112 LABORATORY WORK IX PHYSIOLOGICAL CHEMISTRY. 



estimated after fusion as P 2 5 . From this amount is de- 

 ducted the total P 2 5 determined on another sample of the 

 same urine, and the difference corresponds to the organic 

 phosphorus calculated in terms of P 2 5 . 



TOTAL NITROGEN (KJELDAHL). 



A well-nourished man eliminates under ordinary con- 

 ditions with a mixed diet 10-16 grms. nitrogen. The amount 

 is dependent upon the body weight, the diet, and various 

 factors which may influence the metabolism of protein in the 

 organism. Muscular work has no effect upon the nitrogenous 

 excretion. In round numbers 85 per cent of the nitrogen is 

 in the form of urea; 4 to 5 per cent, NH 3 ; 1 to 2 per cent, 

 uric acid; and the remaining extractives, etc., 8 to 10 per 

 cent. 



Quantitative Estimation. 



The Kjeldahl method for the determination of the total 

 nitrogen of the urine has come into almost universal use. 



The principle consists in the decomposition of all the or- 

 ganic matter by heating with sulphuric acid, whereby all of 

 the carbon and hydrogen become oxidized to C0 2 and H 2 0, 

 and the nitrogen of such compounds which contain it in 

 combination with hydrogen (such as =NH, NH 2 , NH 3 ), 

 but not with oxygen, appears as ammonia. This is lib- 

 erated from the acid solution by saturation with NaOH; 

 the gas is then distilled over into a known quantity of acid, 

 the amount of which thus neutralized being determined by 

 the titration of the acid still remaining. 



Reagents necessary (all nitrogen-free): 



1. Concentrated H 2 S0 4 (2 parts fuming: 3 parts pure 

 concentrated). 



2. Potassium sulphate (powdered). 



