1126 PHYSIOLOGY 



QUANTITATIVE ESTIMATION OF THE CHIEF URINARY 

 CONSTITUENTS 



It may be useful here to summarise the most trustworthy methods which are 

 employed for the estimation of the chief urinary constituents.* 



The TOTAL ' ACIDITY ' of the urine is measured by titrating it against decinormal 

 alkali in the presence of an indicator, such as phenolphthalein. The indistinctness of 

 the end-point is due to the presence of calcium salts and ammonium salts. Folin there- 

 fore recommends that the titration be carried out in the presence of potassium oxalate, 

 which diminishes the error. 



Method. To 25 c.c. urine add 15 to 20 grm. potassium oxalate and 1 to 2 drops of 

 phenolphthalein. Shake thoroughly for one or two minutes, and whilst the solution 



is still cold from the effect of the oxalate, titrate with NaOH until a permanent pink 

 remains. 



TOTAL NITROGEN. In all metabolic experiments, the determination of the total 

 nitrogen of the food, the urine, and the faeces is indispensable. In each case Kjeldahl's 

 method is employed. This method depends on the fact that all the nitrogenous sub- 

 stances met with in the body, when heated for a considerable time with concentrated 

 sulphuric acid, undergo oxidation, the nitrogen being finally converted into ammonia. 

 On adding alkali to the mixture the ammonia is set free from its combination with the 

 sulphuric acid and can be distilled off and received into a vessel containing a known 

 amount of decinormal acid. By titrating this acid after the operation we can determine 

 the quantity of ammonia which has been produced. To carry out this method 5 c.c. 

 of urine are heated with 20 c.c. sulphuric acid and a small quantity of copper sulphate 

 and potassium sulphate. The copper sulphate is to aid the oxidation of the organic 

 substances, the potassium sulphate is to raise the boiling-point of the mixture. The 

 boiling is continued for half an hour. The flask is then cooled and half filled with dis- 

 tilled water. A special form of distillation tube (Fig. 527) is now attached by a rubber 

 cork which fits tightly, but just before this is done an excess of strong caustic soda 

 sufficient to neutralise the concentrated sulphuric acid is run in under the acid. The 

 other end of the distillation tube is at once arranged to dip under the surface of a 



measured quantity of standard acid (e.g. 10 c.c -- H 2 S0 4 ), diluted with water, and con- 



tained in a 600 c.c. Erlenmeyer flask. The flask is then shaken and heated. In about 

 a quarter of an hour the ammonia is completely distilled off, and its amount can be 



determined by titrating the acid in the flask with NaOH, methyl orange being used 

 as indicator. 



UREA. The method usually adopted for estimating the urea is that devised by 

 Hufner. It depends on the fact that urea is decomposed by an alkaline hypobromite 

 with the production of C0 2 and nitrogen. In the presence of an excess of alkali the C0 2 

 is absorbed and the nitrogen may be collected and measured, and serves as an index 

 of the amount of urea present. The reaction which occurs is as follows : 



CO(NH 2 ) 2 + SNaBrO + 2NaOH = 3NaBr + N 2 + Na 2 CO 3 -f 3H 2 O 



60 grm. 22-4 litres = 28 grm. 



372 c.c. 



Actually, however, only 354-33 c.c. nitrogen are evolved by 1 grm. urea. 



The disadvantage of this method is that other substances, such as ammonia, creati- 

 nine, and uric acid, give off a certain amount of their nitrogen with sodium hypobromite, 

 so that the method is not strictly accurate, though enough so for most clinical purposes. 

 In actually carrying out the method 5 c.c. of urine are treated with 25 c.c. of freshly 



*Fuller details will be found in Plimmer's " Practical Physiological Chemistry," 

 from which most of the methods here given are taken. 



