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Potassium dichromate is a versatile oxidising agent and primary standard for volumetric analysis.

Preparation: From chromite ore (FeCr2O4): fusion with Na2CO3 + air to Na2CrO4, acidification to Na2Cr2O7, then metathesis with KCl to K2Cr2O7 (less soluble, crystallises out).

Chromate-dichromate equilibrium: 2$CrO4^{2-}$(yellow) + 2$H^+$ is in equilibrium with $Cr2O7^{2-}$(orange) + H2O. pH-dependent: acid favours orange dichromate, base favours yellow chromate. This is NOT a redox process.

Oxidising reaction in acidic medium: $Cr2O7^{2-}$ + 14$H^+$ + 6$e^-$ to 2$Cr^{3+}$(green) + 7H2O. n-factor = 6. Equivalent weight = 294/6 = 49.

Key oxidation reactions: $Fe^{2+}$ to $Fe^{3+}$ (1:6 molar ratio), $I^-$ to I2, H2S to S, $SO3^{2-}$ to $SO4^{2-}$, ethanol to acetic acid (breathalyser test).

With conc. H2SO4: Produces CrO3 (dark red chromium trioxide), the anhydride of chromic acid. CrO3 + H2O to H2CrO4.

Advantages as primary standard: High purity, stable in air, non-hygroscopic, definite composition, high molecular weight. Indicator: diphenylamine (colourless to blue-violet at endpoint).

Oxide character trend of Cr: CrO ($Cr^{2+}$, basic) to Cr2O3 ($Cr^{3+}$, amphoteric) to CrO3 ($Cr^{6+}$, acidic). Lower oxidation state = more basic, higher = more acidic.