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Potassium permanganate is among the most important transition metal compounds for both JEE and practical chemistry.

Preparation: MnO2 (pyrolusite) fused with KOH and air gives K2MnO4 (green manganate, $Mn^{6+}$). Electrolytic or disproportionation oxidation converts it to KMnO4 (purple, $Mn^{7+}$).

Structure: $MnO4^-$ is tetrahedral. The deep purple colour arises from LMCT ($O^{2-}$ to $Mn^{7+}$) since $Mn^{7+}$ is $d^0$. The colour is NOT from d-d transitions.

Oxidising reactions by medium:

• Acidic: $MnO4^-$ + 8$H^+$ + 5$e^-$ to $Mn^{2+}$ + 4H2O. n-factor = 5. Purple to colourless.
• Neutral: $MnO4^-$ + 2H2O + 3$e^-$ to MnO2 + 4$OH^-$. n-factor = 3. Purple to brown precipitate.
• Strongly alkaline: $MnO4^-$ + $e^-$ to $MnO4^{2-}$. n-factor = 1. Purple to green.

Self-indicator: The purple colour itself signals the endpoint in titrations — no external indicator needed.

Key reactions: With oxalic acid (2:5 ratio, autocatalytic at 60-70 degrees C), with FeSO4 (1:5 ratio), with HCl (produces Cl2 — avoid HCl in permanganate titrations). Baeyer's test uses cold dilute KMnO4 for unsaturation detection.

Practical note: KMnO4 is NOT a primary standard (slowly decomposes). It must be standardised against a primary standard like sodium oxalate.