Important d-Block Compounds — Potassium Dichromate (K2Cr2O7) and Potassium Permanganate (KMnO4) Why these purple and orange salts matter Two classic d-block oxidants appear again and again in NEET Chemistry and in real life: potassium dichromate (K2Cr2O7, orange) and potassium permanganate (KMnO4, deep purple). You will meet them in redox titrations, organic tests (Baeyer’s), and industry (tanning, water treatment). Master their preparation, structures, and how medium (pH) flips their oxidizing products. Color cue: deep purple KMnO4 (left) versus bright orange K2Cr2O7 (right) — a quick visual anchor for exams. High-yield: (1) Chromate ⇌ dichromate is pH controlled (yellow ⇌ orange). (2) KMnO4 products depend on medium: acidic → Mn2+ (n = 5), neutral/slightly alkaline → MnO2 (n = 3), strongly alkaline → MnO4 2− (n = 1). (3) E°: MnO4−/Mn2+ +1.51 V > Cr2O7 2− / Cr 3+ +1.33 V. (4) KMnO4 is a self‑indicator; K2Cr2O7 needs an external indicator. neet-alert Potassium dichromate (K2Cr2O7): Manufacture from chromite Source ore: chromite FeO·Cr2O3 (spinel, often written FeCr2O4). The goal is to first make soluble sodium chromate (Na2CrO4), convert it to sodium dichromate (Na2Cr2O7), then crystallize K2Cr2O7 using KCl. Air/oxygen and sodium carbonate do the initial oxidation during fusion. Chromite → sodium chromate → sodium dichromate → potassium dichromate Fusion of chromite with Na2CO3 in air Greenish-yellow sodium chromate forms; iron becomes Fe2O3. Acidification to dichromate Acidic medium dimerizes chromate to dichromate. Formation of potassium dichromate K2Cr2O7 crystallizes out (lower solubility than sodium salt). Roast/fuse powdered chromite with Na2CO3 in air (oxidation to Na2CrO4). Leach with water to extract Na2CrO4; filter off Fe2O3. Acidify with H2SO4 to get Na2Cr2O7 (solution turns orange). Add KCl; cool to crystallize K2Cr2O7 (orange crystals). Process flow you can remember Structure of Cr2O7 2− and the chromate–dichromate equilibrium Cr2O7 2− consists of two tetrahedral CrO4 units sharing one corner (a μ‑oxo bridge). pH shifts the mono-chromate (yellow) ⇌ dichromate (orange) balance. Adding acid pulls the equilibrium towards dichromate; adding base shifts to chromate. Yellow chromate ⇌ orange dichromate. pH-controlled interconversion 2026-05-26T17:05:04.052Z gpt-image-2 Two corner-sharing CrO4 tetrahedra make Cr2O7 2− . Structure diagram of dichromate ion: two tetrahedral CrO4 units sharing one oxygen (μ-oxo bridge). Show Cr atoms in grey, O in red; terminal vs bridging O labeled. Clean 2D vector style on white. Include small inset color bar: chromate (yellow) ⇌ dichromate (orange) with 'basic' and 'acidic' labels. No embedded text except labels. 2026-05-26T17:05:04.960Z pH-dependent color infographic: 3 beakers from left (basic, yellow CrO4 2− ) to right (acidic, orange Cr2O7 2− ). Include reaction arrow 2CrO4 2− +2H+ ⇌ Cr2O7 2− +H2O beneath. Clean vector, white background, simple labels 'basic', 'neutral', 'acidic'. Chromate (yellow) ⇌ dichromate (orange) shifts with pH. gpt-image-2 Oxidizing behavior of dichromate (acidic medium) E° = +1.33 V; strong oxidizing agent in acidic solution. Cr(VI) to Cr(III) half-reaction in acid Typical oxidations by K2Cr2O7 in acid Fe 2+ → Fe 3+ (classic iron estimations; external indicator needed). I - → I2 (liberates iodine; brown solution). SO2 → SO4 2- (gas scrubbing/analysis). H2S → S (milky sulfur). Primary alcohols → aldehydes/acids; secondary alcohols → ketones (organic lab). In acid, orange dichromate is reduced to green Cr 3+ while oxidizing the reductant — the essence of dichromate titrations. remember Breathalyzer: