122 



Reagents. 



Nitric add. 50 cc of nitric acid (1.40 sp. gr.) in 1,000 cc of water. 



Sodium nitrate wash. 10 grams of sodium nitrate per 1,000 cc of water. 



Phosphomolybdic acid solution. 100 grams of phosphomolybdic acid (Kahlbaum's 

 preferred) in 750 cc of water and 250 cc of nitric acid (1.40 sp. gr.). 'This solution 

 must be freshly prepared not over three or four days old before using. If properly 

 made the evaporated residue from a portion of this solution is never white and readily 

 redissolves in the dilute nitric acid solution in the cold. 



Standard solutions. Standard caustic potash and nitric acid prepared for volu- 

 metric phosphoric acid diluted to 2 volumes. One cubic centimeter of this potassium 

 hydroxid solution is equal to 0.812 mg of potassium oxid. 



Determination. 



Transfer 10 cc of solution to a platinum dish, add 0.25 cc of sulphuric acid (1 to 1). 

 Evaporate to dryness and ignite to whiteness. Dissolve residue in hot water plus a 

 few drops of hydrochloric acid and transfer to a tall 200 cc beaker, add 30 cc phos- 

 phomolybdic acid solution and slowly evaporate to complete dryness on top of a 

 steam bath. 



It requires approximately 22 mg of phosphomolybdic acid, in order to have an 

 excess, for each milligram of potassium oxid present. 



Add 30 cc of nitric acid wash to the dried residue and stir thoroughly in the cold, 

 with a grinding motion with a policeman, allow to settle a moment and decant super- 

 natant liquid at once through a gooch crucible packed with moist filter paper pulp, 

 approximately one-sixteenth inch in thickness. Wash twice by decantation with 

 sodium nitrate wash, transfer precipitate to a gooch and wash with sodium nitrate 

 wash until acid free. Transfer gooch to casserole, run in excess standard alkali solu- 

 tion and add phenolphthalein. Heat to boiling and titrate excess alkali with standard 

 acid. 



Some samples of asbestos seem to hold or "fix" some of the excess acid, making the 

 gooch filter very hard to wash acid free. Hence it is suggested to use a paper pulp 

 filter. It is also desirable to make comparative tests, employing the usual asbestos 

 filter. 



If excess of phosphomolybdic acid has been used, the dried residue has a reddish 

 hue. If excess has not been added the residue is bright yellow. Residue should not 

 appear white. 



In each case, run blanks to ascertain corrections to be made for impurities. 



It is also desired that in sample No. 1 determinations of potash be made, not only 

 by the official method (which provides for direct evaporation of the solution without 

 addition of ammonia and ammonium oxalate), but also by the method applicable to 

 mixed fertilizers, adding ammonia and ammonium oxalate, followed by evaporation 

 and subsequent ignition with sulphuric acid. 



Several chemists have urged that this latter method of procedure be tried, as it is 

 claimed that the official method for potash salts gives too high results owing to impure 

 precipitates. 



The reports of results of cooperating chemists are as follows: 



