XII CHEMISTRY CHAPTER 4

MS. SIDRA JAVEDLead Pigments

Pigments are the substances which are used to give the proper colour to paints.

Lead Pigments

1. White lead Pigment 2. Red lead Pigment (Sandhur)3. Chrome Yellow Pigment 4. Chrome Red Pigment 5. Turner's Yellow Pigment6. Yellow Lead monoxide (Massicot) 7. Red lead monoxide (Litharge or Murda sang)

White lead Pigment❖ Chemical Name: Basic Lead Carbonate

❖ Chemical formula: [2PbCO3.Pb(OH)2] or Pb3(OH)2(CO3)2

❖ Used in white paints as a pigmentWhite color darkens due to formation of PbS

❖ Toxic in nature ❖ It is now replaced by Titanium oxide due to toxicity

White Lead Pigment - PreparationChamber process or Dutch process:❖ Strips of lead are placed in Chambers

(earthenware) ❖ Mixture of acetic acid and steam is

passed for 2-3 days until lead is corroded

2Pb+2CH3COOH+2H2O

→[Pb(OH)2Pb(CH3COO)2] + 2H2

❖ A mixture of CO2 and acetic acid vapors is passed through the corroded lead for 3-4 weeks

❖ White lead is formed which is then scratched, dried and packed.

3[Pb(OH)2.Pb(CH3COO)2 +2H2O+2CO2

→ 2[2PbCO3.Pb(OH)2] + 6CH3COOH

❖ A better white lead is prepared by Passing CO2 into the suspension of Lead oxide in water containing little quantity of Lead acetate

2PbO + 2CO2 + 2H2O + Pb(CH3COO)2 → [2PbCO3.Pb(OH)2] + 2CH3COOH + O2

Red Lead Pigment (Sandhur)

❖ Chemical Formulae & Chemical name: ➢ Pb3O4 - Triplumbic Tetraoxide➢ (2PbO.PbO2) - Lead sesqui oxide

❖ Color: Red❖ Used to prepare paints of different shades from Orange

red to brick red❖ Insoluble in water❖ Soluble in acids

Red Lead pigment - Preparation❖ Prepared by heating PbO

in a revolving furnace with excess of air about 4500C

6PbO + O2 2Pb3O4

4500C

Chrome Yellow Pigment❖ Chemical Formula: PbCrO4

❖ Chemical Name: Lead Chromate

❖ Color: Yellow

❖ Occur in nature as Crocite

❖ Insoluble in water

❖ Soluble in nitric acid and cuastic alkalis

Chrome Yellow Pigment - Preparation❖ Adding solution of Potassium Chromate K2CrO4 to

solution of Lead nitrate Pb(NO3)2

❖ Action of Potassium Chromate on Lead acetate

Pb(NO3)2 + K2CrO4 → PbCrO4 + 2KNO3

Pb(CH3COO)2 + K2CrO4 → PbCrO4 + 2CH3COOK

Chrome Red Pigment❖ Chemical formula: PbCrO4.PbO or Pb2CrO3

❖ Chemical name: Basic Lead Chromate

❖ Color: Dark Red

❖ Used as red pigment

Chrome Red Pigment - Preparation

2PbCrO4 + 2NaOH → Na2CrO4 + Pb2CrO3 + H2O

❖ Prepared by digesting Lead Chromate with NaOH

Turner’s yellow Pigment❖ Chemical formula: PbCl2.4PbO

❖ Prepared by boiling NaCl solution with Litharge i.e. Lead Monoxide

5PbO + H2O + 2NaCl → 2NaOH + PbCl2.4PbO

Yellow Lead monoxide(Massicot)

Red Lead Monoxide(Litharge)

End of Lesson