PRACTICE XVII 



PREPARATION OF AMMONIUM MOLYBDATE SOLUTION* 



Dissolve 100 g. of molybdic acid in 400 cc. of ammonium hydroxid 

 of .96 specific gravity, and pour this solution slowly and with con- 

 stant stirring into 1250 cc. of nitric acid of 1.20 specific gravity. 

 It is best to cool the acid after the addition of each small amount 

 of ammonium hydroxid. Keep the mixture in a warm place several 

 days, or until a portion heated to 40 deposits no yellow precipitate 

 of ammonium phosphomolybdate. 



PRACTICE XVIII 



PREPARATION OF A STANDARD POTASSIUM HYDROXID 



SOLUTION i 



In 400 cc. distilled water dissolve the number of grams of the purest 

 potassium hydroxid to be obtained, sufficient to make 4 liters of a 

 solution, 1 cc. of which will be equivalent to 0.5 mg. of phosphorus. 

 Remove carbonates with barium hydroxid, as in Practice VIII. Then 

 make up to 500 cc. and titrate two 10-cc. portions with standard 

 hydrochloric acid, using phenolphthalein as indicator. Compute the 

 exact weight of potassium hydroxid in the remaining solution, and 

 dilute with a sufficient quantity of water to reduce the strength to 

 exactly 2.0809 g. potassium hydroxid per 100 cc., so that 1 cc. is 

 equivalent to 0.5 mg. phosphorus. 



Mix well, check up by again titrating, and label : Standard Potas- 

 sium Hydroxid (1 cc. = 0.5 mg. P). 



1 To be done by the instructor. 



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