Molybdenum blue

Molybdenum blue is a term applied to:

• reduced heteropolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI), and a hetero atom such as phosphorus or silicon
• reduced isopolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI) formed when solutions of Mo(VI) are reduced
• a blue pigment containing molybdenum(VI) oxide

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⁻. ^[1].
The molybdenum blue pigment is historically documented ^[2]but may not be in use today.

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Heteropoly-molybdenum blues

The first heteropoly molybdate and first heteropolymetallate, yellow ammonium phosphomolybdate, (NH₄)₃ PMo₁₂O₄₀ was discovered by Berzelius in 1826.^[3]. 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 e.g. 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^[3]. The reduction process is reversible and the structure of the anion is essentially unchanged.^[3]

PMo^VI₁₂O₄₀³⁻ + 4e ⇌ PMo^V₄Mo^VI₈O₄₀ ⁷⁻

The structure of the anion, PMo^V₄Mo^VI₈O₄₀ ⁷⁻, has been determined in the solid state and is a β-isomer i.e. with one of the four groups of edgeshared octahedra on the α-Keggin ion rotated through 60 degrees^[4]. Similar structures have been found with silicon, germanium or arsenic hetero-atoms ^[3]
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
    

Uses in quantitative analysis

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 Mo^VI, for example ammonium molybdate, to produce PMo₁₂O₄₀ ³⁻, 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, PMo₁₂O₄₀ ^{7− [4]} 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 analysis of phosphate in sea water. ^[5]
• standard methods for determining phosphorus and silicon content of metals and metal ores. (e.g. BSI^[6] and ISO^[7][8] standards)
• the determination of germanium and arsenic ^[9]

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 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 ^[10] 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. ^{[11] [12] [13]}

Colorimetric determination of some drugs containing o-hydroquinone

Some drugs that contain an o-hydroquinone group react with phosphomolbydic acid to give the heteropoly-molybdenum blue colour ^[14]. Micro quantities of the drugs can be determined.

Uses in qualitative analysis

Examples of simple tests^[15] 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 to detect phospholipids. The spray reagent is prepared as follows:

• Molybdenum(VI) oxide, MoO₃, is dissolved in sulphuric acid
• 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^[16].

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. ^[1] The compounds responsible for the blue colour were not known until 1995^[17]. 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, MoO₄²⁻. 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 [H_xMo₃₆₈ O₁₀₃₂ (H₂O)₂₄₀ (SO₄)₄₈]⁴⁸⁻ (x≈16) ^[18] 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 big wheel

The first publication of the structure of wheel shaped cluster anion, first determined for the nitrosyl derivative by Muller et al.^[17] was announced in New Scientist as "Big Wheel rolls back the molecular frontier"^[19]. 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]¹⁴⁻.^[17] The Mo₁₅₄ type cluster was then shown to be the basic structural type of molybdenum blue compounds obtained under slightly different conditions.^[1]

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^[1]) 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 ^[20]). With 5 more linking MoO₆ octahedra the repeating {Mo₁₁}-type unit is built up.

The spherical vesicle

Along with other aggregates, a hollow, spherical structure self-assembles from approximately 1,165 Mo₁₅₄ wheels. This was termed a vesicle by analogy with lipid vesicles. Unlike lipid vesicles that are stabilised by hydrophobic interactions it is believed that the vesicle is stabilised by an interplay of van der Waals attraction, long-range electrostatic repulsion with further stabilization arising from hydrogen bonding involving water molecules encapsulated between the wheel-shaped clusters and in the vesicles' interior. The radius of the vesicle is 45 nm. ^[21]

Molybdenum blue pigment

A pigment termed molybdenum blue is recorded in 1844 as a mixture of molybdenum with "oxyde of tin or phosphate of lime".^[2] An alternative formulation involves "digesting" molybdenum sulfide with nitric acid to form molybdic acid, which is then mixed with tin filings and a little muriatic acid (HCl)^[2]. This is evaporated and heated with alumina. A 1955 paper states that molybdenum blue is unstable and is not used commercially as a pigment.^[22] The chemistry of these pigments has not been investigated.

