid (considered 

 anhydrous) ; to c, 0'337 gr. of Potassium 

 nitrate ; to d, 0'210 gr. of Nitric acid and 3'000 gr. of Oxalic acid; 

 to e, 0'337 gr. of Potassium nitrate and 3 '000 gr. of Oxalic acid. 

 In each beaker is suspended a slab of marble (see Fig. 23, AT), 

 all the slabs being as nearly as possible of the same size, say 



FIG. 23. Apparatus for in- 

 vestigating the action of Oxalic 

 acid on nitrates. 



