Alizarin

Alizarin, or 1,2-dihydroxyanthraquinone or mordant red, is the red dye originally derived from the root of the madder plant. In 1869, it became the first natural pigment to be duplicated synthetically.

The word alizarin ultimately derives from the Arabic al-usara, juice.

History
Madder has been cultivated as a dyestuff since antiquity in central Asia and Egypt, where it was grown as early as 1500 BC. Cloth dyed with madder root pigment was found in the tomb of the Pharaoh Tutankhamun and in the ruins of Pompeii and ancient Corinth. In the Middle Ages, Charlemagne encouraged madder cultivation. It grew well in the sandy soils of the Netherlands and became an important part of the local economy.

By 1804, the English dye maker George Field had refined the technique to lake madder by treating it with alum, and an alkali, that turns the water-soluble madder extract into a solid, insoluble pigment. This resulting madder lake has a longer-lasting color, and can be used more versatilely, for example by blending it into a paint. Over the following years, it was found that other metal salts, including those containing iron, tin, and chromium, could be use in place of alum to give madder-based pigments of various other colors. This general method of preparing lakes has been known for centuries.

In 1826, the French chemist Pierre-Jean Robiquet found there were two colorants in madder root, the red alizarin and the more rapidly fading purpurin. The alizarin component became the first natural dye to be synthetically duplicated in 1868 when the German chemists Carl Graebe and Carl Liebermann, working for BASF, found a way to produce it from anthracene. About the same time, the English dye chemist William Henry Perkin independently discovered the same synthesis, although the BASF group filed their patent before Perkin by only one day.

The synthetic alizarin could be produced at less than half the cost of the natural product, and the market for madder collapsed virtually overnight. Alizarin itself has been in turn largely replaced today by the more light-resistant quinacridone pigments developed at DuPont in 1958.

Applications
Alizarin red is used in a biochemical assay to determine, quantitatively by colorimetry, the presence of calcific deposition by cells of an osteogenic lineage. As such it is an early stage marker (days 10-16 of in vitro culture) of matrix mineralisation, a crucial step towards the formation of calcified extracellular matrix associated with true bone.

In clinical practice it is also used to stain synovial fluid to assess for basic calcium phosphate crystals.

Alizarin Crimson
At right is displayed the color alizarin crimson.