THE COMPOSITION AND CHARACTERS OF THE URINE 1175 



tive octahedral crystals (envelope-shaped). Insoluble in acetic acid, soluble 

 in hydrochloric acid. 



(4) Ammonium magnesium phosphates (in faintly acid urine). The 

 crsytals have been compared to 



knife-rests or coffin-lids (Fig. 538). 

 They are soluble in acetic acid. 



(5) Calcium hydrogen phosphate. 

 CaHP0 4 . These are rare. They 

 form large prismatic crystals often 

 arranged in rosettes. Easily soluble 

 in dilute acetic acid. On adding a 

 solution of ammonium carbonate, the 

 crystals are eaten away and form an 

 amorphous deposit. 



(6) Tyrosine, fine needles in star- 

 shaped bundles, and cystine, in 



regular hexagonal plates, may occur FlG . 539> Ammonium urate. 



under very rare circumstances. 



(b) In Alkaline Urine. (1) The commonest precipitate cpnsists 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. 



(3) Acid ammonium urate (Fig. 539) 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. 1 



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

 alkali in the presence cf 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 fa i ces 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 



1 Fuller details will be found in Plimmer's Practical Physiological Chemistry, from 

 which most of the methods here given are taken. 



