LESSON XXIII 

 UREA AND CHLORIDES IN URINE 



ESTIMATION OF UREA 



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

 with acetic acid if necessary, and filtering off the flakes of coagulated protein. 

 The hypobromite method of estimation (see p. 141) holds its own, as it is easy 

 and sufficiently exact for clinical purposes. It has entirely replaced the older 

 method of Liebig (titration with mercuric nitrate), which is now of purely 

 historical interest. 



When absolute accuracy is necessary, one or other of many recently 

 introduced methods must be employed. We shall be content with describing 

 two of these. 



(a) Folin's Method. This depends on the fact that urea is decomposed 

 quantitatively into ammonia and carbonic acid by boiling with magnesium 

 chloride solution. The ammonia is estimated by distillation into standard 

 acid and subsequent titration. 



Analysis. Three c.c. of urine, 20 grammes of magnesium chloride and2c.c. 

 of concentrated hydrochloric acid are boiled in a flask, closed by a cork through 

 which a glass tube 20 centimetres in height passes. This acts as a reflux 

 condenser. The boiling is continued for 25 to 30 minutes. After diluting 

 with water the mixture is then transferred to a litre flask, 7 c.c. of 20-per- 

 cent, caustic soda are added, and the ammonia is distilled off and estimated 

 as described in the final operation in Kjeldahl's method (see Appendix, p. 235). 

 Every c.c. of decinormal ammonia in the distillate corresponds to 3 milli- 

 grammes of urea. A small correction has to be made for the ammonia 

 present as such in the original urine. 



(b) Method of Mbrner and Sjtiqvist. The following reagents are necessary: 

 i. A saturated solution of barium chloride containing 5 per cent, of 



barium hydrate. 



ii. A mixture of ether and alcohol in proportion 1 : 2. 



iii. The apparatus, &c., necessary for carrying out Kjeldahl's method of 

 estimating nitrogen (see p. 235). 



Analysis. Five c.c. of urine are mixed with 5 c.c. of the barium mixture 

 and 100 c.c. of the mixture of ether and alcohol. By this means all nitrogenous 

 substances except urea are precipitated. Twenty-four hours later this is filtered 

 off, and the precipitate is washed with 50 c.c. of the ether-alcohol mixture, 

 the filter-pump being used to accelerate the process. The washings are 

 added to the filtrate ; a little magnesia is added to this to drive off ammonia. 



