234 CUVETTE COLORIMETRY 



the specially constructed vessels of Conway and Byrne (1933), 

 Needham and Boell (1939), or Tompkins and Kirk (1942). In this 

 regard, advantages might be claimed for the method of Briiel et al. 

 (1946), in which both diffusion and titration are carried out in the 

 same simple tube, or the Hawes and Skavinski (1942) method, in 

 which the digestion and diffusion are conducted in the same tube. 

 However, the former requires transfer of digest and the latter 

 transfer of acid. The method of Briiel et al. ( 1946) is the most precise 

 of all (0.005 /xg. nitrogen), and the one most thoroughly tested for 

 its reliability. This method is described on page 283. 



Colorimetric Methods. Levy's (1936) method for the direct 

 Nesslerization of the digest has the advantage that all of the chemi- 

 cal operations may be carried out in the same vessel and the diffu- 

 sion process is eliminated. On the other hand, the phenol-hypo- 

 chlorite reaction used by Russell (1944) is highly sensitive, and 

 ammonia, in the quantities that can be determined by Levy's 

 method, can be analyzed with ordinary macro equipment. However, 

 diffusion of the ammonia from the digest is essential for a proper 

 phenolhypochlorite reaction. 



Digestion of Sample for Determination of Total Nitrogen 



The digestion can be conveniently performed in small tubes of 

 resistant glass. In the earlier methods after the sample had been 

 introduced, a small Hengar granule {Hengar Co.) was sometimes 

 added to prevent bumping, the digestion mixture was introduced, 

 and the tubes were placed in a drying oven at 120-130° for a few 

 hours, or at 105-110° overnight, to drive off most of the water. The 

 digestion was continued by heating the tubes in a sand bath, or in 

 a copper or aluminum block with holes drilled so that the tubes 

 could be inserted to a depth not exceeding one third of the tube 

 length. 



Briiel, Holier, Lintlerstr0m-Lang and Rozits Procedure for 

 Digestion of the sample. In the procedure of Briiel et al. ( 1946) , 

 which should be given preference, the digestion tubes are 4 cm. 

 long, 1.8 mm. inner diameter, and 2.4 mm. outer diameter. The 

 tubes are cleaned by boiling in sulfuric acid-chromic acid mixture, 

 rinsed thoroughly with distilled water run through each tube by 

 means of a thin capillary extending to the bottom, and the water is 



