How to do confirmatory test for Fe2+ Ion?

Confirmatory test for Fe2+

What is it used for ?

It is used to detect the presence of Fe2+ ions , also called ferrous ions, of formula Fe2+ in an aqueous solution .

Aqueous solution of Fe2+

Fe2+ ions give their aqueous solution a green coloration which is only perceptible at high concentrations, so it is common for an aqueous solution containing Fe 2+ ions to appear perfectly colorless.

The reagent

As for most metal cations the reagent in general used is sodium hydroxide (soda) but it is possible to use another strong base such as potassium hydroxide (potash) since it is the ion. hydroxide which is involved in the precipitation reaction.

The precipitate

Ferrous ions form a dark green precipitate in the presence of soda .

This precipitate is composed of iron II hydroxide with the chemical formula Fe (OH)2

It is an ionic solid where one iron II ion (Fe2+) combines for two hydroxide ions (OH-)

The precipitation reaction

If we do not take into account the spectator ions then the reaction between the iron II ions and the hydroxide ions can be read:

Fe2+ Ion + hydroxide ion -> Hydroxide Iron II

The reaction equation is written as follows:

Fe2+ (aq) + 2 OH- (aq) -> Fe (OH)2 (s)

According to this equation:

  • The reaction consumes twice as many hydroxide ions as Fe2+ ions
  • As much Fe2+ hydroxide is formed as the Fe2+ ions consumed.

If the test is carried out on a solution of iron II sulfate (SO2- (aq) + Fe2+ (aq)) by adding sodium hydroxide (Na + (aq) + OH- (aq)) then the complete reaction equation can be written:

SO2- (aq) + Fe2+ (aq) + 2Na+ (aq) + 2 OH- (aq) -> Fe (OH)2 (s) + SO2- (aq) + 2Na+ (aq)

Practical performance of the Fe 2+ ion test

  • Take a few milliliters of the test solution.
  • Place this sample solution in a test tube.
  • Add a few drops of soda (with a concentration of 0.1 mol / L for example).
  • Observe the result

Interpretation of the results of Fe 2+ ion test

  • If a dark green precipitate is formed then the presence of iron II ions is confirmed.
  • If we can not see any precipitate then we can deduce either that the test is negative (there are no iron ions) or that their concentration is of iron ions is so low that there is no no precipitation (or the amount of precipitate is so low that it is not visible to the naked eye)
  • If a precipitate is observed which is not blue in color, then this precipitate does not form from the iron II ions. It cannot be ruled out that the solution still contains iron ions but in a lower concentration than another ion, the precipitate of which masks the blue color of iron hydroxide.

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