URINE 559 



excess of silver nitrate over that necessary to precipitate the chlorides reacts with the 

 chromate to form red silver chromate (Ag 2 Cr04). This indicates the end point. 

 The method is accurate when applied to the ash of urine in this manner. 



Procedure. To 10 c.c. of urine in a small platinum or porcelain crucible or dish 

 add about 2 grams of chlorine-free potassium nitrate and evaporate to dryness at 

 iooC. (The evaporation may be conducted over a low flame provided care is taken 

 to prevent loss by spurting.) By means of crucible tongs hold the crucible or dish 

 over a free flame until all carbonaceous matter has disappeared and the fused mass is 

 slightly yellow in color. Cool the residue somewhat and bring it into solution in a 

 small amount (15-25 c.c.) of distilled water acidified with about 10 drops of nitric 

 acid. Transfer the solution to a small beaker, being sure to rinse out the crucible or 

 dish very carefully. Test the reaction of the fluid, and if not already acid in reac- 

 tion to litmus, render it slightly acid with nitric acid. Now neutralize the solution 

 by the addition of calcium carbonate 1 in substance, add 2-5 drops of neutral potas- 

 sium chromate solution to the mixture, and titrate with a standard silver nitrate 

 solution. 



This standard solution should be run in from a burette, stirring the liquid in 

 the beaker after each addition. The end-reaction is reached when the yellow color 

 of the solution changes to a slight orange-red. At this point take the burette read- 

 ing and compute the percentage of chlorine and sodium chloride in the urine 

 examined. 



Calculation. The calculation is made exactly as in the Volhard-Arnold 

 method, page 556, except that the reading is not multiplied by two. 



Calculate the quantity of sodium chloride and chlorine in the twenty-four- 

 hour urine specimen. 



Interpretation. See page 557. 



Calcium and Magnesium 



McCrudden's Methods. 2 Principle. Urine contains magnesium, 

 phosphates and a small amount of iron, each of which will interfere 

 with the accurate determination of its calcium content if proper con- 

 ditions of acidity are not maintained during the precipitation. In the 

 following method the proper acidity is attained through the use of 

 sodium acetate and hydrochloric acid, and this with slow addition of 

 the ammonium oxalate reduces the danger of occlusion of magnesium 

 oxalate, calcium phosphate, or ferric phosphate in the calcium oxalate 

 precipitate. 



The calcium oxalate precipitate is either ignited and weighed as 

 CaO or determined volumetrically by titration with potassium per- 

 manganate. Magnesium is determined in the nitrate from the calcium 

 determination after destruction of the organic matter. It is determined 

 in the usual way by ignition of the magnesium ammonium phosphate 

 precipitate and weighing as the pyrophosphate. 



cessation of effervescence and the presence of some undecomposed calcium 

 carbonate at the bottom of the vessel are the indications of neutralization. 

 2 McCrudden: Jour. Biol. Chem., 7, 83, 1910; 10, 187, 1911. 



