Chromium has three common oxidation states: +2, +3 and +6 with varying colours. Sodium chromate (+6) is soluble and converted to sodium dichromate by treating with an acid. When the yellow solution is acidified it turns to orange.
- Chromium (II) Cr(H2O)62+(aq) is bright blue.
- Chromium (II) is easily oxidised. Chromium metal burns in air to give Chromium (III) oxide.
- Chromium (III) dissolves in an acid solution form a violet coloured ion (H2O)63+(aq).
- The +6 oxidation state is represented by the chromatic ion CrO42-. Sodium chromate Na2CrO4 is yellow.
- The +6 oxidation state is also represented by the dichromate ion Cr2O72-. Sodium dichromate Na2Cr2O7 is orange.
In 1761, Johann Gottlob Lehmann (1719-1767) found an orange-red mineral in the Ural Mountains which he named Siberian red lead. In 1797, Louis Nicholas Vauquelin, isolated natural lead (II) chromate (PbCrO4) while investigating the mineral. He was able to produce chromium oxide (also chromium trioxide), a red crystalline compound (CrO3), by mixing the ore with hydrochloric acid. To prepare CrO3 commercially sulphuric acid is added to concentrated solutions of a dichromate salt:K2Cr2O7 (aq) + 2H2SO4 (l) → 2KHSO4 (aq) + CrO3 (s) + H2O (aq)
Investigations revealed a new metal: Vauquelin proposed the name chrome from the Greek word for colour, a reference to the many brightly coloured compounds of chromium. In 1809 Vauquelin attempted to determine the best conditions for making lead chromate. He obtained it by ionic precipitation using solutions of a lead salt and a chromate. Through his series of experiments, Vauquelin was able to determine that through co-precipitation from solution with lead sulphate the colour could be adjusted, and that the colour could also be varied by changing the temperature of the synthesis which affects the grain size.