THE MOLYBDIC ACID METHOD Jl 



are dissolved in 50 cubic centimeters of aqua regia, made of three 

 parts of hydrochloric acid of 1.12 specific gravity and one part 

 of nitric acid of 1.25 specific gravity, or the solvent may be made 

 of a mixture of 20 cubic centimeters of nitric acid of 1.42 

 specific gravity and 50 cubic centimeters of sulfuric acid of 1.8 

 specific gravity. The boiling should continue for half an hour. 

 The solution is made up to half a liter and filtered. Fifty cubic 

 centimeters of the filtrate containing the phosphoric acid, (with 

 double superphosphates, 25 cubic centimeters), are digested with 

 200 cubic centimeters, of ammonium molybdate solution for 

 three hours at 50 in a water bath and, after cooling, filtered, so 

 that as little as possible of the precipitate is collected upon the 

 filter, which is made of strong paper. 



The yellow precipitate is washed by decantation in the flask 

 nine times with 20 cubic centimeters of molybdic solution 

 diluted with one volume of water and the filter washed out once 

 with the same quantity of liquid. The funnel, with the filter, is 

 immediately placed upon the flask and the portion of the precipi- 

 tate collected in the filter dissolved in five per cent, ammonia, 

 which is easily accomplished by throwing ammonia upon it from 

 a wash bottle. Afterwards the filter is washed with a sufficient 

 quantity of hot water and finally removed. The contents of the 

 flask are neutralized while warm with hydrochloric acid, the acid 

 being added until the precipitate first formed, after continued shak- 

 ing, is again dissolved in the liquid. The solution is then cooled 

 and treated, drop by drop, with constant stirring, with 20 cubic 

 centimeters of magnesia mixture. Finally 25 cubic centimeters of 

 dilute ammonia solution are added, the precipitate is not shaken, 

 and after two hours is filtered through a gooch. 



For the filtering of the ammonium magnesium phosphate by 

 the molybdic method, freshly prepared felts are always employed 

 since the remarkably fine crystalline precipitates will pass through 

 a filter which has once been used. It is necessary also that 

 special precautions be taken in the ignition. The crucible 

 should be heated in a platinum cap, Vhich has the purpose of 

 protecting the contents of the crucible from the access of redu- 

 cing gases during the ignition. After redness has been reached 



