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Redox titration is a scientific laboratory method in which a reaction is caused between a given analyte and a titrant to determine its concentration. Redox indicators or Potentiometers are used during the method.
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Key Terms: Titration, oxidation-reduction reaction, concentration, Redox indicators, Molecular equations, Redox Titration, analytes, titration curve
What is Redox Reaction?
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Redox Titration includes both oxidation and reduction. A type of oxidation-reduction reaction occurs between the titrant and the analyte. Redox Titration is one of the most common laboratory methods of identifying the concentration of unknown analytes.
Redox Titration
To evaluate redox titration, it is necessary to have a corresponding titration curve. It is also convenient in monitoring reaction potential instead of focusing on reacting species.
Redox Titration Involves both Reduction and Oxidation processes, the details about them are explained further in the article.
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Reduction Reaction
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The process of reduction takes place in the following way-
- Firstly, the hydrogen atom is added to the substance.
- Secondly, the oxygen atom is removed from the substance.
- Thirdly, the substance starts accepting electrons.
- Lastly, there's a reduction in the oxidation of the substance.
Reduction Reaction
Read More: Integrated Rate Equation
Oxidation Process and Redox Titration
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Given below are the points that explain oxidation of a substance -
- In the process of oxidation, oxygen is added to the substance.
- Next, hydrogen is removed from the attached species.
- Thirdly, there's a loss of electrons.
- And lastly, oxidation increases in the substance.
Oxidation Process and Redox Titration
From the above two processes, it is clear that due to Redox Titration there's a transfer of electrons from an analyte to the Titrant. To get the endpoint of redox titration starch indicator is used.
For example, iodine solutions are treated with a reducing agent in which diatomic iodine is reduced to iodide ions (I–) due to which the blue color of the iodine solution fades away, this kind of titration is also known as iodometric titration.
Read More: Chemical Kinetics
Example for Redox Titration
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One fine example of Redox Titration is Potassium permanganate Titration against oxalic acid, details about the titration are explained below:
- Potassium Permanganate Titration(KMnO4) against Oxalic Acid (C2H2O4)
This experiment determines the strength of potassium permanganate through the titration process against the standard solution of 0.1M oxalic acid.
- In this titration firstly a standard 250ml oxalic acid solution is prepared.
- After that molecular mass of oxalic acid is determined from the atomic mass of each constituent atom.
- Does the molecular mass of H2C2O4.2H2O = 126
- If the Weight of an Oxalic acid which is required for producing 1000 ml of 1m solution is 126 g, then the weight of oxalic acid which is required for making 250 ml of 0.1 m solution = 126/1000 x 250 x 0.1 = 3.15 g
- Use of standard oxalic acid solution in determining the strength of KMnO4
Oxalic acid is the analyte and titrant is potassium permanganate in the following titration. In this titration, oxalic acid is a reducing agent and KMnO4 is an oxidizing agent.
An acidic medium is created from dilute sulphuric acid due to which the oxidizing power of permanganate ion increases.
| MnO4 + 8H+ + 5e- → Mn2+ + 4H2O |
KMnO4 acts as an indicator and indicates the dark purple color of permanganate ions. MnO4 is reduced to colorless manganous ions (Mn2+) due to the acidic medium. Lastly, the permanganate has a hint of light pink color while reaching its endpoint.
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Molecular Equations
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Molecular equations determine species reactions as their molecular formula. The equations involve subscripts that indicate their solid, liquid, gaseous, or aqueous nature.
Below are the molecular equations for Redox Titration:
| 2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[O] H2C2O4.2H2O + [O] → 2CO2 + 3[H2O] × 5 |
Read More: Gravimetric analysis
Complete Reaction
| 2KMnO4 + 3H2SO4 + 5H2C2O4.2H2O → K2SO4 + 2MnSO4 + 18H2O + 10CO2 |
Ionic Equation
| MnO4 + 8H+ + 5e- → Mn2+ + 4H2O ] 2 C2O24 → 2CO2 + 2e ] 5 |
Complete Reaction
| 2MnO4 + 16H+ + 5C2O24 → 2Mn2 + 8H2O + 10CO2 |
In the above chemical reaction, we can observe that 2 moles of KMnO4 create a reaction with 5 moles of oxalic acid.
Read More: Concentration of a solution
Things to Remember
- The method of titration includes the addition of a known amount of titrants added to an analyte until it reaches its endpoint.
- It contains an oxidizing and reducing agent.
- Some titrants have their own indicators for example potassium permanganate can be titrated against a colorless analyte.
- Standardised solution = Titrant; Analysed Substance = Analyte.
