PHOSPHOEIC ACID AND SESQUIOXIDE OF IRON. 41 



is sufficient to- heat this mixture to 50 or 60 to evolve 

 a steady current of hydrochloric acid gas. 



The amount of sesquioxide of iron may be inferred 

 from the loss of weight, and the result controlled by 

 collecting the chloride of iron which passes over and 

 weighing it. 



22. PHOSPHORIC ACID AND SESQUIOXIDE OF IRON.* 



In order to separate phosphoric acid from sesqui- 

 oxide of iron, the compound is ignited with at least an 

 equal weight of carbonate of potassa and soda (No. 10), 

 the resulting mass exhausted with water, the solution 

 supersaturated with hydrochloric acid and then with 

 ammonia, and the phosphoric acid precipitated by sul- 

 phate of magnesia. 



The residual sesquioxide of iron retains some alkali. 



Or the sesquioxide of iron containing phosphoric 

 acid is dissolved in hydrochloric acid, precipitated by 

 ammonia, and digested with excess of sulphide of am- 

 monium (without previous filtration), until all the 

 sesquioxide is converted into sulphide of iron. When 

 the liquid is no longer green, but of a pure yellow 

 color, it is filtered off, and the phosphoric acid imme- 

 diately precipitated by sulphate of magnesia. 



For the accurate quantitative separation of a small 

 quantity of phosphoric acid from a large quantity of 

 sesquioxide of iron, the latter is dissolved in hydro- 

 chloric acid, and the solution heated to ebullition with 

 sulphite of soda till its color has changed to a bright 

 green, when all the sesquioxide of iron is converted 



* For analyses for practice, the phosphate of sesquioxide of iron 

 is prepared by precipitating sesquichloride of iron with phosphate 

 of soda. Or a mixture of phosphates may be prepared by precipi- 

 tating a solution containing sesquichloride of iron, chloride of 

 calcium, chloride of magnesium, and chloride of manganese. 



4* 



