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AMINO-ACIDS, PROTEINS, UREA AX\D :Ui<K ACID lOl 



Biuret. — This compound, as the name indicates, is a deriva- 

 tive of two molecules of urea from which it is formed by the 

 loss of one molecule of ammonia. The reaction is as follows : 



The substance is colored pink by a very dilute solution of 

 copper sulphate, and the biuret test for proteins (Experiment 

 Study XVIII, 4, c) derives its name from this compound. 



EXPERIMENT STUDY XIX 



Urea 



(i) Determination of Urea, (a) Make a 2 per cent solution of 



urea, (b) Prepare an alkaline solution of sodium hypobromite as 



follows : 



NaOH, 10 per cent solution, 100 c.c. 



Bromine 2 c.c. 



2 NaOH + Br -> NaOBr + NaBr + H2O 



(c) FUl the ureometer tube with the hypobromite solution and then 

 add, by means of a small i c.c. bent pipette, i c.c. of urea solution 

 to the ureometer tube. Allow to stand and measure the gas evolved. 

 The gas is nitrogen according to the following reaction : 



/NH2 

 0C< + 3 NaOBr -> N2 + 2 H2O + CO2 + 3 NaBr 



^NH2 



Urea (mol. wt. 60) Nitrogen (mol. wt. 28) 



The CO2 evolved is absorbed by the alkali of the solution. This 

 reaction is quantitative and the amount of nitrogen gas evolved is 

 an exact measure of the urea decomposed as 60 parts by mass of urea 

 yield 28 parts by mass of nitrogen. The ureometer used in this test 

 is a piece of clinical apparatus so graduated that for i c.c. of urine the 

 nitrogen evolved is read directly in per cent of urea in the urine. 



