Theory & Procedure, Determination of concentration of KMnO₄ solution | Additional Study Material for NEET PDF Download

Objective

Our objective is to determine the strength of KMnO₄ solution by titrating it against a standard solution of;

• Oxalic acid
• Ferrous ammonium sulphate (Mohr’s salt)

The Theory

What is Titration?

Titration is a common laboratory method of qualitative chemical analysis that can be used to determine the unknown concentration of a solution (analyte). The basis of this process is the reaction between the analyte and a solution of unknown concentration (standard solution). The analyte is taken in a conical flask using a pipette and the solution of known concentration is taken in a calibrated burette (titrant).

Some Important Terms in Titration

1. Standard solution

A solution whose concentration is known, is called a standard solution. The substance used to prepare a standard solution is called the primary standard. Oxalic acid and sodium carbonate are some examples.

2. Concentration of a solution

Concentration of a solution is defined as the amount of a solute present in a definite volume of the solvent. Concentration of a solution can be expressed in different ways.

• Normality: Normality of a solution is defined as the number of gram equivalent of solute per litre of the solution. It is denoted by ‘N’.

• Molarity: Molarity of a solution is defined as the number of gram moles of the solute per litre of the solution. It is denoted by ‘M’.

3. End point of a titration

The endpoint of a titration is the point at which the reaction between the titrant and the analyte becomes complete. Generally the endpoint of a titration is determined using indicators. In some cases, either the reactant or the product can serve as the indicator. A best example is the redox titration using potassium permanganate.

Titrations can be classified as:

• Acid-Base Titrations or Acidimetry and Alkalimetry
• Oxidation-Reduction Titrations or Redox Titrations
• Precipitation Titrations
• Complexometric Titrations

We will learn about Redox titrations.

Oxidation-Reduction Titrations or Redox Titrations

The titration based on oxidation and reduction reaction between the titrant and analyte is called Redox titration. Oxidation is the process of the addition of oxygen or removal of hydrogen/electron and reduction involves the process of addition of hydrogen/electrons or removal of oxygen. Oxidizing agents are substances that gain one or more electrons and are reduced. Reducing agents are substances that lose one or more electrons and are oxidized. That is, oxidizing agents are electron acceptors, and reducing agents are electron donors.

In redox systems, the titration method can be adopted to determine the strength of a reductant/oxidant using a redox sensitive indicator. Redox titrations involving potassium permanganate are called permanganometric titrations. In these reactions, MnO^4- ions acts as the self indicator.

Titration of  KMnO₄ against Oxalic acid

Preparation of standard solution of Oxalic acid [250 ml M/10 (0.1 M) solution]

The molecular mass of crystalline oxalic acid is, H₂C₂O₄.2H₂O = 126

Weight of oxalic acid crystals required to prepare 1000 ml of 1 M solution = 126 g

Therefore, weight of oxalic acid required to prepare 250 ml 0.1 M solution =

Determination of strength of KMnO₄ using standard solution of oxalic acid

In this titration KMnO₄ is the titrant and oxalic acid is the analyte. Here, potassium permanganate is the oxidizing agent and oxalic acid is the reducing agent. The reaction between potassium permanganate and oxalic acid is carried out in an acidic medium because permanganate ion in the acidic medium is a very strong oxidizing agent. Acidity is introduced by adding dil. H₂SO₄.

No other indicators are used to determine the endpoint, because KMnO₄ acts as the indicator. Permanganate (MnO^4-) ion has a dark purple colour. In an acidic medium, MnO^4- is reduced to colourless manganous (Mn²⁺) ions. On reaching the end point, the addition of the last single drop of permanganate imparts a light pink colour to the solution. The chemical reaction that takes place during titration can be represented by the chemical equation.

Molecular equation

Ionic equation

Balanced chemical equation

From the balanced chemical equation, it is clear that 2 moles of KMnO₄ reacts with 5 moles of oxalic acid.

According to the molarity equation,

If oxalic acid is to be titrated, add the required amount of dil. H₂SO₄ and heat the flask to 60°-70°C. The purpose of heating is to increase the rate of reaction, which otherwise is slow at room temperature.

Titration of Potassium permanganate (KMnO₄) against Mohr’s salt solution

Preparation of standard solution of Mohr's salt[250 ml M/20 (0.05 M) solution]

The molecular mass of Mohr's salt is, FeSO₄.(NH₄)₂SO₄.6H₂O= 392

Weight of  Mohr's salt required to prepare 1000 ml of 1 M solution = 392 g

Therefore, weight of Mohr's salt required to prepare 250 ml 0.05 M Mohr's salt solution =

Determination of strength of KMnO₄ using standard solution of Mohr's salt

In this titration, potassium permanganate is the oxidizing agent and Mohr’s salt is the reducing agent. Mohr’s salt is a double salt of ferrous sulphate and ammonium sulphate and its composition is FeSO₄.(NH4)₂SO₄.6H₂O. It is a primary standard. Therefore, its standard solution can be prepared directly. Ferrous ions of Mohr’s salt undergo hydrolysis in aqueous solution. To prevent the hydrolysis, Conc. H₂SO₄ needs to be added to the Mohr’s salt crystals during the preparation of its standard solution.

