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The titration is between oxalic acid and potassium permanganate, with $\ce{H2SO4}$ added to oxalic acid. My question is what role other than a dehydrating agent and maybe catalysing does the $\ce{H2SO4}$ play in this titration.

The question came to me while reading a book (Advanced Chemistry by Philip Matthews - Cambridge university press), I was reading about titrations and this question came to me, but it isn't mentioned in the book.

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    $\begingroup$ Write down the balanced equation of the reaction and you'll see right away that you need some extra acid. That's what it is for. $\endgroup$ – Ivan Neretin Oct 12 '15 at 14:39
  • $\begingroup$ @IvanNeretin Why not HNO3 or HCl? $\endgroup$ – Reeshabh Ranjan Nov 30 '16 at 2:24
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    $\begingroup$ @ReeshabhRanjan HNO3 itself may oxidize something. HCl itself may get oxidized by permanganate. $\endgroup$ – Ivan Neretin Nov 30 '16 at 6:13
  • $\begingroup$ how does it act as a dehyradting agent in the titratuion? $\endgroup$ – starunique2016 Aug 30 '18 at 10:06
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The used analytical reaction of purple permanganate to colourless $\ce{Mn^2+}$ occurs under acidic conditions:

$$\ce{MnO4- + 8H+ + 5e- <=> Mn^2+ + 4H2O}$$

Under neutral conditions, permanganate would be reduced to dark brown manganese(IV) oxide:

$$\ce{MnO4- + 4H+ + 3e- <=> MnO2 + 2H2O}$$

Therefore, sulfuric acid is added to make the solution acidic. Actually, excess acid is required since $\ce{H+}$ is consumed during the reaction (8 mol $\ce{H+}$ per 1 mol $\ce{MnO4-}$).

Sulfuric acid is used because it is stable towards oxidation; whereas, for example, hydrochloric acid would be oxidized to chlorine by permanganate.

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Thanks to the comment by Ivan Neretin, I figured out that H2SO4 is actually to provide surplus H+ ions, to keep the reaction going And also it acts as a medium which doesn't interfere with the oxidizing agent to keep the quantitative purpose of the reaction

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There are three reasons:

  1. to provide acidic medium
  2. to oxidize $\ce{Mn^{+2}}$ to $\ce{Mn^{+3}}$
  3. to speed up the reaction
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Sulfuric acid acts as a catalyst in the reaction $\ce{MnO4- + 8H+ + 5Fe2+ -> 5Fe3+ + Mn2+ + 4H2O}$

It initiates a 5e transfer in the ionic equation It used rather than $\ce{HCl}$ because $\ce{HCl}$ is volatile acid which will oxidize to $\ce{Cl}$ in presence of $\ce{KMnO4}$ (strong oxidizing agent). $\ce{H2SO4}$ is stable to oxidization. Also excess $\ce{H+}$ is used because for every 1mole of $\ce{MnO4-}$ , 8 moles of $\ce{H+}$ is consumed.

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H2SO4 increase the acidic content of the solution so as to prevent MnO4 (purple) to reduced to MnO2(dark brow). Hence sulfuric acid is stable in the present of strong oxidising agent. If HCl acid is use it will be oxidised to Cl and make the end point much higher. Furtherore sulfuric acid increase the H+ as it consume.

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protected by Community♦ Mar 29 '18 at 7:51

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