#permanganate

permanganate is such a pretty colour💜

Filtration of manganese dioxide from an oxidation that used potassium permanganate as an oxidant. 

The characteristic purple color of the remaining small amount of permanganate is seen in the filtrate. Even through only a small amount remained from the original amount it still has a quite intense color.  Important to note that contact with skin will result in a long lasting pinkish/purple stain.

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Last time I was trying to “catch” manganate anion by the reduction of permanganate (as I described here). Green colored manganate contains sexavalent manganese. Unfortunately it’s not stable in solutions. In its solid form it can be obtained by a thermal decomposition of potassium permanganate (as a mix with manganese dioxide and some potassium hypomanganate), but once added to water it starts to loose its green color and turns into a mix of potassium permanganate and manganese dioxide (a process called disproportionation):

3K2MnO4 + 2H2O → 2KMnO4 + MnO2 + 4KOH

Basic environment helps to supress this process, so we can enjoy the green color of manganate longer.

Thus, “catching” the green manganate color in a solution is tricky, but let’s go further and try to “catch” the blue color of a pentavalent manganese of hypomanganate ion. It can be obtained by either a reduction or thermal decompositin of potassium permanganate. This compound is even more unstable than manganate. But a strong basic environment at low temperatures may help here.

What you see is a cold strong solution of potassium hydroxide (about 30-40%) with some sodium sulfite (around 10-15%). When I start to add potassium permanganate to this mixture, a blue color of hypomanganate appears. Unfortunately adding more potassium permanganate makes the solution to turn green due to a manganate formation. But the blue color of hypomanganate was clearly present at first:

The process can be described as followed:

Potassium permanganate is often called a “mineral chameleon” (or “chemical chameleon”) as it can show a broad spectrum of color change under certain circumstances. 

You see a (weak) solution of potassium permanganate added to a mix of sugar and sodium hydroxide solutions.

The chemistry behind this process is simple: potassium permanganate oxidizes sugar. Alcohol groups are oxidized to ketone groups:

–C(H)(OH)– + 2OH–  → –C(=O)– + 2H2O + 2e

Manganese (VII) in permanganate ion is reduced to manganate (contains manganese (VI) and then into MnO2 containing Mn(IV). Manganese dioxide or MnO2 is insoluble in water and should form a precipitate, but in low concentration it can form a sol or colloidal solution. It gets dispersed in water without forming a precipitate. This reduction process can be described as followed:

MnO4- + e- → MnO42-

MnO42 + 2e- → MnO2

The reduction of permanganate depends on the acidity. In acid environment it can go further and forms colorless manganese (II) cation. Basic environment contributes to manganate formation. And manganese dioxide “prefers” neutrality. But the choice of reducting agent is also crucial. So playing with pH and the choicer of reducting agent will result in different products.

With the following experiment I wanted to “catch” the manganate anion in a solution and “freeze” the green color of the solution. I used a very weak potassium permanganate solution with some sodium hydroxide to create a basic environment. After giving the flask a good swirl I started to add some sodium sulfite drop by drop:

Permanganate reduces to manganate which results in color change from violet to green. The process can be described as followed:

2KMnO4 + 2Na2SO3 + 2KOH → K2MnO4 + 2K2SO4 + H2O

Another example of this experiment with good explanation can be found here: http://woelen.homescience.net/science/chem/exps/chameleon/index.html

It’s also available as a part of MEL chemistry experiment here - https://melscience.com/en/experiments/chameleon/