References

1. ^ ^{a b c d} From Scheele and Berzelius to Müller: polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches P. Gouzerh, M. Che; L’Actualité Chimique, 2006, 298, 9
2. ^ ^{a b c} A Dictionary of Arts, Manufactures, and Mines: Containing a Clear Exposition of Their Principles Andrew Ure, Published 1844, D. Appleton & Co.
3. ^ ^{a b c d} Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4
4. ^ ^{a b} Structure of a heteropoly blue. The four electron reduced beta-12-molybdophosphate anion, JN Barrows, G. B. Jameson, M. T. Pope, J. Am. Chem. Soc., 1985, 107, 1771
5. ^ A single solution method for the determination of soluble phosphate in sea water, Murphy J. Riley J.P. J. Mar. Biol. Assoc. UK, 1958, 37 ,9-14
6. ^ BS1728-12:1961 ‘’’ Standard Methods for the analysis of aluminium and aluminium alloys. Determination of silicon (absorptiometric molybdenum blue method)’’’ Date of Publication 14/10/61 ISBN 058001569 6
7. ^ ISO 7834:1987 Title: Iron ores -- Determination of arsenic content -- Molybdenum blue spectrophotometric method
8. ^ ISO 8556:1986 Title: Aluminium ores -- Determination of phosphorus content -- Molybdenum blue spectrophotometric method
9. ^ Determination of Phosphorus, Germanium, Silicon, and Arsenic by the Heteropoly Blue Method D. F. Boltz, M.G.Mellon Analytical Chemistry, 19, (1947), 873 DOI: 10.1021/ac60011a019
10. ^ A system of blood analysis O. Folin, H. Wu, The Journal of Biological chemistry (1920), 41, 3, 367
11. ^ Food Analysis S Suzanne Nielson (2003) Springer ISBN 0306474956
12. ^ A new reagent for the determination of sugars M. Somogyi Journal of Biological Chemistry, (1945), 160, 61
13. ^ A photometric adaptation of the Somogyi method for the determination of glucose Nelson N Journal of Biological Chemistry (1944), 153, 375
14. ^ Spectrophotometric determination of microquantities of Carbidopa, Levodopa and alpha-methyldopa using molybdatophosphoric acid P. B. Issopoulos, Pharm. Acta Helv. 64, 82 (1989)
15. ^ Spot test analysis" Ervin Jungreis Encyclopedia of Analytical Chemistry John Wiley and sons(2000)
16. ^ A simple, specific spray for the detection of phospholipids on thin-layer chromatograms Dittmer, J. C., R. L. Lester. J. Lipid Res.5,(1964), 126- 127
17. ^ ^{a b c} [Mo₁₅₄(NO)₁₄O₄₂₀(OH)₂₈(H₂O)₇₀]^{(25±5) -}: A water-soluble big wheel with more than 700 atoms and a relative molecular mass of about 24000, A. Muller, E. Krickemeyer, J. Meyer, H. Bogge, F. Peters, W. Plass, E. Diemann, S. Dillinger, F. Nonnebruch, M. Randerath, C. Menke, Angew. Chem. Int. Ed. Engl., 1995, 34, 19, 2122. The first formula was published with an error limit for the negative charge, the final formula nowadays accepted is [Mo₁₅₄(NO)₁₄O₄₄₈H₁₄(H₂O)₇₀]^28- (see Soluble Molybdenum Blue-"des Pudels Kern", A. Müller, C. Serain, Acc. Chem. Res., 2000, 33, 2).
18. ^ Inorganic Chemistry Goes Protein Size: A Mo₃₆₈ Nano-Hedgehog Initiating Nanochemistry by Symmetry Breaking, A. Müller, E. Beckmann, H. Bögge, A. Dress, Angew. Chem. Int. Ed., 2002, 41, 1162
19. ^ Big Wheel rolls back the molecular frontier, D. Bradley , New Scientist, 1995,148,18
20. ^ En route from the mystery of molybdenum blue via related manipulatable building blocks to aspects of materials science, A. Müller, S. Roy, Coord. Chem. Rev. 2003, 245, 153
21. ^ Self-assembly in aqueous solution of wheel-shaped Mo₁₅₄ oxide clusters into vesicles, T. Liu, E. Diemann, H. Li, A. W. M. Dress, A. Müller, Nature, 2003, 426, 59
22. ^ Inorganic pigments W.G. Huckle, E.Lalor Industrial and Engineering chemistry (1955), 47, 8, 1501