Titration is one of the most crucial chemistry practicals. It helps students understand the basics of physical chemistry in a much deeper way. The process of titration is used to determine the concentration of an unknown solution.
For this purpose, a reagent or a solution of known concentration is used. The volume of this reagent or titrator is known.
For example, in the case of acidic titration, we mix a basic solution of unknown concentration with an acidic solution to determine the equivalence point of reaction. This is the point when both the solutions have an equal number of reactive molecules.
The titration process has wide applications in industries like food, cosmetics, pharmaceuticals etc. However, this article will deal with the titration determination by using Potassium Permanganate with Mohr’s Salt. So, let’s learn about Mohr’s Salt Titration Formula.
What is Mohr’s salt?
Let’s look at some of the general properties of Mohr’s salt.
- Chemical name: In chemistry, Mohr’s salt is referred to as Ammonium Ferrous Sulphate or Ammonium Iron (II) Sulphate.
- Colour and Texture: Mohr’s salt is a light green coloured, inorganic crystalline salt.
- Salt Formula: Fe(SO4)(NH4)2SO4 (anhydrous).
- Hydrated Salt Formula: Fe(SO4)(NH4)2SO4.6H2O. This formula indicates that Mohr’s is a hexahydrate salt.
- The number of cations: Mohr’s salt has two different cations, Fe2 and NH4.
- Nature of salt: This is a double salt of ferrous sulphate and ammonium sulphate.
- Preparation method: We start preparing Mohr’s salt by adding a small amount of sulphuric acid to water.
- Next, an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate is added. This mixture is then subjected to crystallisation. The end-product is light-green coloured crystals of Mohr’s salt. The resulting crystals are fairly small in size.
Experiment: Mohr’s salt titration with potassium dichromate
Let’s look at the much-awaited details of the titration process of Mohr’s salt with potassium dichromate.
- Aim: In this experiment, we will use an M/20 solution of Mohr’s salt. Using the given salt, we will determine the strength and molarity of the given KMnO4 (Potassium Permanganate) solution.
- Theory: This experiment is based on redox titration. Now, in redox titrations, an oxidising agent is titrated against a reducing agent. Our experiment of titrating Mohr’s salt against Potassium Permanganate is also a redox titration. Here, KMNO4 will act as the oxidising agent, while Mohr’s salt will be used as a reducing agent.
Hence, the reaction that will take place here will be a redox reaction with oxidation and reduction simultaneously. Potassium Permanganate can act as an oxidising agent in various mediums such as acidic, neutral and basic.
However, we witness a strong reaction when KMO4 is exposed to an acidic medium. Therefore, we add a small amount of sulphuric acid to our conical flask. Next, we add Mohr’s salt for titration.
Molecular reactions are when two molecules interact with each other, and the resulting molecules change. The atoms break their bonds and form bonds with other atoms to make a new molecule. The reactions involved in this process are:
- Reduction Half Reaction: 2KMnO4 + 3H2SO4 🡪 K2SO4 + 2MnSO4 + 3H2O + 5O
- Oxidation Half Reaction: FeSO4(NH4)2SO4.6H2O + H2SO4 + 5O🡪Fe2(SO4)3 + 2(NH4)2SO4 + 13H2O5
- Finalreaction:2KMnO4 + 10FeSO4(NH4)2SO4.6H2O + 8H2O🡪K2SO4 + 2MnSO4 5Fe2(SO4)3 + 10(NH4)2SO4 + 68H2O
Now, let’s take a look at ionic reactions that take place in the process:
- Oxidation Half: [Fe2 🡪Fe3 -e-]5
- Reduction Half: MnO4- + 8H 5e- 🡪 Mn2 + 4H2O
- Overall Reaction: MnO4- + 8H + 5Fe2 🡪 Mn2 + 5Fe3 + 4H2O
Remember, Potassium Permanganate in this reaction acts as a self-indicator. That is, you will know that your experiment is drawing to a close when the colour of the solution turns from colourless to light pink.
Also, ensure that you stop your experiment as soon as the solution changes colour. For this reason, keep shaking the conical flask frequently to give a chance for Mohr’s solution to mix with the solution and react. If you get a dark pink colour, you will have to start all over again.
Let’s look at the setup and the materials you will need to go through with the experiment.
- Apparatus required: Volumetric flask, beakers, chemical balance, weighing bottle, conical flask, burette, pipette, clamp stand and weight box.
- Chemical required: Dilute sulphuric acid, distilled water and potassium permanganate solution.
Procedure to follow
Let’s look at the step-by-step process of this experiment.
Preparation of 250 ml of M/20 solution of Mohr’s salt:
- Mohr’s salt has a molar mass of 392gmol- per the primary standards.
- Now, 1000 cm3 of potassium permanganate requires 392g of Mohr’s salt. So, we must calculate the amount of Mohr’s salt we will need for 250cm3 of solution. For this reason, we calculate by Mohr’s Salt Titration Formula:
392/20/100 X 250 = 4.9g
- Now, you have to weigh the Mohr’s salt accurately using chemical balance and watch glass.
- Carefully put the weighted Mohr’s salt in the volumetric flask. Use your funnel for this purpose to avoid spills.
- Now, pour in the distilled water in the volumetric flask. Take note of the volume when doing this.
- Once you complete step 5, your standard solution is all good to go.
Procedure to perform the titration
- Take your burette and fill it with the potassium permanganate solution.
- Now, as discussed earlier, add 5ml of sulphuric acid to the conical flask containing Mohr’s salt.
- Now, using a pipette, take out 10 ml of the Mohr’s salt solution in the same conical flask.
- Now, you will require a white tile to correctly spot the colour change. Place the white tile under the conical flask with sulphuric acid and Mohr’s salt solution.
- Note the starting point readings on your burette.
- Drip the KMNO4 solution in the conical flask slowly and keep shaking the flask.
- As soon as you obtain a permanent pink colour in the solution, cork the burette.
- Read the final readings on the burette.
- Repeat this procedure for three readings.
The titration procedure requires patience and stable hands. So, ensure that you’re mentally and physically present in the moment.
Keep a sharp eye on the colour in the conical flask. Remember to use safety gear in the process because you will be working with a strong acid. Do not hurry with the readings and perform your calculations diligently.