Redox Reactions as the Basis of Titrations

Titration

Titration is a very common laboratory technique. It helps in quantitative chemical analysis. It helps in the determination of an unknown concentration of an already known analyte. Moreover, titration also helps in volumetric analysis or measurement of volume. There are different types of titration techniques which is applicable according to the goals and methods. Let us learn about redox titration.

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Browse more Topics under Redox Reactions

• Balance Redox Reactions
• Classical Idea of Redox Reactions
• Oxidation Number
• Redox Reactions and Electrode Potential
• Types of Redox Reactions
• Redox Reactions – Electron Transfer Reactions

Types of  Methods

• Acid-base Titrations
• Redox Titrations
• Precipitation Titrations
• Complexometric Titrations

The most common titration methods that are commonly in use for “quantitative chemical analysis” are Redox and Acid-Base Titration. In the acid-base system, a titration method helps in finding out the strength of one solution against another solution by the use of the pH-sensitive indicator.

Similarly, in a redox system, a titration method helps in the determination of the strength of a particular oxidant or a reductant with the help redox-sensitive indicator. We will now discuss the definition of redox titration and how to use indicators in this technique.

Redox Titrations

Redox Titration is a laboratory technique for measuring the concentration of a particular analyte by initiating a redox reaction between the titrant and the analyte. When the oxidation-reduction reactions happen in a titration method, it is known as redox titration. A redox indicator and/or a potentiometer may be used in redox titration.

A common redox titration is treating an iodine solution with a reducing agent to make iodide while employing a starch indicator to aid identify the endpoint. In this technique, the transfer of electrons occurs in the reacting ions present in the aqueous solutions during the chemical reaction. This is further classified based on reagent used in the redox titration.

Principle of Redox Titration

A redox reaction can only be utilized as the basis for a titration if the following conditions are met:

• The redox reaction must be quick and almost complete (less than 99% success is unacceptable).
• The endpoint must be quantifiable or identifiable using a colour indicator or potentiometry.
• The electron exchange mechanism must be stoichiometric, which means that the redox systems (oxidizing and reducing agents) must be adequate and equal.

1. Reduction Reaction

A substance undergoes a reduction in the following ways:

• The addition of hydrogen.
• The process of removing oxygen.
• Acceptance of electrons.
• A reduction in overall oxidation state.

2. Oxidation Reaction

A substance undergoes oxidation in the following ways:

• The addition of oxygen.
• The hydrogen that was connected to the species was removed.
• Electron donation and loss
• An increase in the oxidation state of the material

Redox Titration Curve

To analyze redox titration, we must first determine the shape of the titration curve. The titration curve in an acid-base titration or a complexation titration indicates how the concentration of H₃O+ (as pH) or Mn+ (as pM) fluctuates as titrant is added. It is more convenient to monitor the potential of the titration reaction rather than the concentration of one species during a redox titration.

Sub-Divisions of Redox Titrations

• Permanganate Titrations
• Dichromate Titrations
• Iodimetric and Iodometric Titrations

1) Permanganate Titrations

Potassium Permanganate is the oxidizing agent in this type of redox titration method. Maintenance of the solution is done with the help of dilute sulphuric acid. Moreover, the addition of Sulphuric acid also helps to increase the hydrogen ions present in the solution.

In this technique, the reagent has intense colour due to the permanganate ion MnO4^–. In this case, the permanganate ion acts as a self-indicator in this method. The solution remains colourless prior to the endpoint. The equation of redox reaction

The result of the endpoint is noticeable when oxidation of the last of the reductant such as Fe²⁺ or C₂O₄^2– occurs. At this point, the solution retains the first lasting tinge of the MnO₄ (pink colour) appears. The concentration can be a minimum of 10^–6 mol L–1. This assures the minimal “overshoot” of the pink colour after the equivalence point.

