CH. XIII.] TOTAL NITROGEN. 323 



is about 10 N. A paraffined bottle, a grease-free burette (see p. 381) and a good 

 supply of recently boiled and cooled distilled water being ready, the bottle is 

 nearly filled with the water, the volume of which is noted. The requisite 

 amount of the strong CO 2 - free soda is added, the solution well mixed, and the 

 burette and soda-lime tubes immediately fitted. It is advisable to seal all the 

 joints with paraffin wax, to prevent the absorption of atmospheric CO 2 , and 

 to ensure the due filling of the burette by suction. It is difficult to fit the 

 upper stopper to an ordinary burette. To overcome this, Messrs. Baird and 

 Tatlock are making burettes fitted with a wide top, similar to that shewn on 

 page 130. Mr. H. W. Hall, of the Cambridge Biochemical Laboratory, has 

 made it possible to use an ordinary burette by the construction of the two-way 

 piece shewn in fig. 39. 



The exact concentration of the soda is determined by titration of a 

 weighed amount of acid potassium phthalate, or by the use of an acid of 

 known normality. 



The acid employed can be hydrochloric or sulphuric. In the case of the 

 aeration methods it is advisable to use the latter, to avoid any risk of loss by 

 volatility. The strength should be somewhat greater than that of the alkali. 

 A great deal of time is saved if the acid be measured from a burette rather than 

 with a pipette. The initial reading is noted, the approximate amount required 

 is run in, and the burette allowed to stand until the final titration is performed. 

 The soda is then run in until the solution goes yellow, a little acid is added until 

 a pink tint appears. The sides of the vessel are washed down with a little 

 distilled water, and the titration completed by the addition of a few drops of 

 the soda. In this way, being relieved from the anxiety that one may overshoot 

 the mark with the alkali, it is not necessary to- run the whole of the alkali in 

 drop by drop, which may take a considerable time. The only danger is that 

 of allowing insufficient time for proper drainage of the alkali burette. 



Calculation. 

 i gram. -molecule of acid neutralises i gram. -molecule of ammonia. 



So 1,000 cc. of N.HC1 (or other acid) neutralises 17 grams. NH 3 and are 

 equivalent to 14 grams, of Nitrogen. 



So i cc. of N. acid = 14 mgms. Nitrogen. 



So i cc. of (A) x N. acid = 14 x (A) mgms. Nitrogen (" acid equivalent.") 



Suppose that the soda employed be (S) normal, 



(S) 



Then i cc. soda = TTT cc. acid (" alkali-acid ratio "). 

 ( A ) 



Let the amount of acid used be (a) cc., and the amount of alkali be (s) ce. 



(S) x (s) 



Then (5) cc. alkali = T-TT cc. acid. 

 ( A ) 



So volume of acid neutralised by the ammonia distilled over is 



(S) x (5)' 

 (a) - / A v ' cc., and the amount of nitrogen is 



I W ~ (Af" J x T 4 x ( A ) m g- 

 A good deal of time is saved if the acid and alkali be labelled with the 



