As the chlorine solution is of concentration approximately 7g dm-3 it is possible to calculate quantities and concentration of other reactants so the experiment will give realistic outcomes. For instance if the concentration of MnO4- is too low in the titration with the Fe2+ then the volume used will be large, possibly requiring more than one burette volume. This introduces unnecessary inaccuracies, of having to repeat the set-up of the burette and also increases the time for the experiment. By working through an example calculation using a reasonable volume titre of approx 20.00 cm3, it is possible to determine the relevant quantities.
From these calculations you can see that one should use at least 0.125g of FeCl2 reducing agent and MnO4- of concentration approximately 0.06 mol dm-3 for the titration.
Method
Equipment:
50 cm3 Burette
50 cm3 Volumetric Pipette
Pipette Filler
250 cm3 Conical Flask
Clamp Stand
White filter paper
Glass rod
Balancing scales
Weighing boat
1. Use a volumetric pipette and pipette filler to deliver 50.0 cm3 of the chlorine solution into a 250 cm3 conical flask. Using the balancing scales and a weighing boat measure out approximately 2.0 g of FeCl2 solid. Record the mass of the weighing boat then add FeCl2 until the mass reads equal to the initial weight of the weighing boat + 2.0 g. Record the weight of FeCl2 - total weight of boat and FeCl2 – initial boat weight. Add to the conical flask containing the Cl2 and mix gently using the glass rod.
2 .Wash a burette with a little of the KMnO4 solution. Then fill the burette, allowing the solution to run into the tip of the burette, ensuring no air bubbles are present. Record the start reading on the burette, making sure you read from the actual level of the solution ignoring the raised meniscus.
3. Add using a volumetric pipette, 25cm3 of 0.5 mol dm-3 HCl to the conical flask containing the Cl2 solution.
4. Arrange the apparatus as shown in fig.1. Run the KMnO4 drop wise out of the burette into the conical flask containing the chlorine solution. Use your left hand to open the tap and your right hand to swirl the flask. The first run should be used as a trial, to determine the approximate end point. This will allow you to anticipate the end point and determine a more accurate result.
5. The end point occurs when a persistent pink tint remains for at least 5 seconds. When this is noticed the end volume of the burette should be recorded. Place the white filter paper under the conical flask, to give a better background on which to spot the colour change.
6. The titre is the change in volume of solution in the burette from the start to the end and is the amount of MnO4- needed to react with the fe2+.
7. Repeat the procedure at least 3 times until you get 2 values within 0.1cm3. Obtain an average titre
Example Table of Results:
Example Calculation
Risk Assessment
When dealing with any chemicals a lab coat and goggles should be worn at all times.
Potassium Permanganate in its solid form is a very strong oxidizer and if it comes into contact with other material may cause a fire. However as we will only be using a dilute aqueous solution this hazard will be minimal. Even in aqueous state it may cause severe digestive tract irritation or eye irritation which is why it is essential to wear goggles. Sulphuric acid is corrosive and an irritant, even though only weak concentrations are being used eye protection and lab coats should be used throughout.
When dealing with glassware extra precautions should be taken, do not use glassware above shoulder level; always fill the burette from below shoulder height.