Redox Reaction
A chemical reaction that involves the oxidation and reduction of chemical species simultaneously is called the Redox reaction. Redox reaction involves the transfer of electrons leading to oxidation of one chemical species and gaining of electrons leading to reduction of other chemical species.
Titration
Titration is an analytical procedure to find out analyte concentration in a liquid by measuring the titrant quantity that reacts with the analyte.
Redox Titration
Redox titration refers to an analytical procedure of titration that involves a redox reaction between the analyte and the titrant simultaneously. This leads to a change in oxidation potential.
The endpoint of the titration is determined using redox indicators or by the potentiometer. Redox indicators are sensitive to the change in oxidation potential and show a colour change on reaching the endpoint. Eg: Titration of Oxalic acid with potassium permanganate.
Titration of oxalic acid with potassium permanganate involves redox reaction wherein Potassium permanganate oxidises oxalic acid and self gets reduced. One can determine the strength of a reductant/oxidant by using indicators.
Potassium permanganate acts as a self indicator in titrations involving Potassium Permanganate are known as Permanganometric titrations.
Determination of molarity/concentration of Potassium permanganate (KMnO4) solution using oxalic acid(C2H2O4)
Aim :
To determine the strength and molarity of KMnO4 solution by carrying out titration against 0.1 M oxalic acid (C2H2O4) standard solution.
Theory :
Potassium permanganate behaves as an excellent oxidizing agent in the alkaline medium as compared to the acidic medium. But for this quantitative analysis dilute sulphuric acid as a medium is used. The oxidizing action of KMNO4 is as follows:
MNO4- + 8H+ + 5e —> Mn2+ + 4 H2O
Nitric acid being self oxidising is not used. HCl is not used because KMnO4 oxidises HCl to Cl2 and gets reduced to Mn2, hindering the actual oxalic acid determination in the solution.
We can carry out a titration using oxalic acid as a reducing agent with potassium permanganate as a titrant in the proposed acidic medium.
The reactions that occur are given in the below image:
Here MnO4- is reduced to Mn2+; C2O42- is oxidised to CO2, and the oxidation number of carbon changes to +4 from +3.
Potassium permanganate is a self indicator. A pale pink colour indicates complete consumption of oxalate ions and the presence of unreacted KMnO4.
This reaction occurs at a higher temperature, so initially, the reaction is slow and gradually gets faster after the oxalic acid solution gets warmed up by dilute sulphuric acid.
Materials and Reagents required
| Materials and Reagents | Quantity |
| Measuring flask (250 mL) | 1 |
| 1.0 M Sulphuric acid | As required |
| Potassium permanganate | As required |
| Burette with glass top | 1 |
| Burette stand | 1 |
| Pipette | 1 |
| Conical flask | 1 |
| Funnel | 1 |
| Glazed tile | 1 |
| Weighing balance | 1 |
| Oxalic acid | As required |
| Burner and wire gauze | 1 |
Analytical procedure:
- Preparation of 0.1 M oxalic acid solution: Weigh 1.26 g of oxalic acid into a 100 mL standard flask and dissolve with little water. Makeup to the volume with adding water. (molecular mass of oxalic acid is 126, so to prepare an M/10 solution of oxalic acid, 12.6 g should be dissolved in one litre of water).
- Titration procedure of oxalic acid solution with potassium permanganate solution
- Fill a clean burette with potassium permanganate solution after rinsing with the same.
- Observe for any air bubble and remove if any
- Transfer 10 mL of 0.1 M oxalic acid solution into a conical flask
- Add 5 mL of 1.0 M Sulphuric acid (prevents the formation of any precipitate of manganese dioxide during titration).
- Heat the oxalic acid solution until it reaches 50°to 60°C
- Place a white glazed tile below the conical flask to observe colour change clearly
- Note down the initial volume of permanganate solution in the burette. (Note upper meniscus reading for coloured solutions).
- Start adding small volumes of permanganate solution to the hot oxalic acid solution.
- Swirl the flask gently.
- Observe the appearance of pale pink colour in the flask.
- Note the reading in the burette as an endpoint.
- Repeat the titration to obtain concurrent readings.
Observation table:
| Trial no | Oxalic acid in mL | Initial burette reading (x) mL | Final burette reading
(y) mL |
Volume of Kmno4 consumed
V (mL) =Y-X |
Calculation:
a1M1V1=a2M2V2…………(1)
a1 (no. of electrons lost per formula unit of oxalic acid in a balanced equation of half cell reaction)=2
a2 (no. of electrons gained per formula unit of potassium permanganate in the balanced equation of half cell reaction)=5
(in a balanced chemical equation, 2 moles of KMnO4 reacts with 5 moles of oxalic acid)
M1 and V1 are Molarity and Volume of Oxalic acid used in the titration, respectively
M2 and V2 are Molarity and Volume of Potassium Permanganateused in the titration, respectively
Applying values a1 and a2 in equation (1), we get
2 M1V1= 5 M2V2
M2 = 2 M1V1/5 V2………….(2)
Calculate the molarity of potassium permanganate solution using equation (2)
Now calculate the strength of potassium permanganate by using the formula Strength = Molarity * Molar mass.
Calculated results:
- Molarity of the potassium permanganate solution used in the titration____.
- Strength of the potassium permanganate solution used in the titration____.
Important terms in analytical chemistry
- A balanced chemical equation refers to a chemical equation wherein the number of the different atoms of elements in the reactants and products is equal.
- Standard solution refers to a solution whose concentration is already known. In the above case, it’s Oxalic acid.
- Normality (N)= Eq (no. of grams of solute)/V(volume of solvent in litres)
- Molarity/Molar concentration (M) = n (moles of solute)/V (litres of solution)
- The Endpoint of the titration is that point wherein the analyte has completely reacted with the titrant
Safety Notes:
- Potassium permanganate can damage eyes, skin, nose, eyes. So always wear gloves while preparing the solution.
- Use slightly hot water to dissolve Potassium Permanganate crystals
- Always wear gloves, goggles and protective clothing while handing Oxalic acids. It is corrosive and can harm the respiratory tract if vapours are inhaled.
Precautions need to be taken during analysis:
- Always use clean standard glasswares (pipettes, burettes, conical flasks of good grade) for analysis
- Make sure to rinse pipette and burette with the solutions they are filled with before filling
- Keep standard solutions away from the titration region to avoid cross contaminations.
- Fill the burette with titrant without any air bubble
- Add titrant slowly to get sharp endpoints and swirl the flask properly on adding titrant each time
- Read upper meniscus measurement for the titrants that have coloured solutions like KMnO4
- Always heat the mixture of oxalic acid and H2SO4 before the titration
- Repeat analysis thrice until consistent readings are obtained
In conclusion
The strength of the KMnO4 solution can be found out by titrating it against a standard solution of oxalic acid. The above titration is an example of redox reactions wherein oxidation and reduction reactions occur simultaneously.