Yellow Paint Pigments

Vincent van Gogh used a pigment known as chrome yellow to achieve the intensity of colour present in such famous 19^th century works of art as his Sunflowers paintings. Chrome yellow is made up of lead (II) chromate, PbCrO_4, and can be produced by mixing solutions of lead (II) nitrate and potassium chromate, then filtering off the lead (II) chromate precipitate.

Unfortunately, chrome yellow paint darkens in the presence of sunlight as Cr(VI) changes to Cr(III). The Cr(III) compounds form as a nanometer-thin coating over the pigment particles that make up the paint.

Because chrome yellow darkens in the presence of sunlight, and because it contains toxic lead, it was replaced with cadmium yellow by the 1950s. Cadmium yellow is actually cadmium sulfide. While cadmium yellow does not tend to change colour in sunlight, it does contain toxic cadmium.

Cadmium pigments are slowly being replaced by azo dyes which are of the general formula R-N=N-R', where R usually contains a benzene ring within its structure .

Reference
Letizia Monico, Geert Van der Snickt, Koen Janssens, Wout De Nolf, Costanza Miliani, Joris Dik, Marie Radepont, Ella Hendriks, Muriel Geldof, Marine Cotte. Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 2. Original Paint Layer Samples. Analytical Chemistry, 2011; 83 (4): 1224 DOI: 10.1021/ac1025122

Further Reading
Naming ionic compounds
Writing the formula of ionic compounds
Writing precipitation reaction equations
Oxidation numbers (states)
Oxidation and Reduction

Study Questions

1. For the compound PbCrO4 give the oxidation number (state) for:
   
• Pb
• Cr
• O
2. Write the formula for:
   
• lead (II) nitrate
• potassium chromate
• cadmium sulfide
3. For the compound potassium chromate, give the oxidation number (state) for:
   • K
   • Cr
   • O
   
   
4. For the reaction between lead (II) nitrate and potassium chromate, write the balanced
   
• molecular equation
• ionic equation
• net ionic equation
5. Name the spectator ions present in the reaction between lead (II) nitrate and potassium chromate.
   
6. Write an electron transfer equation to represent the reaction in which Cr(VI) changes to Cr(III) in the presence of sunlight.
7. For the equation above is:
   • Cr(IV) being oxidized or reduced?
   • Cr(IV) an oxidant or a reductant?
   • Cr(IV) an oxidizing agent or a reducing agent?