Redox Titration

What is Redox Titration?

Redox Titration is a laboratory method of determining the concentration of a given analyte by causing a redox reaction between the titrant and the analyte. These types of titrations sometimes require the use of a potentiometer or a redox indicator.

Redox titration is based on an oxidation-reduction reaction between the titrant and the analyte. It is one of the most common laboratory methods to identify the concentration of unknown analytes.

In order to evaluate redox titrations, the shape of the corresponding titration curve must be obtained. In these types of titration, it proves convenient to monitor the reaction potential instead of monitoring the concentration of a reacting species.

As discussed earlier, redox reactions involve both oxidation and reduction. The key features of reduction and oxidation are discussed below.

Reduction

A substance can undergo reduction can occur via:

• The removal of oxygen.
• The acceptance of electrons.
• A reduction in the overall oxidation state.

Oxidation

The following points describe a substance that has undergone oxidation.

• Removal of hydrogen which was attached to the species.
• The donation/loss of electrons.
• An increase in the oxidation state exhibited by the substance.

Thus, it can be understood that redox titrations involve a transfer of electrons between the given analyte and the titrant. An example of redox titration is the treatment of an iodine solution with a reducing agent. The endpoint of this titration is detected with the help of a starch indicator.

In the example described above, the diatomic iodine is reduced to iodide ions (I), and the iodine solution loses its blue colour. This titration is commonly referred to as iodometric titration.

Redox Titration Example

An example of redox titration is the titration of potassium permanganate (KMnO4) against oxalic acid (C2H2O4). The procedure and details of this titration are discussed below.

Titration of Potassium Permanganate against Oxalic Acid

• Prepare a standard Oxalic acid solution of about 250 ml.
• The molecular mass of oxalic acid is calculated by adding the atomic mass of each constituent atom
• The molecular mass of H2C2O4.2H2O = 126
• Since the weight of oxalic acid that is required to make 1000 ml of 1M solution is 126 g. Hence, the weight of oxalic acid needed to prepare 250 ml of 0.1 M solution = 126/1000 x 250 x 0.1 = 3.15 g

Determining the Strength of KMnO4 using Standard Oxalic Acid Soln

In this titration, the analyte is oxalic acid and the titrant is potassium permanganate. The oxalic acid acts as a reducing agent, and the KMnO4 acts as an oxidizing agent. Since the reaction takes place in an acidic medium, the oxidizing power of the permanganate ion is increased. This acidic medium is created by the addition of dilute sulfuric acid.

$$\begin{array}{l}MnO_{4}^{-}\;+8H^{+}\;+5e^{-}\rightarrow Mn^{2+}\;+4H_{2}O\end{array}$$

KMnO4 acts as an indicator of where the permanganate ions are a deep purple colour. In this redox titration, MnO4 is reduced to colourless manganous ions (Mn2+) in the acidic medium. The last drop of permanganate gives a light pink colour on reaching the endpoint. The following chemical equation can represent the reaction that occurs.

Molecular equation

$$\begin{array}{l}2KMnO_{4}\;+3H_{2}SO_{4}\rightarrow K_{2}SO_{4}\;+2MnSO_{4}\;+3H_{2}O\;+5[O]\end{array}$$

$$\begin{array}{l}H_{2}C_{2}O_{4}.2H_{2}O\;+[O]\rightarrow 2CO_{2}\;+3[H_{2}O]\times 5\end{array}$$

Complete Reaction

$$\begin{array}{l}2KMnO_{4}\;+3H_{2}SO_{4}\;+5H_{2}C_{2}O_{4}.2H_{2}O\rightarrow K_{2}SO_{4}\;+2MnSO_{4}\;+18H_{2}O\;+10CO_{2}\end{array}$$

Ionic equation

$$\begin{array}{l}MnO_{4}^{-}\;+8H^{+}\;+5e^{-}\rightarrow Mn^{2+}\;+4H_{2}O]\times 2\end{array}$$

$$\begin{array}{l}C_{2}O_{4}^{2-}\rightarrow 2CO_{2}\;+2e^{-}]\times 5\end{array}$$

Complete Reaction

$$\begin{array}{l}2MnO_{4}^{-}\;+16H^{+}\;+5C_{2}O_{4}^{2-}\rightarrow 2Mn^{2+}\;+8H_{2}O\;+10CO_{2}\end{array}$$

What are oxidation-reduction reactions?

Oxidation-reduction reactions are chemical reactions that involve the transfer of electrons between the reacting species. These electron transfers are accompanied by a change in the oxidation state of the reactants.

What are oxidizing agents?

An oxidizing agent is an electron-accepting species that is readily reduced in an oxidation-reduction reaction. The oxidation numbers of these species tend to decrease in redox reactions. Examples: nitric acid (HNO3) and hydrogen peroxide (H2O2).

What are reducing agents?

Reducing agents are the electron-donating species that readily undergo oxidation in oxidation-reduction reactions. These species tend to lose electrons in redox reactions and their oxidation number increases. Examples: zinc and lithium.

What is the purpose of titration in chemistry?

Titrating a simple solution with a volume of a regular acid solution (of known concentration) needed to neutralize it may be used to determine its concentration. The aim of the titration is to find the equivalence point, which is the point where chemically equivalent quantities of the reactants are combined.

What does it mean to titrate a solution?

Titration is the incremental addition of a known concentration solution (called a titrant) to a known volume of an unknown concentration solution before the reaction achieves neutralization, which is often signalled by a colour shift.

From the above-balanced chemical reaction, it can be observed that 2 moles of KMnO4 reacts with 5 moles of oxalic acid. To learn more about redox titration and the different types of titration, register with BYJU’S and download the mobile application on your smartphone.

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