Question

Lab Report Sheet:

Colorimetric Determination of Iron

Iron(II) reacts with water by a hydrolysis reaction. In order to prevent this, acid has been added to the standard iron solution. How would your final results change if no acid had been added to the standard iron solution?

Answer 1

Answer 2

In near or neutral pH conditions, Iron(II) reacts with water by a hydrolysis reaction. The product is very easily oxidised from iron(II) to iron(III) like Fe₂O₃. by dissolved air in the solution or contact with air at the surface. [Try making some iron(II) hydoxide by adding OH- ions to Fe2+ soln in a test tube and leave for 30 min to see this].

In an acid solution the Fe2+ ion is really stable and can be kept unchanged for a good while ,long enough to do the expt over a week or so. The acid [H+] stops the complex ion containing OH from being formed.

From what I remember, the iron ion is surrounded by six ligand positions [ is hexadentate] which can be occupied by coordinated water molecules or OH- groups [from water]. This affects the redox potentials.ie ,ease of converting iron(II) to iron(III). In the presence of excess acid, there are no OH- group to coordinate. When iron(III) complexes form, two of these with hexadentate ligands can join to form a bridge complex .. this is why the precipitates of iron hydroxide look gelatinous. At pre-university level chemistry it is simpliied to eg Fe(OH)3. x H2O.