76 METHODS FOR THE ANALYSIS OF URINE 



CHLORIDES 



Volhard- Arnold Method 1 



Principle. The urine is acidified with nitric acid and the 

 chlorides precipitated with a measured excess of standard silver 

 nitrate solution. The silver chloride formed is filtered off and in 

 the filtrate the excess silver nitrate is titrated back with standard 

 ammonium thiocyanate solution. Ferric ammonium sulphate is 

 used as an indicator. A red color due to the formation of ferric 

 thiocyanate indicates that an excess of thiocyanate is present and 

 that the end-point has been reached. 



Procedure. Place 10 c.c. of urine in a 100 c.c. volumetric 

 flask, add 20 to 30 drops of nitric acid (sp. gr. 1.2) and 2 c.c. of a 

 cold saturated solution of ferric alum. If necessary, at this point 

 a few drops of 8 per cent solution of potassium permanganate 

 may be added to dissipate the red color. Now slowly run in a 

 known volume of the standard silver nitrate solution (20 c.c. is 

 ordinarily used) in order to precipitate the chlorine and insure the 

 presence of an excess of silver nitrate. The mixture should be 

 continually shaken during the addition of the standard solution. 

 Allow the flask to stand ten minutes, then fill it to the 100 c.c. 

 graduation with distilled water and thoroughly mix the contents. 

 Now filter the mixture through a dry filter paper, collect 50 c.c. 

 of the filtrate and titrate it with standardized ammonium thio- 

 cyanate solution. The first permanent tinge of red-brown indicates 

 the end-point. Take the burette reading and compute the weight 

 of sodium chloride in the 10 c.c. of urine used. 



Calculation. The number of cubic centimeters of ammonium 

 thiocyanate solution used indicates the excess of standard silver 

 nitrate solution in the 50 c.c. of filtrate titrated. Multiply this 

 reading by 2, inasmuch as only one-half of the filtrate was em- 

 ployed, and subtract this product from the number of cubic 

 centimeters of silver nitrate (20 c.c.) originally used, in order to 

 obtain the actual number of cubic centimeters of silver nitrate 

 utilized in the precipitation of the chlorides in the 10 c.c. of urine 

 employed. 



To obtain the weight in grams of the sodium chloride in the 

 10 c.c. of urine used, multiply the number of cubic centimeters of 

 1 Hawk: Practical Physiological Chemistry, Sixth Edition, 1918. 



