Analytical

Manganometry

Last time I wrote an article about a quantitative chemical analysis called titration analysis, more accurately – neutralization titration. This reaction is based on the protolytic reaction between H3O+ ion released from an acid and OH- ion released from a base. The products of neutralization in general are water and salt. It is used to determine unknown concetration of both the acid and the base. As you can see, in this reaction, neither oxidation nor reduction take place so I decided to go on with titration analysis and discuss oxidation-reduction titrations. What you should know is that this kind of titrations is based on the exchange of electrons between two reactants – the one that loses electrons is oxidized and the other gaining electrons is reduced . When we have the solution of unknown concentration with oxidizable substance (such as oxalic acid) we are able to titrate it using a strong oxidizing (reducible) agent in a solution. In Manganometry, potassium permanganate is used beacuse of its strong oxidizing properties. MnO4–  ion can be easily reduced. Manganese can exist in several oxidation states. In our titration MnO4–  ion (manganese state (VII)) is reduced to colorless Mn(II) in an acidified solution. In a neutral or alkaline solution the reaction will yield Mn(IV) ions. Knowing the fact, that permanganate is violet whereas Mn(II) ion is colorless we assume there´s no need to use an indicator as long as permangate is the indicator itself. Nearly after the equivalnce point was reached the solution turns pink beacuse of an excess of potassium permanganate.

Potassium-permanganate-photo

First calculate the amount of potassium permanganate needed for preparation of 200ml 0,1M solution. Dissolve it in 50cc of distilled water in 150cc beaker and decant into a 200ml volumetric flask. Fill the volume up to 200cc with distilled water.

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When you´re done, take your sample (oxalic acid solution) and pipette 10cc to 100cc titration flask, add 5cc of 2M sulphuric acid and heat the final solution to 80°C, then start the titration. Since Mn(II) ions are autocatalyzators for the reaction in the beginning, after few drops are added, the pink color would fade out slowly but later on with the increasing amount of Mn(II) ions it would be really fast. The equivalence point is reached when the solution remains light pink. Mark the amount of potassium permanganate solution used and repeat the titration four times. Create a table and calculate concentration of your sample.

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