K2Cr2O7 in acidic medium oxidizes ethanol (CCO) from breath; orange Cr(VI) turns green Cr(III). The color change is read as a measure of alcohol. K2Cr2O7 is NOT a self-indicator. Use an external indicator (e.g., diphenylamine sulfonate) in dichromate titrations with Fe 2+ . tip Potassium permanganate (KMnO4): Manufacture from pyrolusite Source ore: pyrolusite (MnO2). First, oxidation with KOH and air gives green potassium manganate (K2MnO4). Then manganate is converted to purple KMnO4 by electrolytic oxidation or by acidification leading to disproportionation. Industrially, both routes are used. Pyrolusite → potassium manganate → potassium permanganate Green manganate (Mn in +6) forms on fusion with KOH and O2. Oxidative fusion to manganate Acidic conversion of manganate to permanganate + MnO2. Disproportionation to permanganate Manufacture path for KMnO4: MnO2 (ore) → green K2MnO4 → purple KMnO4, with colors shown at each step. Structure and color of MnO4− Permanganate ion, MnO4−, is tetrahedral with Mn in the +7 oxidation state. The intense purple color is due to charge-transfer transitions. 2026-05-26T17:05:05.481Z Isolated ion diagram of permanganate: purple tetrahedral MnO4− with Mn at center and four O atoms at corners. Include oxidation state +7 label near Mn. Clean, white background, vector chemistry style. No extra text. Tetrahedral MnO4−: central Mn(VII) with four equivalent Mn–O bonds. gpt-image-2 KMnO4: Products depend on the medium (very high-yield) Acidic medium (strongest) E° = +1.51 V; purple → nearly colorless (pale pink at very high [ Mn 2+ ]). Purple → brown MnO2 precipitate (Mn in +4). Neutral/slightly alkaline Strongly alkaline Purple → green manganate (Mn in +6). 2026-05-26T17:05:05.727Z gpt-image-2 KMnO4 reduction across media: purple → colorless (acid), purple → brown (neutral), purple → green (strong base). Three-panel beaker series: (1) acidic KMnO4 turning colorless (Mn2+), (2) neutral giving brown MnO2 precipitate, (3) strongly alkaline giving green MnO4 2− . Annotate electron counts 5, 3, 1 below panels. Clean vector style, white background. Medium Product of Mn Color change Electrons gained per MnO4 - KMnO4 reduction in different media Case Acidic Mn 2+ Purple → colorless Neutral / slightly alkaline MnO2(s) Purple → brown ppt Strongly alkaline MnO4 2- Purple → green remember KMnO4 is a self‑indicator: the endpoint in titrations is the first permanent faint pink color in the bulk solution. Key reactions you must recognize quickly With common reductants Fe 2+ → Fe 3+ : both K2Cr2O7 (acidic) and KMnO4 (acidic) oxidize Fe 2+ . I - → I2: both oxidants liberate iodine from iodide. SO 3 2- / SO 2 → SO 4 2- : both work in acid; MnO4 - also effective in neutral. H 2 S → S: both convert sulfide to elemental sulfur. C 2 O 4 2- (oxalate) → CO2: classic acidic KMnO4 titration; warm to speed up. Alkenes (e.g., ethene C=C) → 1,2-diols: cold, dilute alkaline KMnO4 (Baeyer’s test). Representative half-reactions and balancing in different media Baeyer’s test: ethene (C=C) → ethan-1,2-diol (HO-CH2-CH2-OH) Purple KMnO4 decolorizes; alkene → vicinal diol Fe 2+ Fe 3+ K2Cr2O7 (acid) or KMnO4 (acid) Iron estimation (volumetric) I - I2 Both (acid) Iodine liberation tests SO 2 / SO 3 2- SO 4 2- K2Cr2O7 (acid); KMnO4 (acid/neutral) Gas scrubbing, analysis H 2 S S (elemental) Both Qualitative sulfide test C 2 O 4 2- (oxalate) CO2 KMnO4 (acid) Permanganate titration Ethene (C=C) Ethan-1,2-diol KMnO4 (cold, alkaline) Baeyer’s test (unsaturation) Reductant Oxidation product Oxidant used Application Reactions of K2Cr2O7 / KMnO4 with reductants System K2Cr2O7 