SWEDISH DETERMINATION OF PHOSPHORIC ACID 75 



* 



occasionally shaken) at the ordinary temperature of the room for 

 two hours, the mixture is filtered. 



2. The Determination. To 25 cubic centimeters of the super- 

 phosphate solution thus prepared (or a quantity of the 

 sample equal to one-tenth gram phosphorus pentoxid) add a 

 quantity of molybdic solution sufficient for complete precipitation, 

 leave standing for four hours in a beaker covered with a watch- 

 glass; decant the solution through a small filter, wash the pre- 

 cipitate first by decantation, then on the filter, with a mixture con- 

 taining 100 parts fnolybdic solution, 20 parts nitric acid of 

 1.2 specific gravity, and 80 parts water, until a few drops 

 put into alcohol, to which some dilute sulfuric acid has been added, 

 do not any longer cause turbidity. The molybdic precipitate is 

 now washed with a little water from the filter into a beaker 

 and particles adhering to the filter are dissolved by a hot mix- 

 ture of one part ammonia and three parts water, which is allowed 

 to flow into the beaker till the precipitate is finally completely 

 dissolved in it. To the clear solution add dilute hydrochloric acid 

 while stirring till the yellow precipitate formed by the acid is no 

 longer immediately dissolved; then add from six to eight cubic 

 centimeters of ammonia through the filter. The volume of the 

 solution is not to exceed 75 cubic centimeters. It is cooled com- 

 pletely and one cubic centimeter of magnesia mixture is added 

 from a burette for every centigram of phosphorus pentoxid 

 which it is expected to contain, and finally one-quarter* of 

 its volume of ammonia is added. The precipitate may be filtered 

 after four hours, and washed on the filter, preferably by means of 

 suction, with a mixture of one part ammonia and three parts 

 water till the filtrate is entirely free from chlorin. After drying, 

 heat the precipitate, first gently, then stronger, and finally with a 

 blast for a few minutes and then weigh it. 



Treated with hydrochloric acid it must leave no insoluble residue 

 (SiO 2 ), nor should hydrogen sulfid cause any precipitation in the 

 solution thus formed (MoO 3 ). 



(b) Total Phosphoric Acid. i. In Superphosphates. For the 

 determination of total phosphoric acid, treat a weighed quantity 

 of the superphosphate with nitric acid, if necessary to bring a dif- 



