52 BOTANICAL MICROTECHNIQUE. 



7. Phosphoric Acid, H,PO< , and its Salts. 



77 The following reactions are adapted for the micro- 

 chemical recognition of phosphoric acid : 



1. Nitric acid and ammonium molybdate. This reagent, 

 first introduced into microchemistry by Hansen (I, 96), 

 causes the formation of regular crystals which represent 

 chiefly a combination of the octahedron and the cube, and 

 are colored an intense yellow. There is commonly no 

 danger of confusing these with the isomorphic compounds 

 of arsenic acid, so far as the study of vegetable objects is 

 concerned. 



It is convenient to use as the reagent a solution which 

 contains 12 ccm. of officinal nitric acid of specific gravity 

 1. 1 8, to one gram of ammonium molybdate. In the pres- 

 ence of small quantities of acid the precipitate is formed 

 only after slight warming (to 4O-5o C.), and then often 

 only after some time. 



The sections to be tested are best burned before the 

 addition of the reagent, since otherwise the reaction may 

 be hindered by the presence of certain organic substances, 

 as, for example, potassium tartrate. Besides, the phosphoric 

 acids combined with the nuclein or otherwise organically 

 united, as, for instance, the phosphoric acids contained in 

 the globoids, are not directly shown by this reagent, but 

 only in the ash (cf. Schimper II, 215). This reagent may 

 be applied directly to the ash prepared by heating upon the 

 cover-glass. Thus is obtained at once with the ashes of 

 sections of not too young stems of Stapelia picta a strong 

 reaction, which occurs only after some hours in sections 

 prepared from alcoholic material which contain sphaerites 

 of calcium phosphate (cf. 96). 



2. The addition of magnesium sulphate and ammonium 

 chloride produces with salts of phosphoric acid a crystalline 

 precipitate of ammonio-magnesium phosphate, which is 

 practically insoluble in ammonia and ammonium chloride 

 solutions. These crystals, some of the most characteristic 



