d Block Elements : Potassium dichromate

 

Potassium dichromate, (K2Cr2O7)

Potassium dichromate is one of the most important compound of chromium, and also among dichromates. In this compound Cr is in the hexavalent (+6) state.

Preparation : It can be prepared by any of the following methods,

(i) From potassium chromate : Potassium dichromate can be obtained by adding a calculated amount of sulphuric acid to a saturated solution of potassium chromate.

\underset { potassium\quad chromate\quad (Yellow) }{ 2{ K }_{ 2 }Cr{ O }_{ 4 } } +\quad H_{ 2 }S{ O }_{ 4 }\rightarrow

\underset { potassium\quad dichromate\quad (orange) }{ K_{ 2 }Cr{ O }_{ 7 } } +K_{ 2 }S{ O }_{ 4 }+H_{ 2 }{ O }

K2Cr2O7 Crystals can be obtained by concentrating the solution and crystallisation.

 

(ii) Manufacture from chromite ore : K2Cr2O7 is generally manufactured from chromite ore (FeCr2O4). The process involves the following steps.

(a) Preparation of sodium chromate. Finely powdered chromite ore is mixed with soda ash and quicklime. The mixture is then roasted in a reverberatory furnace in the presence of air. Yellow mass due to the formation of sodium chromate is obtained.

          4FeCr2O4 + O2 → 2Fe2O3 + 4Cr2O3

          4Cr2O3 + 8Na2CO3 + 6O2 → 8N2CrO4 + 8CO2(g)

             4FeCr2O4 + 8Na­2CO3 + 7O2 → 2Fe2O3 + 8CO2(g) + 8Na2CrO4

                                                                                      Sodium chromate

The yellow mass is extracted with water, and filtered. The filtrate contains sodium chromate.

The reaction may also be carried out by using NaOH instead of Na2CO3.The reaction in that case is,

          4FeCr2O4 + 16NaOH + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8H2O 

(b) Conversion of chromate into dichromate. Sodium chromate solution obtained in step (a) is treated with concentrated sulphuric acid when it is converted into sodium dichromate.

          2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

          Sodium chromate       Sodium dichromate

On concentration, the less soluble sodium sulphate, Na2SO4.10H2O crystallizes out. This is filtered hot and allowed to cool when sodium dichromate, Na2Cr2O7.2H2O, separates out on standing.

 

(c) Concentration of sodium dichromate to potassium dichromate. Hot concentrated solution of sodium dichromate is treated with a calculated amount of potassium chloride. When potassium dichromate being less soluble crystallizes out on cooling.

                   Na2CrO7 + 2KCl → K2Cr2O7 + 2NaCl

                   Sod. Dichromate             pot. dichromate

 

Physical properties

(i) Potassium dichromate forms orange-red coloured crystals.

(ii) It melts at 699 K.

(iii) It is very stable in air (near room temperature) and is generally, used as a primary standard in the volumetric analysis.

(iv)It is soluble in water though the solubility is limited.

 

Chemical properties

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

                   4K2Cr2O7 (s) \underrightarrow { \quad \Delta \quad } 4K2CrO4(s) + 2Cr2O3(s) + 3O2          

 

(ii) Action of acids

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

                   K2Cr2O7(aq) + conc.H2SO4 → KHSO4 (aq) + 2CrO3 (s) + H2O

On heating a dichromate-sulphuric acid mixture, oxygen gas is given out.

                   2K2Cr2O7 + 8H2SO4 → 2K2SO4 + 2Cr2 (SO4)3 + 8H2O + 3O2

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

          K2Cr2O7(aq) + 14HCl(aq) → 2CrCl3(aq) + 2KCl(aq) + 7H2O + 3Cl2(g)         

 

(iii) Action of alkalies : With alkalies, it gives chromates. For example, with KOH,

          K2Cr2O4 + 2KOH → 2K2CrO4 +H2O

                   Orange                yellow       

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

          2K2CrO4 + H2SO4 → K2Cr2O7 + K2SO4 + H2O

Actually, in dichromate solution, the Cr2O72– ions are in equilibrium with Cr2O72– ions.

