36 Biological Chemistry. 



pounds which need not be considered here in any very 

 great detail. The most general method consists in treat- 

 ing the substance with nitric acid at high temperatures 

 with excess of the acid. This operation is generally 

 performed by enclosing the substance with the acid in a 

 sealed glass tube and heating the mixture to a high 

 temperature in a " bomb " furnace. The organic matter 

 is thereby destroyed by oxidation. If silver nitrate is 

 present, the halogens are precipitated as silver halides ; and 

 if barium chloride is present, sulphur which is oxidized 

 to sulphuric acid in the reaction is precipitated as barium 

 sulphate. These elements can also be estimated, in the 

 case of non-volatile substances, by various dry incinera- 

 tion processes for example, by fusion with alkalis and 

 sodium peroxide and separation from the fusion mixture 

 after destruction of the organic matter as silver halide 

 compounds or barium sulphate. Phosphorus may be readily 

 estimated quantitatively by the use of the method already 

 described for its detection (see p. 23). The organic 

 matter is destroyed by a mixture of nitric and sulphuric 

 acids, and the phosphorus precipitated as ammonium phos- 

 phomolybdate. The precipitate thus produced is filtered off 

 and dissolved in an excess of standard (generally half- 

 normal) caustic soda and the solution is heated ; the 

 ammonium salt is thereby converted into the sodium salt, 

 and the ammonia is volatilized. The excess of standard 

 alkali is then determined by titration with standard acid 

 (1 c.c. N/2 sodium hydroxide used up is equivalent 

 to 1*268 mg. phosphorus). This method is known as 

 Neumann's wet-ashing process.* 



* This method does not always yield correct results. It is better to dissolve 

 the yellow phosphomolybdate precipitate in ammonia, and then to reprecipi- 

 tate the phosphate by magnesia mixture and continue the estimation in the 

 ordinary manner. 



