290 URINE. [CH. XII. 



2. The sodium hypobromite is prepared as follows : dissolve 100 grams. 

 of caustic soda in 250 cc. of water. Cool, and slowly add 25 cc. of bromine, 

 cooling under the tap as the bromine is added. The reaction is as follows : 



2 NaHO + Br 2 = NaBrO + NaBr + H 2 O. 



It must be freshly prepared before use as it undergoes the following 

 decomposition : 



3 NaBrO = 2 NaBr + NaBrO 3 . 



3. As a test for urea the reaction with hypobromite is only useful in a 

 negative sense; that is to say, if an effervescence is not obtained urea is 

 absent, but if an effervescence is obtained it does not necessarily follow that 

 urea is present. 



341. To some of the urea solution add a solution of mercuric 

 nitrate. A white precipitate of mercuric oxide combined with urea 

 and mercuric nitrate takes place. To the mixture thus obtained 

 add a saturated solution of sodium chloride, drop by drop. The 

 precipitate dissolves, to reappear on a further addition of mercuric 

 nitrate. 



NOTES. i. The precipitate consists of urea and mercuric nitrate and 

 one, two or three molecules of mercuric oxide, depending on the concentration 

 of the two solutions. 



2. The solubility in NaCl is due to the formation of mercuric chloride, 

 which is only very feebly ionised in neutral solutions. 



342. Treat a solution of urea with Millon's reagent, and heat. 

 A white precipitate is formed, owing to the presence of mercuric 

 nitrate in the reagent. There is also an evolution of gas due to the 

 action of the nitrous acid on the urea. 



343- Specific urease test for urea. To 4 or 5 cc. of a 



dilute solution of urea add 4 or 5 drops of phenol red. The colour 

 obtained is generally slightly pinkish. Add traces of very dilute 

 acetic acid by means of a glass rod until the reaction is very faintly 

 acid to the indicator. Warm to about 45 C. Add a large "knife 

 point " of finely ground Soya bean meal, shake and keep the solution 

 warm. The colour changes to a reddish purple, owing to the 

 enzyme converting neutral urea to alkaline ammonium carbonate. 



NOTES. i. In applying the test it is important to see that the reaction 

 is only faintly acid to the indicator. For if a considerable amount of acid 

 and only a small amount of urea be present, the amount of ammonium carbon- 

 ate formed may not be sufficient to bring the reaction to the point where a 

 pink colour is given with the indicator. 



2. Proteins only interfere by acting'as buffers. It is not usually neces- 

 sary to remove them. 



