57 2 PHYSIOLOGICAL CHEMISTRY 



i 



solution (sp. gr. 0.90). Let stand for some time (two hours is usually enough) 

 then filter and wash the precipitate with 2.5 per cent ammonia until practically 

 free from chlorides. Ignite to whiteness or to a grayish-white ash and weigh. 

 Multiply the weight of magnesium pyrophosphate thus obtained by 0.637 to get 

 the weight of P 2 O 5 . 



Calculation. Calculate as explained above. 



Chlorides 



i. Volhard-Arnold Method. 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 am- 

 monium sulphate is used as an indicator. A red color due to the forma- 

 tion 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 20-30 

 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 1 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 10 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 thiocyanate solution. 2 The first per- 

 manent 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 employed, 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. 



1 Standard silver nitrate solution may be prepared by dissolving 29.042 grams of silver 

 nitrate in i liter of distilled water. Each cubic centimeter of this solution is equivalent to- 

 o.oio gram of sodium chloride or to 0.006 gram of chlorine. 



2 This solution is made of such a strength that i c.c. of it is equal to i c.c. of the stand- 

 ard silver nitrate solution used. To prepare the solution dissolve 13 grams of ammonium 

 thiocyanate, NH^SCN, in a little less than a liter of water. In a small flask place 20 c.c. 

 of the standard silver nitrate solution, 5 c.c. of the ferric alum solution and 4 c.c. of nitric 

 acid (sp. gr. 1.2),' add water to make the total volume 100 c.c. and thoroughly mix the con- 

 tents of the flask. Now nfn in the ammonium thiocyanate solution from a burette until a 

 permanent red-brown tinge is produced. This is the end-reaction and indicates that the 

 last trace of silver nitrate has been precipitated. Take the burette reading and calculate 

 the amount of water necessary to use in diluting the ammonium thiocyanate in order that 

 10 cc, of this solution may be exactly equal to 10 c.c. of the silver nitrate solution. Make 

 this dilution and titrate again to be certain that the solution is of the proper strength. 



