What role does sulfuric acid play in the titration of oxalic acid and potassium permanganate?

This article explains the oxalic acid titration curve and the role of sulfuric acid in the titration of oxalic acid and potassium permanganate.

Introduction：

Titration, often referred to as titrimetry, is a popular quantitative and volumetric method used in lab settings to determine an analyte's concentration in a mixture in the presence of a suitable indicator. The titration curve as in the topic oxalic acid titration curve is a curve represented as a graphical representation of the amount of titrate added during a titration process versus the pH value (or any other value under consideration) being changed.

Titration of Oxalic Acid with Potassium Permanganate:

The titration of oxalic acid C₂H₂O₄ with potassium permanganate KMnO₄ in the presence of sulfuric acid is an example of redox titration (transfer of electrons in between the reacting entities which leads to change in oxidation state) and is a common method to determine the concentration of oxalic acid present in a solution. If this reaction is carried out in normal conditions, it will result in the formation of dark brown manganese (IV) oxide due to a deficiency of protons. The reduction half-reaction with permanganate can be represented as:

MnO₄^- + 4H⁺ + 3e ⇌ MnO₂ + 2H₂O

A diluted form of sulfuric acid H₂SO₄ is introduced in the solution of two participants to avoid producing MnO₂. As KMnO₄ is an oxidizing agent an acidic environment amplifies its function than an alkaline one. This is exactly what sulfuric does here. The role of sulfuric acid in this reaction is to provide an acidic medium by providing surplus H⁺ ions and help during the protonating oxalic acid. Due to sulfuric acid, the reduction half-reaction can be shown as:

MnO₄^- + 8H⁺ + 5e ⇌ Mn²⁺ + 4H₂O

Moreover, oxalic acid serves as a reducing agent in this acidic solution while KMnO₄ continues to get absorbed and also functions as a self-indicator, negating the requirement for an additional indicator.  

Why? When permanganate is added to a solution containing a reducing agent, it becomes decolorized because solutions containing MnO^4– ions are purple while solutions carrying Mn²⁺ ions are colorless. The solution becomes purple (persistent pink color) when there is an excess of potassium permanganate in this medium. Thus, in an acidic solution, KMnO₄ acts as a self-indicator. During the reaction, oxalic acid is oxidized to CO₂ by KMnO₄, which itself gets reduced to MnSO₄. The overall reaction by combining the above two half-reactions can be stated as follows:

5C₂H₂O₄+ 2KMnO₄ +8H₂SO₄ →10CO₂ +2MnSO₄+8H₂O+K₂SO₄

Note from the equation that for every 5 moles of oxalic acid C₂H₂O₄ in this titration reaction, 2 moles of potassium permanganate 2KMnO₄ are required to complete the reaction where 8 moles of sulfuric acid H₂SO are being used to boost the  C₂H₂O₄ in its protonation. The reaction is redox because oxalic acid is oxidized to carbon dioxide and potassium permanganate is reduced to manganese (II) sulfate MnSO_4.

Oxalic acid titration curve:

A typical titration curve contains several regions; an initial region, buffer region, equivalence point, post-equivalence region, and endpoint. In the above reaction, the addition of permanganate causes a gentle upward slope in this curve which is marked as its initial region. As is added in oxalic acid, the oxalate ions C₂O₄(²⁻)  will start to resist the change and will act as a buffer. This region is termed a buffer region and is characterized as a relatively flat or slowly changing pH region in the curve of oxalic acid titration.

When stoichiometrically equivalent amounts of potassium permanganate and oxalic acid have reacted, an equivalence region is achieved at which the curve abruptly goes up indicating that the oxalic acid has lost all its ions against the pH change and is converted into salts. This leads first, to a sharp increase in the pH values and then a gradual increase indicating a post-equivalence region.

In post-equivalence, potassium permanganate has effectively dominated the solution. At the final stage, the endpoint of this curve is reached just after the post-equivalence region at which potassium permanganate changes its color showing somewhere in between persistent pink and purple.

In this redox titration process, the quantity of oxalic acid can be measured by using the stoichiometry of the balanced redox equation as mentioned above and the volume/concentration of the KMnO₄ solution used during the entire titration process.

Conclusion:

The titration of oxalic acid with the help of KMnO₄ and in the presence of sulfuric acid leads to a redox reaction. Here, sulfuric acid facilitates oxalic acid by providing an acidic medium for the reaction.  The oxalic acid titration curve explains the buffer action of oxalic acid, gradual pH increase, equivalence regions, and endpoint which is signaled by an indicator, in this case, potassium permanganate itself performed its role as an indicator.