Potassium Permanganate ($\text{KMnO}_4$): Preparation and Properties

Potassium permanganate, $\text{KMnO}_4$, is manufactured industrially from pyrolusite ore, $\text{MnO}_2$ (manganese's principal natural ore, introduced earlier in this chapter), in two successive stages.

Stage 1: Oxidative fusion to potassium manganate. Finely powdered pyrolusite is fused with potassium hydroxide in the presence of an oxidizing agent -- either atmospheric oxygen or, in an older laboratory-scale variant, potassium nitrate -- converting manganese from the $+4$ state (in $\text{MnO}_2$) to the $+6$ state, as the dark green manganate ion:

Stage 2: Oxidation to potassium permanganate. The green potassium manganate solution is then oxidized further, from manganese's $+6$ state to its $+7$ state, either by passing chlorine gas through the alkaline solution,

or, in the modern industrial process, by electrolytic oxidation of the manganate solution, in which the manganate ion is oxidized directly at the anode:

Potassium permanganate, being far less soluble in cold water than potassium manganate, is then isolated by crystallization.

Structure and appearance. The permanganate ion, $\text{MnO}_4^-$, is tetrahedral, with manganese in the $+7$ oxidation state; the solid salt forms dark purple, almost black, crystals with a characteristic metallic lustre, and its aqueous solutions are an intense purple even at low concentration.

Oxidizing properties -- three different media, three different products. Potassium permanganate is one of the most versatile oxidizing agents available, because the number of electrons it gains -- and hence the reduction product formed -- depends on the pH of the medium:

• In acidic medium, $\text{Mn}^{7+}$ is reduced all the way to the nearly colourless $\text{Mn}^{2+}$ ion, a five-electron change: $\text{MnO}_4^- + 8\text{H}^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}$ (n-factor $=5$). This is the medium used in the classic titration against oxalate ion, developed as a full worked equation in a later exercise.
• In neutral or faintly alkaline medium, $\text{Mn}^{7+}$ is reduced only as far as $\text{Mn}^{4+}$, precipitating as brown, insoluble manganese dioxide, a three-electron change: $\text{MnO}_4^- + 2\text{H}_2\text{O} + 3e^- \rightarrow \text{MnO}_2 + 4\text{OH}^-$ (n-factor $=3$), also developed further in a later exercise. …