Potassium Manganate

• Potassium manganate(VII) is usually used in neutral or alkaline solution in organic chemistry. Acidified potassium manganate(VII) tends to be a rather destructively strong oxidising agent, breaking carbon-carbon bonds.

• Ethene, for example, is oxidised to ethane-1,2-diol.

  

  To be honest, this isn't a good test for a carbon-carbon double bond, because anything which is even mildly reducing would have the same effect on the potassium manganate(VII) solution.

  You could, however, use this reaction simply as a means of making the diol.

Potassium manganate(VII) solution is used to find the concentration of all sorts of reducing agents. It is always used in acidic solution.It oxidises iron(II) ions to iron(III) ions

• hydrogen peroxide solution to oxygen

  

• ethanedioic acid to carbon dioxide (This reaction has to be done hot.)

  

• sulphite ions (sulphate(IV) ions) to sulphate ions (sulphate(VI) ions)

  

  the half-equation for the manganate(VII) ions in acidic solution is

Using potassium manganate(VII) as an oxidising agent in organic chemistry.

Potassium manganate(VII) is usually used in neutral or alkaline solution in organic chemistry. Acidified potassium manganate(VII) tends to be a rather destructively strong oxidising agent, breaking carbon-carbon bonds.

The potassium manganate(VII) solution is usually made mildly alkaline with sodium carbonate solution, and the typical colour changes are:

Doing the titration

The potassium manganate(VII) solution always goes into the burette, and the other solution in the flask is acidified with dilute sulphuric acid.

As the potassium manganate(VII) solution is run into the flask it becomes colourless. The end point is the first faint trace of permanent pink in the solution showing that there is a tiny excess of manganate(VII) ions present.

There are two things you need to be aware of:

• Potassium manganate(VII) can't be used in titrations in the presence of ions like chloride or bromide which it oxidises. An unknown amount of the potassium manganate(VII) would be used in side reactions, and so the titration result would be inaccurate.

  This is why you don't acidify the solution with hydrochloric acid.

• Potassium manganate(VII) isn't a primary standard. That means that it can't be made up to give a stable solution of accurately known concentration.

  It is so strongly coloured that it is impossible to see when all the crystals you have used have dissolved, and over a period of time it oxidises the water it is dissolved in to oxygen.

  Bottles of potassium manganate(VII) solution usually have a brown precipitate around the top. This is manganese(IV) oxide - and is produced when the manganate(VII) ions react with the water.

You have to make up a solution which is approximately what you want, and then standardise it by doing a titration. This is often done with ethanedioic acid solution, because this is a primary standard.