Preparation Properties and Uses of KMnO4 and K2Cr2O7

Potassium Permanganate, KMnO4:

Preparation:

Potassium permanganate is prepared from pyrolusite (MnO2). Pyrolusite is fused with caustic potash (KOH) or K2CO3 in the presence of oxidizing agents like potassium nitrate or potassium chlorate (KClO3) in a reverberatory furnace, as a result, green mass of K2MnO4 is formed.

KNO3 —–> KNO2    +    [O]

MnO2  +  2KOH  + [O]  ——> K2MnO4  + H2O

                                                 Or

MnO2  +  K2CO3  + [O] ——->  K2MnO4  + CO2

The green mass of K2MnO4 is extracted with water and oxidized to KMnO4 by passing chlorine or ozone or CO2 or by electrically.

K2MnO4  + Cl2 ——-> KMnO4  + KCl

K2MnO4   + H2O  +  O3 ———-> KMnO4  + KOH   +  O2

K2MnO4   +  CO2 ———> KMnO4  + K2CO3   +  MnO2

As a result of oxidation, the green colour of solution changes into purple. The purple solution containing KMnO4 is concentrated by evaporation which on cooling gives crystals of KMnO4.

Properties of Potassium Permanganate:

  • Potassium permanganate is a violet crystalline solid, soluble in water and its aqueous solution is purple colour.
  • On heating, KMnO4 changes into K2MnO4 and oxygen gas is evolved.

KMnO4 ——-> K2MnO4  +  MnO2  + O2

  • On heating with alkalies, KMnO4 changes into K2MnO4 and oxygen gas is evolved.

KMnO4  +   KOH ——–> K2MnO4  +  H2O  + O2

  • KMnO4 act as a powerful oxidizing agent in acidic, neutral and alkaline medium. e.g. In acidic medium,

KMnO4  +   H2SO4 ——-> K2SO4  + MnSO4  +  H2O  + O

In neutral medium,

KMnO4 + H2O ——-> KOH  +  MnO2  + O

In alkaline medium,

KMnO4  +   KOH ——–> K2MnO4  +  H2O  + O

  • It oxidizes ferrous sulphate to ferric sulphate

KMnO4 + H2SO4 + FeSO4 ———> K2SO4 + MnSO4 + Fe 2(SO4)3 + H2O

  • It oxidizes potassium iodide to iodine.

KMnO4 + H2SO4 + KI ——-> K2SO4 + MnSO4+ H2O + I2

  • It oxidizes oxalic acid to water and carbon dioxide.

KMnO4  +  H2SO4  + H2C2O4 ——–> K2SO4  +  MnSO4  +  H2O  + CO2.

  • It oxidizes hydrogen sulphide to sulphur.

KMnO4 + H2SO4 + H2S ——–>  K2SO4 + MnSO4 + H2O + S

  • It oxidizes hydrochloric acid to chlorine.

KMnO4 + H2SO4 + HCl——–> K2SO4 + MnSO4 + H2O + Cl2

  • It oxidizes sulphur dioxide to sulphuric acid.

KMnO4 + SO2 + H2O ——-> K2SO4 + MnSO4 + H2SO4

Uses: KMnO4 is used –

  • As an oxidizing agent in the laboratory and industry.
  • For volumetric estimation of ferrous salts, oxalates and other reducing agents.
  • In dry cell.
  • As laboratory reagent (Baeyer’s reagents).
  • As disinfectant for water.

Potassium Dichromate, K2Cr2O7:

Potassium dichromate is prepared from chromite ore of iron, (FeCr2O4). The various steps involved are as follows—

i) Preparation of sodium chromate: The powdered ore is heated with sodium hydroxide in the presence of air in a reverberatory furnace to produce sodium chromate.

4 FeCr2O4 + l6NaOH + 7O2 ——–> 8Na2CrO4 + 2Fe2O3 +8H2O

ii) Conversion of sodium chromate into sodium dichromate: Sodium chromate is extracted with water and acidified with sulphuric acid to get sodium dichromate.

2 Na2CrO4 + H2SO4 ——–> Na 2Cr 2O7  + Na 2SO4 + H2O

On cooling, sodium sulphate separates out as Na2SO4 .10H2O and the solution contains sodium dichromate in it.

iii) Conversion of sodium dichromate into potassium dichromate: The solution containing sodium dichromate is treated with potassium chloride.

Na2Cr2O7+ 2KCl ———> K 2Cr 2O7 + 2NaCl

Sodium chloride being least soluble separates out and is removed by filtration. On cooling, potassium dichromate crystallizes out in the form of orange coloured crystals. Potassium dichromate is fairly soluble in water.

The structure of dichromate ion is shown below. It consists of two tetrahedra sharing an oxygen atom at the common corner.

The dichromate ion (Cr 2O7-2) and chromate ion (CrO4-2) exist in equilibrium with each other at a pH of about 4. They are interconvertible by changing the pH. The chromate ions on addition of acid change into dichromate ion while the dichromate ions on addition of alkali change into chromate ion.

Fig: Structure of dichromate ion

Properties of Potassium Dichromate:

  • It is an orange crystalline solid. It is soluble in hot water but moderately soluble in cold water.
  • Action of Heat. On heating, K2Cr2O7 decomposes to give potassium chromate and chromic oxide.

K2Cr2O7 ——>  K2CrO4  +  Cr2O3  + O2

  • Oxidizing Properties. It acts as strong oxidizing agent in acidic medium.

K2Cr2O7  +  H2SO4 ——-> K2SO4  +  Cr2(SO4)3  +  H2O  + O.

Some of the oxidizing reactions of K2Cr2O7 are given below:

(a) It oxidizes potassium iodide to iodine.

K2Cr2O7  +  H2SO4  + KI ——> K2SO4  +  Cr2(SO4)3  +  H2O  + I2

(b) It oxidizes acidified ferrous sulphate to ferric sulphate.

K2Cr2O7  +  H2SO4 + FeSO4 ——>  K2SO4  +  Cr2(SO4)3  + Fe2(SO4)3 +  H2O

(c) It oxidizes hydrogen sulphide to sulphur.

K2Cr2O7  +  H2SO4  + H2S                       K2SO4  +  Cr2(SO4)3  +  H2O  + S.

(d) It oxidizes oxalic acid to water and carbon dioxide.

K2Cr2O7  +  H2SO4  + H2C2O4 ——–> K2SO4  +  Cr2(SO4)3  +  H2O  + CO2.

(e) It oxidizes hydrochloric acid to chlorine.

K2Cr2O7  +  H2SO4  + HCl ——-> K2SO4  +  Cr2(SO4)3  +  H2O  + Cl2

  • Chromyl Chloride Test: When potassium dichromate is heated with any chloride salt (NaCl) and conc. H2SO4, orange red fumes of chromyl chloride are formed. This reaction is called chromyl chloride test and is used to detect chloride ions.

K2Cr2O7  +  H2SO4  + NaCl ——–> KHSO4  + NaHSO4  +  CrO2Cl2 +  H2O.

Uses: It is used –

  • As an oxidizing agent in the volumetric analysis for the quantitative estimation of iron, oxalate ions etc.
  • In photography for hardening of gelatin films.
  • In dyeing as mordants.
  • For the detection of chlorine.
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