Redox Reactions

Redox Reactions as the Basis of Titrations

All of us know that wine is made from the fermentation of grape juice using specific yeast cells. However, do you know wine requires a particular amount of fruit acid? So how is this acid content measured in industries? The determination of acid content is done by titration. Yes, the lab technique we all have done as well as read about.

Moreover, a specialized titration technique known as redox titration is used for analysis of wines for sulfur dioxide. This is just one example. Similarly, many other industrial processes require titration with respect to redox reactions. So let us know more about redox reactions as the basis of titration.

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Titration

It is a very common laboratory technique. Titration helps in the 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.

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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 redox system, a titration method helps in 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

When the oxidation-reduction reactions happen in a titration method, it is known as a redox titration. In this technique, transfer of electrons occurs in the reacting ions present in the aqueous solutions during the chemical reaction. This is further classified on the basis of reagent used in the 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 the 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

Titration

The result of the endpoint is noticeable when oxidation of the last of the reductant such as Fe2+ or C2O42– occurs. At this point, the solution retains the first lasting tinge of the MnO4 (pink colour) appears. The concentration can be 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 electron 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 addition of dilute sulphuric acid. The equation of the reaction is

Titration

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

Titration

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 reaction is of Cu(II)titration

The ability of iodine to produce intense blue colour with starch as the substance and its capacity to react with thiosulphate ions (S2O32– ) forms the basis of this method. The specific reaction with (S2O32–) is also a redox reaction

titration

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 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.

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 S2O32– + I2 → S4O62– + 2I

S2O32– + 2Br2 + 5H2O → 2SO42– + 2Br + 10 H+

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

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

Solution:  Option 2 is the correct answer. Bromine is stronger oxidant than iodine. Bromine is a stronger oxidizing agent in comparison to iodine. Therefore, bromine oxidizes S of S2O32– to 2SO42– whereas I2 oxidizes S to S4O62–. Upon calculation, you will find that the oxidation number of S4O62– is less than 2SO42–.

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