Folin-Ciocalteau phenol reagent consists of a mixture of the heteropoly
acids, phosphomolybdic and phosphotungstic acids in
which the molybdenum and the tungsten are in the 6+ oxidation
state. On reaction with a reductant, the molybdenum blue and
the tungsten blue are formed and the mean oxidation state of the
metals is between 5 and 6.