Chemical amounts (A2)

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Dilute sulphuric acid is added in the permanganate titration against oxalic acid. Please can you tell me why? Can hydrochloric acid or nitric acid be used instead ?

Jenkin replies ...
The sulphuric acid is needed as a source of hydrogen ions. The ionic equation for the reaction is
 
2MnO₄^- + 5C₂O₄^2- + 16H⁺ -> 2Mn²⁺ + 10CO₂ + 8H₂O
 
This shows us that lots of H⁺ are needed, more than the oxalic acid (which we should really be calling ethanedioic acid) itself can provide.
 
If hydrochloric acid were to be used, there is the possibility that the permanganate could oxidise the Cl^- ions to chlorine gas. Nitric acid is itself a strong oxidising agent and could complicate things by reacting with the ethanedioic acid, releasing nasty oxides of nitrogen as well as causing inaccurate titration results. So, in practice, sulphuric acid is always used.

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updated: 14 December 2006