§ 52. IRON. 67 



potassic ferricyanide ; ferric salts give no precipitate with 

 this reagent. 



Ferric salts give a deep red color with potassic sulpho- 

 cyanate ; this reaction is exceedingly delicate. Nitric 

 acid causes the color to disappear after a while, and am- 

 nionic hydrate destroys it immediately. Ferrous salts 

 give no color with this reagent. 



Ferrous salts are converted into ferric compounds when 

 heated with nitric acid. 



Quantitative estimation. — Iron may be determined by 

 a gravimetric or a volumetric process. In the former 

 case it is weighed as sesquioxide, Fe^Oj. 



a. Add ammonic hydrate in excess to the hot solution, 

 in which the iron has been completely oxidized by heating 

 with nitric acid, if any ferrous oxide Avas present, heat 

 the' mixture almost to boiling, and then let it stand imtil 

 the larger part of the liquid can be decanted into the fil- 

 ter ; wash the precipitate several times by decantation, 

 and afterwards on the filter, until a drop of the washings 

 leaves no residue on evaporation on platinum foil, and 

 ignite the precipitate and filter separately. The residue 

 is pure ferric oxide, and contains 70"! ^ of iron. 



If the substance analyzed contained silica, this precipi- 

 tate is liable to be contaminated with it, and should be 

 digested with concentrated hydrochloric acid, after hav- 

 ing been gently ignited ; if silica is present, it will remain 

 undissolved, and may be filtered out and weighed. 



h. The volumetric process, with potassic permanganate, 

 is particularly convenient for the determination of iron 

 in the presence of aluminium. The iron is converted into 

 a ferrous salt, and then it is ascertained how much of a 

 solution of permanganate of known strength is required, 

 to oxidize the ferrous to a ferric salt. 



To make the solution of permanganate, dissolve about 

 8 grms. of the crystallized potassic permanganate of the 



