Iron is searched to the third group, together with chromium and aluminum. His research deserves special precautions. Often iron is a pollutant outside in the systematic search of the cations.
If the iron was present in the initial substance it is presumed that it is entirely, or almost, in the form of Fe 2+. This is because we have moved from the second analytic group, or a solfidrica solution. Fe 3+ is able to oxidize the ion sulfide to elemental sulfur (S °), and then reduced to Fe 2+.
(E ° Fe 3+ / Fe 2+ = 0.77 V | E ° S ° / S 2- = - 0.4 V)
It makes a control on the solution coming from the second group to see if it is present Fe 2+ (Fe 3+ or more difficult). This is necessary because, if it is present, the solution will be treated with concentrated HNO 3. We will understand the reason for this later.
Preliminary analysis (search for ferrous salts)
It picks up an aliquot of the solution coming from the second group to control the presence of Fe 2+.
Wise with potassium ferricyanide
There is a very reliable way to reveal it. Our rate are added a few drops of potassium ferricyanide. If present Fe 2+ precipitates immediately ferrous ferricyanide, blue.
Had he been present Fe 3+ would form a soluble complex of ferric ferricyanide, Fe [Fe (CN) 6], which would be colored red-brown soft solution.
Iron Research at the third analytical group
If in the preliminary analysis, we detected the presence of Fe (II), then the solution of the third group is treated with HNO 3 before being basified to precipitate the hydroxides. This treatment is specific for iron. In fact, the Fe (II) has the characteristic, this time not useful, to form amino-complex, and therefore not be able to quantitatively precipitate it as hydroxide.
For this reason it to oxidise Fe (III) with concentrated HNO 3:
3Fe 2+ + NO 3 - + 4H + 3Fe 3+ + NO + 2H 2 O
that does not have the same characteristic feature.
Subsequently, as we advance, the Fe (III) is precipitated as hydroxide (alkaline environment for NH 3)
A time separated from aluminum and chromium, the rust-colored precipitate of ferric hydroxide are made to specific recognition assays.
Recognition of Iron
For both assays that we will see we need a preliminary step. The intention is to Fe 3+ in solution, and to this end it is the precipitate of ferric hydroxide with dilute hydrochloric acid (2N).
The hydroxide is easily brought in solution:
Fe (OH) 3 + 3H + Fe 3+ + 3H 2 O
In fact, the Fe 3+ ion in solution coordinates around if 6 water molecules (esaacquoferrico ion).
Fe +3 + 6H 2 O Fe (H 2 O) 6 3+
The solution thus obtained should be colorless, but if the esaacquoferrico ion undergoes partial hydrolysis is possible to obtain slightly colored solution from yellow to red rust. Obviously we speak of a clear solution but colorful, there should be no precipitate.
Fe (H 2 O) 6 3+ [Fe (H 2 O) 5 (OH)] 2+ + H +
Wise with potassium ferrocyanide K 4 [Fe (CN) 6]
To the solution containing Fe 3+ are added a few drops of reagent. Precipitates ferric ferrocyanide, blue staining.
It creates a two-phase system by adding ether to our solution. Then they bring in two strokes solution of potassium thiocyanate. It forms a complex that is extracted from the organic solvent (ether).
[Fe (H 2 O) 5 (OH)] 2+ + NCS - [Fe (H 2 O) 5 NCS] 2+ + OH -
It is ferric thiocyanate, red precipitate.
The complex is not very strong and then operate using a reagent excess (KCNS) to shift the balance of complex formation to the right. If these is a large excess, however, it is a more complex form, colored red and soluble in the aqueous phase (K 3 Fe (SCN) 6).
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