Hypomanganate, Mn (V)
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:
MnO−4 + SO2−3 + H2O → MnO3−4 + SO2−4 + 2 H+
