4 82 



A MANUAL OF PHYSIOLOGY 



its bore, and then closed again. Any urine which has passed into 

 the tube B is washed out with water, and B is then filled with 

 hypobromite solution. A is now filled up with urine to the top of 

 the graduation. By opening the stopcock, i c.c. of urine (or less 

 if the urine is concentrated) is permitted to pass into B and to mix 

 with the hypobromite solution. The nitrogen collects in B, and 

 when it has ceased to come off, the meniscus of the liquid is read 

 off. The corresponding degree on the scale gives the amount of 

 urea in grammes contained in the quantity of urine employed. 



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 (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 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 con- 

 tinue 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 determining 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 granu- 

 lated zinc. Shake the flask two or three times. Add also about 1 2 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 oxida- 

 tion. Commercial ' liver of sulphur ' will do quite well. Imme- 

 diately connect the distilling-flask with the worm, as shown in 

 Fig. 1 80, and distil the ammonia over into 50 c.c. of standard 

 (decinormal) sulphuric acid (see footnote, p. 439) 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 



FIG. 179. DOREMUS 

 UREOMETER. 



