Deduce the concentration of a saturated solution of chlorine

Authors Avatar

CHRISTOPHER TRANTER

Aims: Deduce the concentration of a saturated solution of chlorine

In order to determine the concentration of aqueous chlorine one must firstly reduce the chlorine to its ions, using a reducing agent, in this case Fe(II). I will be using FeCl2 powder for a source of Fe2+.

2FeCl2 (2) + Cl2(aq)   2FeCl3 (aq)

  [ EQUATION 1]

2Fe2+ (aq) + Cl2 (aq)      2Fe3+ (aq) + 2Cl-(aq)

  2          :       1        ratio

Now if one can calculate the amount of reducing agent left over, it will be possible to calculate the initial Cl2 solution concentration. It is possible to do this by titration of the Fe2+ with Potassium Manganate.

[ EQUATION 2]

MnO4- (aq) +  5Fe2+(aq)   +8H+(aq)        Mn2+ (aq)  + 5Fe3+(aq) + 4H2O(l)    

  1          :       5        ratio

 

Acidic conditions are necessary, because in neutral or alkaline conditions Potassium Manganate is reduced no farther than Mn+4 and a source of H+ is also needed. Sulphuric acid is a good source of H+, and the SO4-2 ions are not reactive. Hydrochloric acid would react with the KMnO4, and chlorine gas would be evolved. Nitric acid is not suitable as it can become involved in redox processes where the NO3- ion is reduced.

The end-point of the titration is detected by the first persisting pale pink colour change from purple. Even at the end-point the pink colour will fade after a short period as the MnO4- ions react with Mn2+ ions.

Join now!

As the chlorine solution is of concentration approximately 7g dm-3 it is possible to calculate quantities and concentration of other reactants so the experiment will give realistic outcomes. For instance if the concentration of MnO4- is too low in the titration with the Fe2+ then the volume used will be large, possibly requiring more than one burette volume. This introduces unnecessary inaccuracies, of having to repeat the set-up of the burette and also increases the time for the experiment. By working through an example calculation using a reasonable volume titre of approx 20.00 cm3, it is possible to determine the relevant ...

This is a preview of the whole essay