Molybdenum blue

Molybdenum blue is a term applied to:

The "heteropoly-molybdenum blues", are used extensively in analytical chemistry and as catalysts. The formation of "isopoly-molybdenum blues" which are intense blue has been used as a sensitive test for reducing reagents. They have recently been shown to contain very large anionic species based on the so-called "big wheel" containing 154 Mo atoms, with a formula [Mo₁₅₄O₄₆₂H₁₄(H₂O)₇₀]¹⁴⁻.

The molybdenum blue pigment is historically documented but may not be in use today.

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 heteroatom, other heteroatoms 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, PMo
12O3−
40, 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.

The structure of the anion, PMoV
4MoVI
8O7−
40, 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°). Similar structures have been found with silicon, germanium or arsenic heteroatoms.

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 determination of phosphorus, arsenic, silicon and germanium are examples of the use of heteropoly-molybdenum blue in analytical chemistry. The following example describes the determination of phosphorus. A sample containing the phosphate is mixed with an acid solution of Mo^VI, for example ammonium molybdate, to produce PMo
12O3−
40, which has an α-Keggin structure. This anion is then reduced by, for example, ascorbic acid or SnCl₂, to form the blue coloured β-keggin ion, PMo
12O7−
40. The amount of the blue coloured ion produced is proportional to the amount of phosphate present and the absorption can be measured using a colorimeter to determine the amount of phosphorus. Examples of procedures are:

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