OPTIONAL VOLUMETRIC METHOD 159 



These investigations have led to the adoption of the volumetric 

 method in the present form, and established its position as a pro- 

 cess which can be relied upon to give concordant and reliable re- 

 sults. For all ordinary analytical, routine and control work it may 

 be confidently used. In cases of controversy, and especially be- 

 fore the courts, it is advisable to check the data obtained by the 

 volumetric method against the results of the official gravimetric 

 determination. The principal credit for perfecting this method 

 is due to Kilgore, as is clearly shown by a study of the references 

 .given. 



143. Optional Volumetric Method. The principles on which this 

 rapid and accurate process is based have been described in detail 

 in the discussion of the Pemberton process. The method of con- 

 ducting the determination and the reagents employed as prescribed 

 by the official chemists are as follows : 



Reagents Used: Molybdic Acid. To 100 cubic centimeters of 

 the molybdic acid solution used in the official gravimetric method 

 already described, add five cubic centimeters of nitric acid of 1.42 

 specific gravity. If cloudy this solution should be filtered before 

 using. 



Potassium or Ammonium Nitrate Solution. Dissolve three 

 grams of the salt in 100 cubic centimeters of water. 



Nitric Acid. Dilute 100 cubic centimeters of nitric acid of 1.42 

 specific gravity to 1000 cubic centimeters with water. 



Potassium Hydro.vid. This solution contains 18.171 grams of 

 KOH in one liter. It is prepared by diluting 323.81 cubic centi- 

 meters of a normal solution of pure potassium hydroxid, free of 

 carbonates, to one liter. One milligram of the solution is equiva- 

 lent to one milligram of phosphorus pentoxid. 



Standard Nitric Acid Solution. The strength of this solution 

 is the same as, or one-half that of, the standard alkali solution, 

 and is determined by titrating against that solution, using phenol- 

 phthalein as indicator. 



Phenolphthalein Solution. One gram of phenolphthalein is 

 dissolved in 100 cubic centimeters of alcohol. 



Total Phosphoric Acid. Methods of Making Solution. Dis- 

 rsolve according to the official methods described in paragraph 61, 