vs KMnO4 — compare and conquer Source (manufacture) Chromite FeO·Cr2O3 → Na2CrO4 → Na2Cr2O7 → K2Cr2O7 Pyrolusite MnO2 → K2MnO4 → KMnO4 Oxidation state (central atom) Cr in +6 (reduced to +3 in acid) Mn in +7 (reduced to +2/+4/+6 depending on medium) Structure of active ion Cr2O7 2− : two corner-sharing CrO4 tetrahedra (μ‑oxo) MnO4 − : tetrahedral Solution color Orange (chromate yellow; dichromate orange) Deep purple Standard potential in acid Cr2O7 2− / Cr 3+ : E° = +1.33 V MnO4 − / Mn 2+ : E° = +1.51 V Indicator behavior Needs external indicator Self-indicator (faint pink endpoint) Flagship uses Chrome tanning; breathalyzer; oxidant in labs/dyes Volumetric titrations; Baeyer’s test; water purification Feature K2Cr2O7 KMnO4 Quick comparison table Aspect Industrial and real-life uses Where they show up outside exams Chrome tanning (K2Cr2O7 precursor for Cr(III) tanning baths in leather industry). Textile/dye industry (oxidation steps, mordanting with chromium compounds). Breathalyzer (K2Cr2O7 color change tracks ethanol). Water purification (KMnO4 oxidizes Fe 2+ , Mn 2+ , H2S; controls taste/odour). Medical antiseptic (dilute KMnO4 solution, ~1% for some external uses). Fruit storage: KMnO4 sachets adsorb ethylene, delaying ripening. 2026-05-26T17:05:05.877Z gpt-image-2 Chrome tanning: conversion of raw hide collagen into durable leather using chromium(III) generated from dichromate processing. Process schematic: raw hide → deliming → pickling (acid) → chrome tanning bath (Cr(III) complexes) → basification → finished leather. Show K2Cr2O7 as precursor reduced to Cr(III). Clean flowchart, icons for each step, vector style, no dense text. Wrong. In dichromate, Cr is +6 (usually reduced to +3 in acid; n = 6 per Cr2O7 2− ). In permanganate, Mn is +7 and the n‑factor changes with medium (5 in acid, 3 in neutral/slightly alkaline, 1 in strongly alkaline). K2Cr2O7 and KMnO4 have the same oxidation state and identical n-factors in redox titrations. KMnO4 always gives manganate (green) in alkaline solutions. Only in strongly alkaline medium does MnO4− accept 1 e− to become green MnO4 2− . In neutral or only slightly alkaline solution, KMnO4 is reduced to brown MnO2 (n = 3). “A–N–A: 5–3–1” → Acid–Neutral–Alkaline electron counts for KMnO4 are 5, 3, 1. Colors: Acid (purple → colorless), Neutral (purple → brown), strong Alkali (purple → green). Chromite ore Iron chromium oxide (FeO·Cr2O3; spinel, FeCr2O4) used to make chromates/dichromates. Manganese dioxide (MnO2) ore used to manufacture KMnO4. Pyrolusite CrO4 2− (yellow) tetrahedral oxyanion of chromium(VI). Chromate Cr2O7 2− (orange) from two corner-sharing chromate tetrahedra; dominant in acid. Dichromate μ-oxo bridge A single oxygen atom bridging two metal centers (as in Cr2O7 2− ). MnO4 2− (green) with Mn in +6; interconverts with permanganate. Manganate MnO4 − (deep purple) tetrahedral oxyanion of manganese(VII). Permanganate Chrome tanning Leather tanning using Cr(III) complexes; K2Cr2O7 is a common precursor. Self-indicator A reagent whose own color change marks the endpoint (e.g., KMnO4 in titrations). Volumetric analysis Quantitative analysis by titration; KMnO4 is widely used as an oxidimetric titrant. Test for unsaturation: cold, dilute alkaline KMnO4 converts alkenes to 1,2-diols with decolorization and brown MnO2. Baeyer’s test Breathalyzer Device using K2Cr2O7 in acid to oxidize ethanol from breath; orange → green indicates alcohol. Quick glossary SMILES (organic examples mentioned): ethanol (CCO), ethene (C=C), ethan-1,2-diol (OCCO), oxalic acid (OC(=O)C(=O)O). All preserved key equations (verify and memorize)