5 1 4 EXCRETION 



9. Estimation of the Total Nitrogen. It is sometimes more important 

 to determine the total nitrogen of the urine than the urea alone. This 

 is conveniently done by Kjeldahl's method (or some modification of 

 it), which can also be applied to the estimation of the nitrogen in the 

 fseces, or in any of the solids or liquids of the body. It depends on the 

 oxidation of the nitrogenous matter (or, rather, in the case of urine, 

 mainly its hydrolysis) in such a way that the nitrogen is all represented 

 as ammonia. The ammonia is then distilled over, collected and esti- 

 mated, and from its amount the nitrogen is easily calculated. In urine 

 the method can be carried out by adding to a measured quantity of it 

 (say 5 c.c.) four times its volume of strong sulphuric acid, and boiling 

 in a long-necked flask (capacity 200 c.c.), after the addition of a globule 

 of mercury (about 0*1 c.c.), which hastens oxidation and obviates 

 bumping. A part of the mercuric sulphate formed remains in solution ; 

 the rest forms a crystalline deposit. The heating should continue for 

 half an hour, or until the liquid is decolourized. It should be kept 

 gently boiling. This completes the process of oxidation ; and the next 

 step is to liberate the ammonia from the substances with which it is 

 united in the solution, and to distil it over. Dilute the liquid with 

 water, after cooling, up to about 150 c.c., and pour into a larger long- 

 necked flask. Add enough of a solution of sodium hydroxide (specific 

 gravity about 1-25) to render the liquid alkaline, avoiding excess, as 

 this favours bumping. The proper quantity can be found by deter- 

 mining beforehand how much of the alkali is needed to neutralize the 

 acid used for oxidation, and a little more than this amount should be 

 added. Twenty c.c. of strong sulphuric acid needs about 75 c.c. of 

 40 per cent, sodium hydroxide to neutralize it. Bumping may further 

 be prevented by the addition of a little granulated zinc. Shake the 

 flask two or three times. Add also about 12 c.c. of a concentrated 

 solution of potassium sulphide (i part to i parts water), which favours 

 the setting free of the ammonia from the amino-compounds of mercury 

 that have been formed during oxidation . Commercial ' liver of sulphur ' 

 will do quite well. Immediately connect the distilling-flask with a 

 worm or Liebig's condenser, and distil the ammonia over into 50 c.c. 

 of standard (decinormal) sulphuric acid (see footnote, p. 473) con- 

 tained in a flask into which a glass tube connected with the lower end 

 of the worm dips. Heat the distilling-flask at first gently, then strongly, 

 and boil for three-quarters of an hour, or until about two-thirds of the 

 liquid has passed over. Then lift the tube out of the standard acid, and 

 continue the distillation for two or thiee minutes longer. The ammonia 

 is now all united with the standard acid, a certain amount of which is 

 left over. By determining this amount we arrive at the quantity com- 

 bined with ammonia, and therefore at the quantity of ammonia. Fill 

 a burette with a decinormal solution of potassium or sodium hydroxide. 

 Add a little methyl-orange solution to the standard sulphuric acid, to 

 serve as indicator. Then run in the potassium or sodium hydroxide 

 till the pink tinge gives place to a permanent but just recognizable 

 yellow. Let x be the number of c.c. run in. Since i c.c. of any deci- 

 normal solution is equivalent to i c.c. of any other, x represents also the 

 number of c.c. of the standard sulphuric acid left uncombined with 

 ammonia; and 50 x, the quantity combined with ammonia. Then, 

 i c.c. of decinormal sodium or potassium hydroxide being equivalent 

 to i c.c. of decinormal ammonium hydroxide, and i c.c. of decinormal 

 ammonium hydroxide containing 0-0014 gramme nitrogen, we get 

 (50 #)x 0*0014 as the quantity of nitrogen in 5 c.c. of urine. 



Instead of mercury, potassium sulphate and copper sulphate may be 

 added to the sulphuric acid in order to aid the decomposition in the 



