PREPARATION OF SAMPLE 133 



dilute, especially on ebullition. The best solvent, however, for 

 calcium phosphates in the uranium method is incontestably hy- 

 drochloric acid which also very easily dissolves the iron and 

 aluminum phosphates often found with calcium phosphates. 



(3) Nitric Acid. In many laboratories nitric acid is preferred 

 in order to avoid, in part, the solution of ferric oxid which inter- 

 feres with the determination of phosphoric acid in certain pro- 

 cesses. Since it does not act in this way for the citro-magne- 

 sium uranium method, it is preferable to employ hydrochloric acid, 

 especially because it dissolves the iron completely and thus per- 

 mits the operator to judge of the success of the solvent action by 

 the completely white color of the residue. 



(4) Pyritic Phosphates. Certain phosphates contain pyrites 

 which hydrochloric acid does not readily dissolve, and there is 

 left, consequently, a residue more or less colored. In this case it 

 is necessary to add some nitric acid and to- prolong the boiling 

 until the pyrite has disappeared, since it might retain a small 

 quantity of phosphoric acid in the state of iron phosphate. 



(5) Sulfuric Acid. Some chemists decompose the phosphates 

 by means of dilute sulfuric acid. This method, which is cer- 

 tainly able to give good results for certain products and for cer- 

 tain processes, presents numerous inconveniences tending to 

 render its use objectionable for volumetric purposes. The cal- 

 cium sulfate formed, requires prolonged washings which lead 

 to chances of fatal error. 



If an aluminum phosphate be under examination, containing 

 only very little or no lime, sulfuric acid is to be preferred to 

 hydrochloric and nitric acids, since it attacks amblygonite, which, 

 as has been before stated, resists the action of the other two acids. 

 But these are cases which are met with very rarely, and which 

 can always be treated by the general method of previously fusing 

 the material with a mixture of sodium and potassium carbonate. 



In the great majority of cases the decomposition by hydro- 

 chloric acid is very easily accomplished by simply boiling in a 

 glass vessel, and without effecting the separation of the silica. 

 This operation is only necessary after the substance has been 

 fused with alkaline carbonates, or in case of substances which 