In this titration, the MnO^4- ion is reduced to Mn2+ in the presence of acid and Fe²⁺ ions of Mohr’s salt is oxidized to Fe³⁺

The chemical reaction that occurs in this titration can be represented by the following chemical equations.

Molecular equation

Ionic equation

In ionic form the reaction can be represented as,

Balanced chemical equation

From the overall balanced chemical equation, it is clear that 2 moles of potassium permanganate react with 10 moles of Mohr’s salt.

Learning Outcome

• Students understand the terms- volumetric analysis, morarity, molality normality and redox titration.
• Students acquire the knowledge to calculate the strength of KMnO₄ using molarity equation.
• Students understand the purpose of addition of dil. H₂SO₄ and the purpose of heating of oxalic acid before titration.
• Students acquire the skill to prepare standard solutions of oxalic acid and Mohr’s salt.
• Students understand the apparatus used for a titration.
• Students acquire the skill to perform the redox-titration in the real lab after understanding the different steps.

A. To determine of the strength of the given KMnO₄ solution using standard oxalic acid solution

Materials Required

Real Lab Procedure

Preparation of standard solution of oxalic acid [250 ml M/10 (0.1 M) solution]

• Using an electronic balance, first weigh exactly 3.15g of oxalic acid crystals in a weighing bottle.
• Transfer these into a 250ml beaker.
• Then wash the weighing bottle 2 or 3 times with distilled water and transfer all the washings into the beaker.
• Dissolve the oxalic acid crystals in the beaker by gentle stirring with a clean glass rod.
• When the oxalic acid crystals in the beaker are completely dissolved, transfer the entire solution from the beaker into a 250ml standard flask through a funnel and a glass rod.
•  Wash the beaker 2 to 3 times with distilled water and transfer all the washings into the standard flask.
• Finally wash the funnel thoroughly with distilled water to transfer the drops of the solution on the sides of the funnel into the standard flask.
•  Add enough distilled water to the standard flask so that the level is just below the calibration mark on it.
• Add the last few drops of distilled water with a pipette until the lower level of the meniscus just touches the mark on the standard flask.
•  Stopper the measuring flask and shake gently to make the solution uniform throughout.

Determination of strength of given KMnO₄

• Take a burette and wash it with distilled water.
• Rinse and fill the burette with the given KMnO₄ solution and set the initial burette reading as zero.
• Clamp it vertically to the burette stand.
• Rinse the pipette with water and then with the given oxalic acid solution.
• Then pipette out 20ml of the given oxalic acid solution into a conical flask and add one test tube (~20ml) full of dil.H2SO4 into it.
• Heat the contents of the conical flask to 60-70°C.
• Titrate it against the KMnO₄ solution taken in the burette till the colour of the solution in the conical flask changes from colourless to light pink.
• Note down the final burette reading.
• Repeat the titration until concordant values are obtained.

Observation

The readings are recorded in a tabular form as shown and the molarity of the given KMnO₄ solution can be calculated using the molarity equation given in the theory.

The Result

The strength of the given KMnO₄ solution = ............g/litre

B. To determine the strength of the given KMnO₄ solution using standard solution of Mohr’s salt

Materials Required

Real Lab Procedure

Preparation of standard solution of Mohr’s salt (Ferrous Ammonium Sulphate).[250 ml M/20 (0.05 M) solution]

• Using an electronic balance weigh exactly 4.9g of Mohr’s salt crystals in a weighing bottle.
• Transfer these into a 250ml beaker.
• Add 5ml conc.H₂SO₄ into the beaker.
• Wash the weighing bottle well with distilled water and transfer all the washings into the beaker.
• Dissolve Mohr’s salt crystals in the beaker by gentle stirring with a clean glass rod.
• When the crystals in the beaker are completely dissolved, transfer the entire solution from the beaker into a 250ml standard flask through a funnel and a glass rod.
• Wash the beaker thoroughly with distilled water and transfer all the washings into the standard flask.
• Finally, wash the glass rod and the funnel thoroughly with distilled water to transfer the solution on the sides of the funnel into the standard flask.
• Add enough distilled water to the standard flask so that the level is just below the calibration mark on it.
• Add the last few drops of distilled water with a pipette until the lower level of the meniscus just touches the mark on the standard flask.
• Stopper the measuring flask and shake gently to make the solution uniform throughout.

