Molybdenum Blue - Heteropoly-molybdenum Blues

Heteropoly-molybdenum Blues

The first heteropoly molybdate and first heteropolymetallate, yellow ammonium phosphomolybdate, (NH₄)₃PMo₁₂O₄₀ was discovered by Berzelius in 1826. The phosphorus atom in the anion is termed the hetero-atom, other hetero-atoms are silicon and arsenic. The heteropoly-molybdenum blues have structures based on the Keggin structure. The blue colour arises because the near-colourless anion, such as the phosphomolybdate anion, PMo₁₂O₄₀3−, can accept more electrons (i.e. be reduced) to form an intensely-coloured mixed-valence complex. This can occur in one electron or two electron steps. The reduction process is reversible and the structure of the anion is essentially unchanged.

PMoVI₁₂O₄₀3− + 4e ⇌ PMoV₄MoVI₈O₄₀7−

The structure of the anion, PMoV₄MoVI₈O₄₀7−, has been determined in the solid state and is a β-isomer (i.e. with one of the four groups of edge-shared octahedra on the α-Keggin ion rotated through 60 degrees). Similar structures have been found with silicon, germanium or arsenic hetero-atoms.

The intense blue colour of the reduced anion is the basis for the use of heteropoly-molybdenum blues in quantitative and qualitative analytical techniques. This property is exploited as follows:

• the sample to be analysed is reacted to produce the reduced blue heteropoly-molybdate in order to:
  • detect the presence of a hetero atom in e.g. a spot test
  • measure the amount of a hetero atom present in the sample colorimetrically
• the sample is added to a solution of the near colourless, un-reduced complex in order to:
  • detect the presence of a reducing compound e.g. a reducing sugar such as glucose
  • measure the amount of a reducing compound in a two step procedure