464 PHYSIOLOGICAL CHEMISTRY. 



burned separately and its ash added to the contents of the crucible, 

 which is moderately heated before weighing. 



Experiment 73. Determine the amount of sodium chloride solution volu- 

 metrically by means of potassium sulphocyanate as follows: Place 5.837 

 grammes (about 5.7 c.c.) of urine into a 150 c.c. flask, add 2 c.c. of nitric acid, 

 20 c.c. of water, and 15 c.c. of deci-normal silver nitrate solution. Mix well, 

 add some ferric alum solution, and then from a burette deci-normal potassium 

 sulphocyanate solution until, after frequent shaking of the flask, the white pre- 

 cipitate coheres in curdy masses, and the liquid assumes a red tint. The 

 number of c.c. of sulphocyanate required is then deducted from the 15 c.c. of 

 silver nitrate added at first, and the difference shows the number of c.c. of silver 

 nitrate required to precipitate the sodium chloride in the urine. Normal urine 

 requires from 8 to 11 c.c. silver solution, each c.c. corresponding to 0.1 per 

 cent, of sodium chloride. (For explanation of the method see page 269.) 



Phosphorie acid is found in urine, in part (about two-thirds) com- 

 bined with alkalies, and in part (about one-third) with lime and 

 magnesia. These phosphates have in acid or neutral urine the com- 

 position ]S T aH 2 PO 4 , Na 2 HPO 4 , CaHPO 4 , CaH 4 (PO 4 ) 2 , MgHPO 4 ; in 

 alkaline urine compounds of the composition Na 3 PO 4 , Ca 3 (PO 4 ) 2 , 

 Mg 3 (PO 4 ) 2 , MgNH 4 PO 4 may be present. 



By adding any alkali the phosphates of calcium and magnesium 

 (generally termed earthy phosphates) are precipitated, the phosphates 

 of sodium or possibly potassium remain dissolved. 



The so-called earthy phosphates (phosphates of calcium and magne- 

 sium) may be approximately determined by adding a few drops of an 

 alkaline hydroxide to about 50 c.c. of urine, heating to the boiling- 

 point, collecting on a filter, washing, igniting, and weighing in a 

 platinum crucible. 



Experiment 74. Add to 50 c.c. of urine a few drops of calcium chloride solu- 

 tion and then water of ammonia. Phosphoric acid is completely precipitated, 

 chiefly as tricalcium phosphate, Ca 8 (PO 4 ) 2 , containing, however, a very small 

 quantity of magnesium ammonium phosphate. Collect the precipitate on a 

 filter, wash well, dry, ignite and weigh it. Calculate the phosphoric acid from 

 the tricalcium phosphate, without reference to the small amount of magnesium 

 phosphate. 



Experiment 75. Add to 100 c.c. clear urine 5 c.c. hydrochloric acid ; boil, 

 and then add barium chloride to complete precipitation. Set aside for one 

 hour, filter, wash well, dry, ignite and weigh. Calculate from the weight of 

 barium sulphate, thus obtained, the percentage of sulphuric acid present in the 

 urine examined. 



The methods for estimating urea and uric acid have been described 

 in the preceding chapter. 