The equivalence point is where reductant and oxidant are equal with respect to the mole stoichiometry or the total number of electrons lost and electrons gained in oxidation and reduction reaction will be equal. The potassium permanganate titration helps in the estimation of oxalic acid, hydrogen peroxide, ferrous salts, oxalates and more. However, it is very important to always standardize the solution prior to use.

2) Dichromate Titrations

In this method, potassium dichromate acts as the oxidant in the acidic medium. It is necessary to maintain the acidity of the medium by the addition of dilute sulphuric acid. The equation of the reaction is

In this situation, there is no substantial auto colour change as seen in the MnO4^– titration. Cr₂O₇ ^2– is not a self-indicator. However, Cr₂O₇ ^2– oxidizes the indicator substance diphenylamine soon after achieving the equivalence point thereby producing an intense blue colour. The change in signals the end point of the titration. We can use the potassium dichromate solution in titrations directly. This method helps in the estimation of ferrous salts and iodides.

3) Iodometric Titrations

This is an interesting but common method. In this case, free iodine reduction to iodide ions occurs as well as iodine ion oxidation to free iodine occurs. The oxidation and reduction reactions are

The solution acts as an indicator. The use of this method is limited to the reagents capable of oxidizing I^– ions. One of the examples of such a reaction is of Cu(II)

The ability of iodine to produce intense blue colour with starch as the substance and its capacity to react with thiosulphate ions (S₂O₃^2– ) forms the basis of this method. The specific reaction with (S₂O₃^2–) is also a redox reaction

In this case, I₂ is insoluble in nature with water but it remains in the solution in the form of KI₃ that contains KI. After the addition of starch, the iodine in the reaction liberates as iodide ions producing an intense blue colour. However, the colour disappears on the consumption of iodine by the thiosulphate ions. The endpoint of the reaction is easily noticeable. Thus, it is easy to determine the concentration of the unknown solution by stoichiometric calculation.

Redox Titration Example

Titration of Potassium Permanganate against Oxalic Acid

• Make a normal Oxalic acid solution of approximately 250 ml.
• The atomic mass of each constituent atom is added to get the molecular mass of oxalic acid. H₂C₂O₄.2H₂O (Oxalic acid) has a molecular mass of 126.
• Because the weight of oxalic acid needed to generate 1000 ml of 1M solution is 126 g. As a result, the amount of oxalic acid required to make 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 Solution

2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄.2H₂O –> K₂SO₄ + 2MnSO₄ + 18H₂O + 10CO₂

The analyte in this titration is oxalic acid, while the titrant is potassium permanganate. The reducing agent is oxalic acid, and the oxidizing agent is KMnO₄. Because the reaction takes place in an acidic media, the permanganate ion’s oxidizing power is boosted. The addition of weak sulfuric acid produces this acidic medium. KMnO₄ works as a marker for permanganate ions that are a rich purple colour. When the last drop of permanganate reaches the terminal, it turns a pale pink colour.

A Solved Question for You

Q: Refer to the reactions below to observe how thiosulphate reacts in a different manner with respect to iodine and bromine

2 S₂O₃^2– + I₂ → S₄O₆^2– + 2I^–

S₂O₃^2– + 2Br₂ + 5H₂O → 2SO₄^2– + 2Br^– + 10 H⁺

Select the statement that justifies the dual behaviour of thiosulphate according to the above reactions.

1. Iodine is a stronger oxidant than Bromine.
2. Bromine is a stronger oxidant than iodine.
3. Iodine undergoes reduction and bromine undergoes oxidation.
4. Thiosulphate undergoes oxidation and reduction by bromine and iodine, respectively.

Solution:  Option 2 is the correct answer. Bromine is a stronger oxidant than iodine. Bromine is a stronger oxidizing agent in comparison to iodine. Therefore, bromine oxidizes S of S₂O₃^2– to 2SO₄^2– whereas I₂ oxidizes S to S₄O₆^2–. Upon calculation, you will find that the oxidation number of S₄O₆^2– is less than 2SO₄^2–.