322 



ANALYSIS OF URINE. 



[CH. XIII. 



the boiling point. The mixture is diluted with water, made alkaline by the 

 addition of sodium hydroxide and the ammonia distilled into a measured 

 amount of standard acid. The amount of this neutralised by the ammonia is 

 found by subsequent titration with standard alkali. Knowing the amount 

 of ammonia formed from the volume of urine taken, the percentage of nitrogen 

 can be readily calculated. 



The technique adopted varies with the material and with the inclinations 

 of the operator. There are four main methods of distillation, viz., by direct 

 boiling, by steam, by boiling with alcohol, and by the author's method of 

 combined boiling with alcohol and aeration. 



The main objection to direct boiling is that the mixture is apt to bump 

 very violently, and that a certain loss of ammonia may occur when adding the 

 alkali. The bumping is much less with potash than with soda, but the price 

 is prohibitive. 



Steam distillation is much safer, and, although it is slow, it needs very 

 little attention. It is the best method when considerable amounts of sulphuric 

 acid have to be used for the incineration. 



Distillation with alcohol is very smooth and relatively rapid, for the 

 alcohol boils quickly and carries the ammonia over with it. 



The combination of aeration with alcohol distillation (Ex. 397) was 

 originally devised for the estimation of very small amounts of nitrogen, but 

 it is so rapid and accurate that the author now uses it for the estimation of total 

 nitrogen in urine. Either 0-5 or i cc. of the urine is measured with an Ostwald 

 pipette, the incineration is completed in 15 mins., and the result is obtained in 

 less than 30 mins. The results agree exactly with those obtained when 5 cc. 

 of urine are taken. 



Standard acids and alkalies and indicators. It is essential to use CO 2 -free 

 soda for the back titrations. The end point is extremely sharp if methyl red 



is used as an indicator. With 

 ordinary soda the yellow tint 

 obtained when the solution is 

 just alkaline soon changes to 

 a pink, and there is great un- 

 certainty as to how far to pro- 

 ceed with the titration. This 

 is fatal to good results. With 

 CO 2 - free soda, on the other 

 hand, the change of tint is 

 brought about by the addition 

 of less than a drop of 0*02 N. 

 soda, and is permanent for a 

 considerable time. The titra- 

 tion can be conducted by arti- 



A...,. ficial light, which is not the 



case with methyl orange. The 

 preparation of CO 2 - free soda 

 is described on page 26. The 

 strength required varies with 

 the method adopted. For 

 ordinary work the author pre- 

 fers to use about o-i N. 

 strength, but for the micro- 

 method it is better to use 0-03 to 0-05 N. As a rough guide it may be 

 stated that the strong soda prepared according to the directions on page 25 



B. 



Fig 39- 



Hall's two-way tube 

 for burettes. 



