42 



PRACTICE 18. 

 PREPARATION OF AMMONIUM MOLYBDATE SOLUTION.* 



Dissolve loo gm. of molybic acid in 400 cc. of ammounium hydroxid of 

 .96 specific gravity and pour this solution slowly and with constant 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 4O 9 

 deposits no yellow precipitate of ammonium phosphomolybdate. 



PRACTICE 19. 

 PREPARATION OF A STANDARD POTASSIUM HYDROXID SOLUTION.* 



Dissolve the number of grams of chemically pure potassium hydroxid 

 (usually about 85 % ) sufficient to make 4 liters of a solution, one cc. of 

 which will be equivalent to .5 mg. of phosphorus, in 400 cc. distilled water. 

 Remove carbonates with barium hydroxid, as in Practice 14. Then make 

 up to 500 cc. and titrate 10 cc. portions with standard hydrochloric acid, 

 using phenolphthalein as indicator. Compute the exact weight of potas- 

 sium hydroxid in the remaining solution and dilute with a sufficient quan- 

 tity of water to reduce the strength to exactly 2.0809 S m - potassium hy- 

 droxid per 100 cc., so that I cc. is equivalent to .5 mg. phosphorus. 



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

 Hydroxid (ice. 5 mg. P.) 



PRACTICE 20. 



PREPARATION OF A STANDARD NITRIC ACID SOLUTION.* 



Determine the specific gravity of concentrated nitric acid. Measure out 

 sufficient to make 4 liters of solution of a strength equivalent per cc. to the 

 standard potassium hydroxid solution. Dilute with ammonia-free water 

 to 3^2 liters and titrate 25 cc. portions of the standard potassium hydroxid 

 with the dilute nitric acid, using phenolphthalein as indicator. Then add 

 sufficient ammonia-free water to make the nitric acid of the same titrating 

 strength as the standard alkali. Mix thoroughly and check up by another 

 titration. 



*To be done by the instructor. 



