720 PHYSIOLOGICAL CHEMISTRY. 



Experiment 99. ( Volumetric determination of phosphoric acid.) Soluble 

 uranium salts give with phosphates a dirty-white precipitate of uranium phos- 

 phate : Na 2 HPO 4 + UO 2 (NO 3 ) 2 = UO 2 HPO 4 + 2NaNO 3 . The precipitate is 

 soluble in mineral acids, insoluble in acetic acid. Tincture of cochineal is not 

 affected by uranium phosphate, but is colored greenish by soluble uranium 

 salts. These reactions are used for determining phosphoric acid, thus : 



Make up a solution of sodium acetate, 100 grammes ; acetic acid (glacial), 

 30 grammes ; and water to make 1000 c.c. 



Prepare a volumetric solution of nitrate or acetate of uranium so adjusted 

 that 1 liter is equivalent to 5 grammes of P 2 O 5 . To 100 c.c. of filtered urine 

 add 5 c.c. of the acetate solution and a few drops of solution of cochineal. Heat 

 to boiling and titrate with uranium solution until the liquid assumes a green 

 color. The number of c.c. required multiplied by 0.005 indicates the quantity 

 of P 2 O 5 in the urine used. 



Sulphur in the urine. Sulphur is present in the urine in three 

 forms : 



Neutral (unoxidized) sulphur ; cystine, etc. 

 Oxidized (acid) sulphur : 



a. Inorganic (preformed) sulphates, Na, K, etc. 



b. Ethereal (conjugate) sulphates ; sulphuric acid in combination 



with skatole, indole, phenol, etc. 



The origin of the urinary sulphur is the protein metabolism, while 

 a small portion may arise from the ingested sulphates. The total 

 amount has little clinical importance. It is increased in fever and 

 with a meat diet. The inorganic sulphates are largely in excess, 

 their amount being about ten times that of the ethereal sulphates. 



Experiment 100. 1. Demonstrate neutral sulphur by adding HC1 to urine 

 with a fragment of zinc ; hydrogen sulphide will be evolved and will blacken 

 lead acetate paper. 



2. Demonstrate inorganic sulphates by adding barium chloride solution to 

 urine acidified with acetic acid ; a white precipitate of barium sulphate will 

 form. Filter this solution ; and, 



3. Demonstrate ethereal sulphates by adding HC1 and barium chloride solu- 

 tion to the filtrate. On boiling the organic sulphates will be broken up and a 

 second precipitate of barium sulphate will form. 



Neutral sulphur in the urine. While there is normally present 

 about 10 per cent, of the total sulphur in this form, the bodies which 

 contain it are so far almost unknown. Sulphocyanates are found 

 in small amounts. 



Pathologically the best known body is cystine, which is believed 

 to indicate an inability on the part of the body to completely break 

 down the protein residues. 



Cystine, diamino-dithw-dipropionic acid, ^' 2 is 



