Roses are red, sunflowers are yellow

Following on from indium and thallium back in the p-block, some more colourful chemistry today with rhodium and chromium. I’m rather enjoying finding ways of getting some colour into a black and white design!

William Hyde Wollaston was an English chemist who made his fortune by purifying and selling platinum. It was while extracting the residue from his platinum process in 1804 that he produced a dark red powder which dissolved in water to give a beautiful rose-coloured solution. This proved to be the chloride of sodium with a new metal which he called rhodium, from the Greek rodon meaning rose.

Rhodium and chromium are both transition elements, and a characteristic of this family of elements is their ability to form coloured compounds. As a result many transition element compounds find use as pigments in paints and as pottery glazes. Chrome yellow, lead(II) chromate, is one such pigment, and varies in colour from lemon yellow to orange, depending on the particle size and presence of sulfates. Chrome yellow was first synthesised in the early nineteenth century, by French chemist Louis Vauquelin, and later attracted the eye of Vincent van Gogh who used it in his famous Sunflowers paintings. Unfortunately some preparations of chrome yellow gradually darken on exposure to sunlight, running the risk that without proper care and conservation the golden yellow sunflowers may end up a murky orange.

With thanks to Stephen Wilkes for the idea for chromium.

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