PRACTICAL EXERCISES 425 



the water m the burette is read oft. The pinchcock having been 

 closed, the bottle B is now tilted so that the urine in the thimble 

 is gradually mixed with the hypobromite solution, and the nitrogen 

 given off is added to the air in the burette and its connections. 

 The level of the water in the burette is therefore depressed. When 

 gas ceases to be given off, and a short time has been allowed for the 

 whole to cool, the tube is raised till the level of the water is once 

 more the same inside and out. The level is again read off; the 

 difference of the two readings gives the volume of nitrogen at the 

 temperature of the air and the barometric pressure. In order that 

 the temperature of the water may be the same as that of the air, 

 the cylinder should be filled a considerable time before the observa- 

 tions are begun. 



8. Estimation of the Total Nitrogen. It is sometimes more im- 

 portant to determine the total nitrogen of the urine than the urea 

 alone ; and 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 faeces, or in any of the solids or liquids of the body. 

 It depends on the oxidation of the nitrogenous matter in such a 

 way that the nitrogen is all represented as ammonia. The ammonia 

 is then distilled over, collected and estimated, 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 o'i 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. 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 hydrate (specific gravity about 1-25) to render the liquid 

 alkaline, avoiding excess, as this favours bumping. The proper 

 quantity can be found by determining beforehand how much of the 

 alkali is needed to neutralize the acid used for oxidation, and this 

 amount should be added. 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 ij parts water), which favours the setting free of 

 the ammonia from the amido-compounds of mercury that have been 

 formed during oxidation. Commercial ' liver of sulphur ' will do 

 quite well. Immediately connect the distilling-flask with the worm, 

 as shown in Fig. 132, and distil the ammonia over into 50 c.c. of 

 standard (decinormal) sulphuric acid contained 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 



