potassium dichromate

Aug 26, 2022, 16:45 IST

Potassium Dichromate, K2Cr2O7

Preparation of Potassium Dichromate

 It is the most important compound of Cr (VI). It is manufactured from chromite ore. Chromite ore is first converted into sodium dichromate. 

It is manufactured from chromite ore [FeCr2O4] which is first converted into sodium dichromate Sodium dichromate is hygroscopic and more soluble than poto dichromate .The hot saturated solution of Na2Cr2O7 is mixed with KCl, NaCl precipitates out while hot because of its less solubility in comp. to KCl.  When the mother liquor cooled, the orange crystals of K2Cr2O7 separates out.

Properties Potassium Dichromate

It is orange-red coloured crystalline compound. It is moderately soluble in cold water but freely soluble in hot water It melts at 398°C. On heating strongly, it decomposes liberating oxygen.

Potassium Dichromate Action on alkali

On heating with alkali, it is converted into chromate i.e., the colour changes from orange to yellow.  On acidifying, the yellow colour again changes back to orange.Thus dichromate and chromate ions are inter convertible in aq. solution depending upon the pH of solution being in equilibrium at pH 4.0.  The oxidation state of Cr atom is same in both – the dichromate and chromate ion. The chromate ion is tetrahedral, the dichromate ion being to tetrahedrons sharing one corner.

Potassium Dichromate Reaction with H2O2

Acidified solution of   ions develops a deep blue colour with H2O2 in presence of little ether due to formation of perchromic acid (perodie of chromium).

Chromyl chloride test of Potassium Dichromate 

This is a test of chloride. When a mixture of a metal chloride and potassium dichromate is heated with conc. H2SO4, orange-red vapours of chromyl chloride are evolved.When chromyl chloride vapours are passed through NaOH solution, yellow coloured solution is obtained.

Uses of Potassium Dichromate

i)   As oxidizing agent in laboratory. 

ii)   For volumetric estimation of reducing agents like oxalic acid, Fe2+, salts I SO32 ions etc.

iii)   For prep. of other chromium compounds eq. chrome alum, chrome yellow, chrome red, zinc yellow etc.

iv)   In photography for hardening of gelatin film.

v)   In dying by mordant dye for providing Cr(OH)3 as more mordant are those substances which have more than one group attached with it and are capable of forming bonds with dyes as well as the object to be dyed.  In fact they react firstly with the fabric or leather forming a layer thought.  This layer, now depending on its chemical nature be stained (dyed) with suitable colouring materials.  Use of mordant become crucial for the reason that some dyes are not able enough to bound with fabric or leather permanently.  As they are exposed to sunlight, acids or alkalis, their colours fade away.  Use of mordant makes them acid, base or u v fast.

vi)   Chromic acid (K2Cr2O7 + H2SO4) is used as cleansing agent for glass ware in lab.

vii)  IN chromyl chloride test.

viii) In deflection of H2O2.