Phosphomolybdic acid

Phosphomolybdic acid, also known as dodeca molybdophosphoric acid or PMA is a component of Masson’s trichrome stain. It is a yellow-green compound, freely soluble in water and polar organic solvents such as ethanol. It is used as a reagent in thin layer chromatography for staining phenolics, hydrocarbon waxes, alkaloids and steroids.
Conjugated, unsaturated compounds reduce PMA to molybdenum blue. The color intensifies with increasing number of double bonds in the molecule being stained.

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