Determination of strength of KMnO₄

• Take a burette and wash it with distilled water.
•  Rinse and fill the burette with the given KMnO₄ solution and set the initial burette reading as zero.
• Clamp it vertically to the burette stand.
• Rinse the pipette with water and then with the given Mohr’s salt solution.
• Pipette out 20ml of the given Mohr’s salt solution into a conical flask and add one test tube (~20ml) full of dil.H₂SO₄ into it.
• Titrate it against the KMnO₄ solution taken in the burette till the colour of the solution in the conical flask changes from colourless to light pink.
• Note down the final burette reading.
• Repeat the titration until concordant values are obtained.

Observation

The readings are recorded in a tabular form as shown and the molarity of the given KMnO₄ solution can be calculated using the molarity equation given in the theory.

The Result

The strength of the given KMnO₄ solution = .............g/litre

Simulator Procedure (as performed through the Online Labs)

Titrate: Mohr's Salt

• Select the titrate 'Mohr's Salt' from the 'Titrate' drop down list.
• You can adjust the drops rate using the 'Drops rate' slider.
• Use the respective sliders to select the molarity and volume of the titrate.
• Drag the test tube containing Dil. H2SO4 to the conical flask to pour the solution in it.
• Drag the conical flask and place it on tile.
• Click on the 'Start' button or on the 'Nozzle' of the burette to start the titration.
• The zoomed view of the burette reading is shown.
• You can see the volume of KMnO4 used for the titration by clicking on the 'Show the volume of titrant' check box.
• The end point is reached when a pink colour develops, at this point stop the titration either click on the 'Stop' button or click on the 'Nozzle' of the burette.
• You can see the chemical equation of the reaction on the side menu.
• Find out the number of moles of the titrate (n1) and that of the titrant (n2) from the chemical equation and enter the values in the respective text boxes and verify the values.
• Calculate the molarity of the titrant using the given equation and enter the value in the corresponding text box and verify the value.
• The molar mass of the titrant is shown on the side menu.
• Calculate the strength of the given titrant (in g/lit) and enter the value in the corresponding text box and verify the result.
• You can alternatively use the embedded worksheet to find the values by entering the data in the specific fields.
• To redo the experiment click the 'Reset' button.

Note: Click on the ‘HELP’ button to see the instructions.

Titrate: Oxalic Acid

• Select the titrate 'Oxalic Acid' from the 'Titrate' drop down list.
• You can adjust the drops rate using the 'Drops rate' slider.
• Use the respective sliders to select the molarity and volume of the titrate.
• Drag the test tube containing Dil. H₂SO₄ to the conical flask to pour the solution in it.
• Drag the conical flask and place it over the Bunsen burner.
• Light the burner by clicking on the knob and heat it till the temperature reacher 70^oC to 80^oC.
• Drag the conical flask and place it on tile.
• Click on the 'Start' button or on the 'Nozzle' of the burette to start the titration.
• The zoomed view of the burette reading is shown.
• You can see the volume of KMnO₄ used for the titration by clicking on the 'Show the volume of titrant' check box.
• The end point is reached when a pink colour develops, at this point stop the titration either click on the 'Stop' button or click on the 'Nozzle' of the burette.
• You can see the chemical equation of the reaction on the side menu.
• Find out the number of moles of the titrate (n1) and that of the titrant (n2) from the chemical equation and enter the values in the respective text boxes and verify the values.
• Calculate the molarity of the titrant using the given equation and enter the value in the corresponding text box and verify the value.
• The molar mass of the titrant is shown on the side menu.
• Calculate the strength of the given titrant (in g/lit) and enter the value in the corresponding text box and verify the result.
• You can alternatively use the embedded worksheet to find the values by entering the data in the specific fields.
• To redo the experiment click the 'Reset' button.

Note: Click on the ‘HELP’ button to see the instructions.

Precautions

• Handle the apparatus and chemicals carefully.
• Rinse the pipette and burette first with distilled water then with the corresponding solutions.
• Close the pipette with the index finger.
• Always keep the lower end of the pipette in the liquid when sucking it.
• Do not blow out the last drop of the solution from the jet end of the pipette.
• KMnO4 solution is always taken in the burette.
• Avoid the use of burette having a rubber tap as KMnO₄ reacts with rubber.
• Carefully fill the burette with the solution and see that the stopcock does not leak.
• Do not allow any air bubbles to remain inside the burette.
• Read the upper meniscus while taking burette reading with KMnO₄ solution.
• Let no drops of solution be at the tip of the burette at the end point.
• Add about an equal volume of dil.H₂SO₄ to the solution to be titrated before adding KMnO₄.
• If oxalic acid or some oxalate is to be titrated, add required amount of dil.H₂SO₄ and heat the flask to 60°-70°C.
• In case of ferrous salt, no warming is required.
• No external indicator is required for KMnO₄ titration because KMnO₄ acts as self indicator.
• Do not rinse the titration flask with the solution.
• Give a rotatory motion to the titration flask throughout the titration.
• Place the titration flask containing solution on a white tile to see the colour change correctly.