Chrome Yellow

• In 1797, Louis Vauquelin, a French chemist, discovered chromium.
• The colours of his samples of chromium varied from orange-yellow to orange-red; these samples were a form of lead chromate known as crocodite.
• Crocodite was not readily available in Europe until the early 19th century, when a new source of crocodite was discovered in the Var region of France.
• Within a few years, painters were using the pigment as a dark lemon yellow colour which lasted much longer than the pigments previously available.
• In 1809, Vauquelin carried out a series of investigations to determine the best way of producing crocodite in the laboratory.
• He eventually came up with the following precipitation reaction:
  Pb(NO₃) _2(aq) + Na₂CrO_4(aq) 2NaNO_3(aq) + PbCrO_4(s)
  
• Or more simply:
  Pb²⁺_(aq) + CrO₄^2-_(aq) PbCrO_4(s)
  
• Chrome yellow was the first of a range of metal chromates containing lead, barium, strontium or zinc, which can be used to produce pigments with different shades of yellow.
• Chromium yellow is still used today in printing; it is also used to paint the road with double yellow lines!

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