Read More: Quantitative chemical analysis
Sample Questions
Ques. What is the definition of Redox Titration? (1 Mark)
Ans. Redox titration is particularly based on reactions caused by redox in-between Elite and titrant. For example treatment of an iodine solution with the use of a reducing agent.
Ques. What is an iodometric titration? (2 Marks)
Ans. Iodometric titration occurs when an iodide solution is treated using a reduced agent which creates iodide content that is determined until its endpoint. The purple color of the iodine vanishes when the iodine is fully consumed in the reaction.
Ques. What type of indicator can be used in redox titration? (3 Marks)
Ans. The indicators that are used in redox titration are mentioned below-
- Indicators like organic redox systems of methylene blue.
- Complex metals including bipyridine and phenanthroline.
- Organic compounds.
Ques. In redox titration, the compound of known concentration is called analyte or titrant while the standard solution is called titrant or titrator. (3 Marks)
A.) True
B.) False
Ans. When performing an acid-base titration in a laboratory, we certainly must always have a way to detect if the neutralization reaction has occurred by some visual means. This is the reason scientists generally use an indicator which has a distinct difference in color when in an acidic or a basic medium.
A common indicator which we usually use in the laboratories for strong acid-strong base titrations is phenolphthalein.
The standard solution is the solution in a titration whose concentration is known to us. It is also called the titrant or titrator. It is gradually added to the solution whose concentration we do not know. This solution needs to be analyzed hence we call it an analyte.
Keeping this in mind we can conclude that the correct answer of the question given is Option (A). True
Note: While performing the experiment sometimes it so happens that a drop of pink appears and then vanishes. This is not the equivalence point. The base must be added till the pink color is persistent in the conical flask.
Ques. The end point of iodometric titrations is detected by adding starch just near the end point and not at the beginning of titration. Which statement is not valid for this fact? (5 Marks)
A.Starch forms a complex with I2.
B.Due to slow decomposition of complex, a diffuse endpoint is obtained.
C.Iodometric titrations are carried out in a strong alkaline medium where starch is either hydrolysed or decomposed.
D.Iodometric titrations are made in a weak acidic medium where starch is neither hydrolysed nor decomposed.
Ans. Iodometry is the method of the volumetric chemical analysis, where the appearance and disappearance of the iodine in the solution indicates the end point. The procedure of the Iodometry is based on the oxidation of the iodide into iodine.
The amount of the oxidising agents is determined by the Iodometric titrations. There are four types of titrations commonly used these are:
- Acid- base titrations.
- Complexometric titrations
- Redox titrations
- Precipitation titrations
In order to determine the end point of the reaction starch is used as an indicator. The formation of the Blue black complex indicates the appearance of the end point of the reaction. This can be chemically represented as:
I2+Na2S2O3 → 2NaI + Na2S2O4
Thus the option C. Iodometric titrations are carried out in strong alkaline medium where starch is either hydrolysed or decomposed, is the correct answer.
Note: Iodometric titrations are commonly used to analyse the concentration of the oxidizing agents in water samples, to study the oxygen saturation in the ecological studies or to analyse the active chlorine content in the swimming pool.
Ques. In the titration K2Cr2O7 and FeSO4, the following data is obtained, V1mL of M1K2Cr2O7 requires V2mL of M2FeSO4. Which of the following relations is true for the above titration? (5 Marks)
A) 6M1V1 = M2V2
B) M1V1 = 6M2V2
C) M1V1 = M2V2
D) 3M1V1 = 4M2V2
Ans. This is an oxidation-reduction titration as Potassium dichromate (K2Cr2O7) is an oxidizing agent and oxidized Iron(II) sulphate (FeSO4) to Iron(III) sulphate {Fe2(SO4)3} in the presence of sulphuric acid (H2SO4) in the aqueous medium in which ions are free to move and react.
The balanced chemical equation for this reaction is:
K2Cr2O7 + 7H2SO4 + 6FeSO4, → K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O
.From the above balanced equation, it is clear from the coefficients of substances that for every one mole of Potassium dichromate, six moles of Iron (II) sulphate are used.
Finding the number of moles of Potassium dichromate (K2Cr2O7) = concentration × volume = M1V1 ---------(i)
Finding the number of moles of Iron (II) sulphate (FeSO4) = concentration × volume = M2V2 ---------(ii)
From the balanced chemical equation:
The number of moles of Iron (II) sulphate (FeSO4) = 6 x The number of moles of Potassium dichromate (K2Cr2O7) ⇒ M2V2 = 6M1V1
Therefore, option A. is the correct answer.
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