Redox Titrations | A-level Chemistry | OCR, AQA, Edexcel

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The key points covered in this video include:
1. Recap titrations
2. manganate(VI) ions as an oxidising agent
3. Iron(II) - potassium permanganate titrations
4. Iron(II) - potassium dichromate(VI) titrations
5. Iodine - sodium thiosulphate titrations
6. Unknown redox titration questions
Titrations
Titrations are used to find unknown concentrations of substances, e.g. acids and bases. If we know the volume in the flask and the reaction equation for whatever is taking place (e.g. neutralisation) then we can calculate the concentration. Watch our video on titration curves to see more examples of these calculations.
Manganate(VII) ions as an oxidising agent
In our video on redox reactions we see that manganate(VII) ions are readily reduced to form Mn^2+ ions in acidic conditions. Usually we use potassium permanganate as a source of manganate(VII) ions. It’s a useful oxidising agent for titrations as the MnO4- ions are a different colour to the Mn^2+ ions.
Iron(II) and Potassium Manganate Titration
The first common redox titration we will look at is the oxidation of Fe^2+ to find the concentration of iron. Equation. End point: When we have used all the Fe^2+ we get a build-up of purple MnO4- ions. Uses: We can use this titration to work out the percentage composition of iron in a sample as we assume that all of the iron has formed 2+ ions in the solution.
Iron(II) and Potassium Dichromate(VI)
Cr2O7^2- ions will also oxidise Fe^2+ so can be used to find the concentration of iron. There are 2 half-equations, the oxidation of iron(II) and reduction of chromium(VI). Overall equation. End point. There is not a clear colour change at the endpoint here, so a redox indicator like diphenylamine sulfonate is used which gives a violet-blue coloured end point.
Iodine Titrations
Another useful type of titration uses the reduction of iodine to form iodide ions. It’s a useful oxidising agent as if we include starch, I2 and starch are a different colour to the I- ions and starch.
Iodine and Thiosulphate Titration
We will now look at the redox titration where sodium thiosulphate reduces iodine. Equation. End point: When we have used all the I2, the starch returns to a straw yellow colour. Uses: We often use this titration to work out the concentration of an oxidising agent by first letting it oxidise excess I- ions. This is our only source of I2 so we can use the titration to work out the amount of I2 present. We then work backwards to find the concentration of the oxidising agent.
Unfamiliar Examples
You need to be able to apply what you know about redox titrations to unfamiliar situations, let’s go through an example to see the method.
Summary
Titrations are used to find unknwn concentrations
Manganate(VI) ions are a useful oxidising agent as they are readily reduced and have a distinct colour change
Iron(II) - potassium permanganate titrations use the reaction.
To find unknown concentrations of iron(II)
Iodine - sodium thiosulphate titrations use the equations.
To find unknown concentrations of oxidising agents X
We’ve also looked at how to tackle unknown redox titration questions

[@abhinandtitus2191]

Please make videos and share resources for CIE A-levels also.

[@ray4736]

10:23 the reduction of dichromate goes to chromium 3+ ions not 2+ ions. You have changed this in the final overall equation but not in the individual reduction half equation.

[@issaabdalla281]

they corrected it literally 2 seconds bruh

[@theteenagersguidetomakingf3090]

Hi SnapRevise, thank you so much for yet another very clear and concise video! I just noticed that at 14:11, in the ionic equation for the reaction of thiosulphate and iodine, the thiosulphate ion should have a '2-' charge I think, not a '1-' charge as in the video, as otherwise the charges don't balance? Thanks once again!

[@laurawilson9713]

You cannot use HCl. It has to be sulfuric

[@ys-sy7xo]

OCR ?

[@Ayeshaness]

why is it written -1 for the thiosulfate ion when its actually -2

[@matheeshapeiris9798]

It’s wrong look the charges are not equal in both sides.

[@SaniaKumar-pd4nd]

is this A2?