                   Cr2O72– + H2O → 2 Cr2O42– + 2H+

 

(iv) Oxidising nature : In neutral or in acidic solution, potassium dichromate acts as an excellent oxidising agent, and Cr2O72–gets reduced to Cr3+. The standard electrode potential for the reaction, Cr2O72– + 14+ + 6e– → 2Cr+3 +7H2O is +1.31 V. This indicates that dichromate ion is a fairly strong oxidising agent, especially in strongly acidic solutions. That is 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

K2CrO7 + 4H2SO4 → K2SO4 + Cr2 (SO4)3 + 4H2O + 3[O]

2FeSO4 + H2SO4 + [O] → Fe2 [SO4]3 + H2O × 3

K2CrO7 + 6FeSO4 + 7H2SO4 → K2SO4 + Cr2 (SO4)3 + 3Fe2(SO4)3+ 7H2O         

Ionic equation : Cr2O72– + 14+ + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O

 

(b) Sulphites to sulphates and arsenites to arsenates.

K2Cr2O7 + 4H2SO4 → K2SO4 +Cr2 (SO4)3 + 4H2O + [O]

Na2SO3 + [O] → Na2SO4] × 3 

K2Cr2O7 + 4H2SO4 + 3Na2SO3 → K­2SO4 + Cr2 (SO4)3 + 3Na2SO4 + 4H2O

Ionic equation : Cr2O72– + 8H+ + 3SO42– + 4H2O

Similarly, arsenites are oxidised to arsenates.

Cr2O72– + 8H+ + 3AsO33– → 2Cr3+ + 3AsO43– + 4H2O

 

(c) Hydrogen halides to halogens.                 

K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2 (SO4)3 + 4H2O + 3[O]

2HX + O → H2O + [X2] × 3

K2Cr2O7 + 4J2SO4 + 6HX → K2SO4 + Cr2 (SO4)3 + 7H2O + 3X2

Where, X may be Cl, Br, I.

Ionic equation : Cr2O72– + 8H+ + 6HX → 2Cr3+ +3X2 + 7H2O

 

(d) Iodides to iodine

          K2Cr2O7 + H2SO4 → K2SO4 + Cr2 (SO4)3 + 4H2O + [O]           

          2KI + H2O + [O] → 2KOH + I2] × 3

          2KOH + H2SO4 → K2SO4 + 2H2O] × 3

          K2Cr2O7 + 7H2SO4 + 6KI → 4K2SO4 + Cr2(SO4)3 + 3I2 + 7H2O

Ionic equation : Cr2O72– + 14H+ + 6I– → 2Cr3+ + 7H2O + 3I2

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

 

(e) It oxidises H2S to S.

          K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2 (SO4)3 + 4H2O + 3[O]

          H2S + [O] → H2O + [S] × 3

          K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2 (SO4)3 + 7H2O + 3S          

Ionic equation : Cr2O72– + 8H+ + 3H2S → 2Cr3+ +3S +7H2O

 

(v) Formation of insoluble chromates : With soluble salts of lead, barium etc., potassium dichromate gives insoluble chromates. Lead chromate is an important yellow pigment.

          2Pb(NO3)2 + K2Cr2O7 + H2O → 2PbCrO4 + 2KNO3 + 2HNO3

         

(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.

K2Cr2O7 + 4NaCl + 6H2SO4  \underrightarrow { \quad heat\quad } 2KHSO4 + 4NaHSO4 +2CrO2Cl2

                                                                   Chromyl chloride

                                                               (orange-red vapours)

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

          CrO2Cl2 + 3H2O → 2HCl + H2CrO4

                                     Chromic acid (yellow solution)

Chromyl chloride test can be used for the detection of chloride ion is any mixture.

Uses : Potassium dichromate is used as,

(i)      An oxidising agent

(ii)     In chrome tanning

(iii)    The raw material for preparing large number of chromium compounds

(iv)    Primary standard in the volumetric analysis.

 

Structures of Chromate and Dichromate Ions

Chromates and dichromates are the salts of chromic acid (H2CrO4). In solution, these ions exist in equilibrium with each other. Chromate ion has four oxygen atoms arranged tetrahedrally around Cr atom. (see Fig). Dichromate ion involves a Cr–O–Cr bond as shown in Fig.

 

d Block Elements : Potassium Permanganate