1258 PHYSIOLOGY 



in regular hexagonal plates, may occur under very rare circum- 

 stances. 



(6) IN ALKALINE URINE 



(1) The commonest precipitate consists of earthy phosphates, 

 amorphous, easily soluble in dilute acetic acid. 



(2) Ammonium magnesium phosphate or triple phosphate is 

 common in urine which has undergone ammoniacal fermentation. 



FIG. 523. Deposit of ' triple ' phosphate 

 and ammonium urate. (FuNKE.) 



FIG. 524. Ammonium urate. 



(3) Acid ammonium urate (Fig. 524) may also occur in alkaline 

 urine. On treatment with HC1 it is dissolved and uric acid in crystals 

 slowly separates out. 



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 decinor- 

 mal alkali in the presence of an indicator, such as phenolphthalein. The indistinct- 

 ness of the end-point is due to the presence of calcium salts and ammonium salt?. 

 Folin therefore 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 to 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 substances met with in the body, when heated for a considerable 

 time with concentrated sulphuric acid, undergo oxidation, the nitrogen being 



* Fuller details will be found in Plimmer's " Practical Physiological Chemis- 

 try," from which most of the methods here given are taken. 



