Molybdenum blue
Encyclopedia
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.
, (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.
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:
, to produce PMo12O403− , 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, PMo12O407− . 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:
The comparison of the measured absorption against readings taken for analyses of standard solutions means that a detailed understanding of the structure of the blue complex was unnecessary.
and compared to standards prepared from reacting sugar solutions of known concentration, to determine the amount of reducing-sugar present.
The Folin-Wu method uses a reagent that contains sodium tungstate
. The exact nature of the blue complex in this procedure is not known.
The Somogyi-Nelson method uses an arsenomolybdate complex formed by the reaction of ammonium molybdate
, (NH4)6 Mo7O24, with sodium arsenate, Na2HAsO7.
group react with phosphomolbydic acid to give the heteropoly-molybdenum blue colour . Micro quantities of the drugs can be determined.
or by detection of the heteroatom
Dittmer's spray reagent for phospholipids is used in thin layer chromatography
to detect phospholipids. The spray reagent is prepared as follows:
When applied to the TLC plate, compounds containing phosphate ester show up immediately as blue spots.
− . Dissolving molybdate salts in strong acid produces "molybdic acid", MoO3.2H2O. In between these extremes of pH polymeric ions are produced which are mostly built from MoO6 octahedral units sharing corners and edges. Examples include Mo7O246− , Mo8O264− and Mo36O112(H2O)168− which contain the {(Mo)Mo5}-type unit comprising a central MoO7 pentagonal bipyramid
sharing edges with 5 MoO6 octahedra. The later unit occurs also in the giant mixed-valence molybdenum blue species [HxMo368 O1032 (H2O)240 (SO4)48]48− (x≈16) as well as in the cluster described in the next section. The molybdenum blue species are obtained by reduction of acidifed molybdate(VI) solutions.
− . The Mo154 type cluster was then shown to be the basic structural type of molybdenum blue compounds obtained under slightly different conditions.
The structure of the big wheel is constructed from units containing 11 Mo atoms ({Mo11}-type units), 14 of which are linked together to form the {Mo154}-type cluster that has an external diameter of 3.4 nm. (12 {Mo11}-type units are also involved in the construction of higher symmetrical spherical systems called Keplerates) These units consist of a central MoO7 bipyramid sharing edges with 5 MoO6 octahedra (an illustration of this is on page 155 of the review ). With 5 more linking MoO6 octahedra the repeating {Mo11}-type unit is built up.
- reduced heteropolymolybdate complexes, polyoxometalatePolyoxometalateIn chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form a large, closed 3-dimensional framework....
s containing Mo(V), Mo(VI), and a hetero atom such as phosphorus or silicon - reduced isopolymolybdate complexes, polyoxometalatePolyoxometalateIn chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form a large, closed 3-dimensional framework....
s containing Mo(V), Mo(VI) formed when solutions of Mo(VI) are reduced - a blue pigment containing molybdenum(VI) oxideMolybdenum(VI) oxideMolybdenum trioxide is chemical compound with the formula MoO3. This compound is produced on the largest scale of any molybdenum compound. It occurs as the rare mineral molybdite...
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.
Heteropoly-molybdenum blues
The first heteropoly molybdate and first heteropolymetallate, yellow ammonium phosphomolybdateAmmonium phosphomolybdate
Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic acid with the chemical formula 3PMo12O40. It contains the remarkable phosphomolybdate ion complex.-Synthesis:...
, (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
Keggin structure
Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α–Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom , M is the addenda atom , and O represents oxygen...
. The blue colour arises because the near-colourless anion, such as the phosphomolybdate anion, PMo12O403
- PMoVI12O403
− + 4e PMoV4MoVI8O407−
The structure of the anion, PMoV4MoVI8O407
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 colorimetricallyColorimeterFor articles on Colorimeter see:* Colorimeter * Tristimulus colorimeter...
- 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 sugarReducing sugarA reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...
such as glucose - measure the amount of a reducing compound in a two step procedure
- detect the presence of a reducing compound e.g. a reducing sugar
Colorimetric determination of P, As, Si and Ge
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 MoVI, for example ammonium molybdateAmmonium molybdate
Ammonium heptamolybdate is an odourless crystalline compound ranging in colour from white to yellow-green. It is usually encountered as the tetrahydrate, whose chemical formula is 6Mo7O24·4H2O...
, to produce PMo12O403
Keggin structure
Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α–Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom , M is the addenda atom , and O represents oxygen...
. This anion is then reduced by, for example, ascorbic acid
Ascorbic acid
Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form of vitamin C. The name is derived from a- and scorbutus , the...
or SnCl2
Tin(II) chloride
Tin chloride is a white crystalline solid with the formula 2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent , and in electrolytic baths for tin-plating...
, to form the blue coloured β-keggin ion, PMo12O407
Colorimeter
For articles on Colorimeter see:* Colorimeter * Tristimulus colorimeter...
to determine the amount of phosphorus. Examples of procedures are:
- the analysis of phosphate in sea water.
