URINE: QUANTITATIVE ANALYSIS. 395 



casserole on a hot plate or on a gauze and heat the contents almost 

 to the boiling-point. 1 To the hot solution add a standard solution of 

 argentic nitrate (see page 396) in slight excess. 2 Filter off the sil- 

 ver chloride while the solution is still hot and wash the precipitate 

 thoroughly with distilled water. To the filtrate, add i c.c. of a 

 saturated solution of ferric ammonium sulphate and then titrate with 

 a standard solution of ammonium thiocyanate (see page 397) until 

 the clear, slightly yellow fluid (or the opalescent, milky fluid, in 

 case there is much excess of argentic nitrate) changes to a slight red- 

 dish-brown color. The color of the end-point varies with the in- 

 dividual. The exact end-point reached is not so important as is the 

 securing of the same end-point in a series of determinations as 

 that obtained in the standardization of the standard solutions used. 



Calculation. The standard solution of argentic nitrate should be 

 made up so that I c.c. equals o.oio gram of sodium chloride and 

 i c.c. of the ammonium thiocyanate should be equivalent to i c.c. of 

 the argentic nitrate solution (see pp. 396 and 397). Then, if the 

 number of cubic centimeters of ammonium thiocyanate used be sub- 

 tracted from the number of cubic centimeters of argentic nitrate, 

 the difference is the number of cubic centimeters of argentic nitrate 

 actually used in the precipitation of chlorine as silver chloride. 

 This number, multiplied by o.oio, gives the weight in grams of the 

 sodium chloride in the 10 c.c. of urine used. If it is desired to 

 express the result in percentage of sodium chloride; move the deci- 

 mal point one place to the right. 



In a similar manner the weight or percentage .of chlorine may be 

 computed, using the factor 0.006 as explained in Mohr's method, 

 page 396. Calculate the quantity of sodium chloride and of chlo- 

 rine in the twenty- four hour urine specimen. 



2. Mohr's Method. To 10 c.c. of urine in a small platinum or 

 porcelain crucible or dish add about 2 grams of chlorine-free potas- 

 sium nitrate and evaporate to dryness at 100 C. (The evapora- 

 tion 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- 



1 If there is a slight precipitate, due to silicic acid from the casserole, this is 

 filtered off and the filtrate collected in a 200 c.c. beaker. 



2 This point is most easily recognized by keeping the solution hot and in 

 constant agitation while adding the argentic nitrate so that the silver chloride 

 formed coagulates and sinks, leaving a clear, supernatant fluid. 



