Titrimetric Determination of Fe III

The reduction potential of the Sn(lV)/Sn(ll) couple in acid solution, although not know precisely, shows that Sn(Il) is likely to reduce Fe(lll) to Fe(ll) but SnCl2 solutions are not very stable and are kept in contact with Sn. However, when an excess of SnClj solution is added to an Fe(lII) solution, it reduces it to Fe(II). Before this is titrated eg against dichromate, the excess Sn(ll) has to be removed by reaction with HgCl2 solution carefully when the excess Sn(ll) is removed as can be seen by the formation of Hg(I) chloride precipitate. 

Weigh out 8 g purest iron(lll) chloride hexahydrate, into a 400 cm beaker. Add water containing a few cm cone. HCl and dissolve by stirring. Transfer to a volumetric flask and make up to 250 cm Pipette a 25.0 cm aliquot into a 250 cm conical flask, add 3 cm cone. HCl and then add, dropwise 0.25 M tin(II) chloride solution until the yellow colour just disappears and then gradually add a saturated mercury(Il) chloride solution to remove the excess tin(II) when a slight silky mercury(l) chloride is formed. Allow to stand for a minute, add 5 cm of orthophosphoric acid, a few drops of N-methyldiphenylamine indicator and titrate with standard M/60 dichromate solution as above. 

When the solution of the mixture is titrated against standard dichromate solution, only Fe(II) reacts, being oxidised to Fe(III). However, when an identical aliquot is treated with SnCl2 solution as above and then titrated against the same dichromate solution, the titre gives the combined Fe(II) and Fe(IIl). The latter can be calculated from the difference between the two titres. 

Using purest solids, weigh out accurately 9.80 g of (NH4)2S04.FeS04.6H20 and 6.03 g (NH4)Fe(S04)2.12H20, acidify with dil. sulphuric acid and make up to 250 cm in a volumetric flask. 

Prepare a standard dichromate solution by weighing accurately between 0.9 and 1.1 g of the dried purest potassium dichromate and making up to 250 cm with water. 

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