- standard methods for determining phosphorus and silicon content of metals and metal ores. (e.g. BSI and ISO standards)
- the determination of germanium and arsenic
The comparison of the measured absorption against readings taken for analyses of standard solutions means that a detailed understanding of the structure of the blue complex was unnecessary.
Colorimetric determination of glucose
The Folin-Wu and the Somogyi-Nelson methods are both based on the same principles. In the first step glucose (or a reducing sugar) is oxidised using a solution of Cu(II) ion which in the process is reduced to Cu(I). In the second step the Cu(I) ions are then oxidised back to Cu(II) using a colourless hetero-polymolybdate complex, which is, in the process, reduced to give the characteristic blue colour. Finally the absorption of the hetero-poly molybdenum blue is measured using a colorimeterColorimeter
For articles on Colorimeter see:* Colorimeter * Tristimulus colorimeter...
and compared to standards prepared from reacting sugar solutions of known concentration, to determine the amount of reducing-sugar present.
The Folin-Wu method uses a reagent that contains sodium tungstate
Sodium tungstate
Sodium tungstate, Na2WO4, a tungstate of sodium, is useful as a source of tungsten. It is prepared from tungsten ores used to manufacture tungsten by reducing it....
. The exact nature of the blue complex in this procedure is not known.
The Somogyi-Nelson method uses an arsenomolybdate complex formed by the reaction of ammonium molybdate
Ammonium molybdate
Ammonium heptamolybdate is an odourless crystalline compound ranging in colour from white to yellow-green. It is usually encountered as the tetrahydrate, whose chemical formula is 6Mo7O24·4H2O...
, (NH4)6 Mo7O24, with sodium arsenate, Na2HAsO7.
Colorimetric determination of some drugs containing o-hydroquinone
Some drugs that contain an o-hydroquinoneHydroquinone
Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, having the chemical formula C6H42. Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid...
group react with phosphomolbydic acid to give the heteropoly-molybdenum blue colour . Micro quantities of the drugs can be determined.
Uses in qualitative analysis
Examples of simple tests are shown below that rely on the production of the molybdenum blue colour either due to reduction:- tests for Sn(II) Sb(III)
- tests for organic reducing agents
or by detection of the heteroatom
- silicate
- phosphate
Dittmer's spray reagent for phospholipids is used in thin layer chromatography
Thin layer chromatography
Thin layer chromatography is a chromatography technique used to separate mixtures. Thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose...
to detect phospholipids. The spray reagent is prepared as follows:
- Molybdenum(VI) oxideMolybdenum(VI) oxideMolybdenum trioxide is chemical compound with the formula MoO3. This compound is produced on the largest scale of any molybdenum compound. It occurs as the rare mineral molybdite...
, MoO3, is dissolved in sulfuric acidSulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates... - A second solution is made up from molybdenum metal dissolved in some of the first solution.
- The spray is made up of a diluted mixture of the first and second solutions.
When applied to the TLC plate, compounds containing phosphate ester show up immediately as blue spots.
Isopoly molybdenum blues
The isopoly-molybdenum blues have been known for many years. They are the cause of the “blue waters” found near Idaho Springs, known to Native Americans. They were first documented by Scheele and Berzelius. The compounds responsible for the blue colour were not known until 1995. Before then it was well known that there were polymolybdates of Mo(VI). Molybdenum(VI)oxide, MoO3, when dissolved in aqueous alkali forms the tetrahedral molybdate anion, MoO42Pentagonal bipyramid molecular geometry
In chemistry, a pentagonal bipyramid is a molecular geometry with one atom at the centre with seven ligands at the corners of a pentagonal dipyramid. A perfect pentagonal bipyramid belongs to the molecular point group D5h ....
sharing edges with 5 MoO6 octahedra. The later unit occurs also in the giant mixed-valence molybdenum blue species [HxMo368 O1032 (H2O)240 (SO4)48]48
The big wheel
The first publication of the structure of wheel shaped cluster anion, first determined for the nitrosyl derivative by Achim Müller et al. was announced in New Scientist as "Big Wheel rolls back the molecular frontier". Further work by the same group then refined the initial findings and determined the structure of the wheel produced in molybdate solutions as [Mo154O462H14(H2O)70]14The structure of the big wheel is constructed from units containing 11 Mo atoms ({Mo11}-type units), 14 of which are linked together to form the {Mo154}-type cluster that has an external diameter of 3.4 nm. (12 {Mo11}-type units are also involved in the construction of higher symmetrical spherical systems called Keplerates) These units consist of a central MoO7 bipyramid sharing edges with 5 MoO6 octahedra (an illustration of this is on page 155 of the review ). With 5 more linking MoO6 octahedra the repeating {Mo11}-type unit is built up.