Chemical properties

(i)       Action of heat: Potassium dichromate when heated strongly. This decomposes to release oxygen.              

(ii) Action of acids

(a) In the cold, with concentrated H₂SO₄, red crystals of the chromium trioxide separate out.

On heating the dichromate-sulphuric acid mixture, oxygen gas is released.

(b) With the HCl, on heating chromic chloride is formed and Cl₂ is released.

(iii) Action of alkalies : With alkalies, it gives result to chromates. Hear its shown with KOH,

On acidifying, the colour again changes to orange-red owing to the formation of dichromate.

In fact, in dichromate solution, the Cr₂O₇^2- ions are in equilibrium with CrO₄^2- ions.

(iv) Oxidising nature : In neutral or in acidic solution, the potassium dichromate acts as an outstanding oxidising agent, and Cr₂O₇^2- gets reduced to Cr³⁺. The standard electrode potential for the reaction,

is +1.31V.

   This indicates that dichromate ion is a fairly strong oxidising agent, especially in strongly acidic solutions. This is the reason why potassium dichromate is widely used as an oxidising agent, for quantitative estimation of the reducing agents such as, Fe²⁺. It oxidises,

(a) Ferrous salts to ferric salts

Ionic equation

(b) The sulphites to sulphates and arsenites to arsenates.

Ionic equation

Likewise, arsenites are oxidised to arsenates.

(c) Hydrogen halides to halogens.

where, X may be Cl, Br, I.

Ionic equation :

(d) Iodides to iodine

Ionic equation : 

Therefore, when KI is added to an acidified solution of K­₂Cr₂O₇ iodine gets liberated.

(e) It oxidises H₂S to S.

Ionic equation

(v) Formation of insoluble chromates : With soluble salts of lead, barium and more, the potassium dichromate gives insoluble chromates. Lead chromate is the significant yellow pigment.

         (vi) Chromyl chloride test : When potassium dichromate is heated with conc. H₂SO₄ in the presence of a soluble chloride salt, the orange-red vapours of chromyl chloride (CrO₂Cl₂) are formed.

Chromyl chloride vapours when passed through water give yellow-coloured solution containing chromic acid.

The chromyl chloride test can be used for detection of chloride ion is any mixture.

Uses : Potassium dichromate is used as,

(i) An oxidising agent

(ii) In chrome tanning

(iii) The basic raw meterial for the preparing large number of chromium compounds

(iv) The primary standard in volumetric analysis.

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