166 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



LESSON XXIII 

 UBEA AND CHL0BIDE8 IN URINE 



ESTIMATION OF UBEA 



If albumin is present it must be first separated by boiling after acidulation 

 with acetic acid if necessary, and filtering ofif the flakes of coagulated proteid. 



The three chief methods of estimating urea are — 



(a.) The mercuric nitrate or Liebig's method. 



(6) The hypobromite, or Hiifner's method. 



(c) The method of Morner and Sjoqvist. 



a. Liebig's Method. — The combination between urea and mercury has 

 the formula (CON2H4)2Hg(N03)2(HgO)3. It forms a white precipitate, in- 

 soluble in water and weak alkaline solutions. It is therefore necessary to 

 prepare a standard solution of mercuric nitrate, and to have an indicator by 

 which to detect the point when all the urea has entered into combination 

 with the mercury, and the latter slightly predominates. This indicator is 

 sodium carbonate, which gives a yellow colour with the excess of mercury, 

 owing to the formation of hydrated mercuric oxide. 



Theoretically, 100 parts of urea should require 720 parts of mercuric 

 oxide, but practically 772 of the latter are necessary to remove all the 

 urea, and at the same time show the yellow colour with alkali ; consequently 

 the solution of mercuric nitrate must be of empirical strength in order to 

 give accurate results. 



The following solutions must be prepared — 



i. Standard mercuric nitrate solution. Dissolve 77'2 grammes of red oxide 

 of mercury (weighed after it has been dried over a water-bath), or 71-5 gr. 

 of the metal itself, in dilute nitric acid. Expel excess of acid by evapo- 

 rating the liquid to a syrupy consistence. Make up to 1,000 c.c. with distilled 

 water, adding the water gradually. This solution is of such a strength that 

 19 c.c. will precipitate 10 c.c. of a 2-per-cent urea solution. Add 52-6 c.c. of 

 water to the litre of the mercuric nitrate solution and shake well ; then 20 c.c. 

 (instead of 19) = 10 c.c. 2-per-cent. urea solution, i.e. 1 c.c. = '01 gr. urea. 



ii. Baryta mixture. This is a mixture of two volumes of solution of 

 barium hydrate with one of solution of barium nitrate, both saturated in the 

 cold. 



Analysis. — Take 40 c.c. of urine. Add to this 20 c.c. baryta mixture and 

 filter off the precipitate of barium salts (phosphates and sulphates). Take 

 15 c.c. of the filtrate (this corresponds to 10 c.c. of urine) in a beaker. Run 

 into it the mercuric nitrate solution fi:om a burette, until it is found that, on 

 mixing a drop of the mixture with a drop of a saturated solution of sodium 



