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?
   
   

Xanthophylls

Plant pigments are an important source of non-toxic compounds for use as food or cosmetic colouring agents. The coloured xanthophylls in capsicum are important sources of pigments that can replace carcinogenic synthetic dyes.


Xanthopylls are very similar to carotenes but often contain hydroxyl groups.
Cryptoxanthin, shown to the right, is an example of a xanthophyll.

β-carotene, shown on the left, is an example of a carotene.

Until now, the common method for extracting red pigments from dried fruit of Capsicum has used hexane as the extraction solvent. Current extraction processes are limiting in that the red pigment can only be recovered from American paprika varieties or other mild cultivars.

A study from the New Mexico State University presents a process for the efficient extraction of these pigments using a "green chemistry" method that generates an oleoresin from dried capsicum fruit with virtually the same xanthophyll composition as the hexane extraction method.

Reference
Richard D. Richins, Laura Hernandez, Barry Dungan, Shane Hambly, F. Omar Holguin, and Mary A. O'Connell. A 'Green' Extraction Protocol to Recover Red Pigments from Hot Capsicum Fruit. HortScience, 2010; 45: 1084-1087

Further Reading
Functional Groups
Empirical and Molecular Formula
Molecular Mass (Formula Weight)
Percentage Composition
Oxidation of Alcohols
Intermolecular Forces

Study Questions

1. Circle the hydroxyl group in cryptoxanthin.
2. Write the molecular formula for cryptoxanthin.
3. What is the empirical formula for cryptoxanthin?
4. What percentage by mass of carbon is present in cryptoxanthin?
5. Write the molecular formula for β-carotene.
6. What is the empirical formula for β-carotene?
7. What percentage by mass of hydrogen is present in β-carotene?
8. Which molecule, β-carotene or cryptoxanthin, would be the most polar? Explain your answer.
9. If you were given a solution that contained either cryptoxanthin or β-carotene, how might you be able to decide which of the compounds is present in the solution? Explain your answer.
10. If cryptoxanthin were to react with a strong oxidizing agent, what product(s) might you expect from the reaction? Explain your answer.