Molybdenum heteropoly blues

Instead of being determined volumetrically, however, the ammonium or quinoline molybdophosphates are usually reduced to another molybdenum complex, molybdenum blue (heteropoly blue), which is then analysed spectrophotometrically (see Section II.B.2.C). 

SiMouO ]4-.134 The structure of the heteropoly blue species (four-electron-reduced PM012O40]3-) formed in the molybdenum blue determination of phosphorus has been reported135 and a review of molybdate heteropoly blues has appeared.130 A stability index of polyanion structures has been discussed.137 The presence of two PMo, anions in aqueous solutions of molybdate and phosphate has been demonstrated by 31P NMR.138 Solid state NMR (nonspinning and MAS techniques) has been used for characterization of heteropolyanions.139 Molybdenum-95 NMR spectra of some polymolybdates have been reported.140... 

The manganate ion is not reduced by bromide ion but is reduced slowly by iodide ion and quickly by vanadyl(IV) or hexacyano-ferrate(II) ions. When the latter two ions are used as reductants, especially with the potassium complex, green products are obtained rapidly and in high yield. The green species is unstable in solution and is apparently in equilibrium with the reactants. With potassium salts, the solubility of the product is low, and the reaction is driven to completion. Potentiometric titrations show that a one-electron reduction occurs to produce the green species, which has been characterized by analysis and optical and e.s.r. spectroscopy. It is a mixed-valence species similar to the heteropoly blues of molybdenum and tungsten. E.s.r. spectra suggest that the extra electron is fairly well trapped on a specific vanadium atom, and the complex is therefore a class II mixed-valence species.8... 

Folin-Denis method Reduction of complex polymeric ions formed from phosphomolybdic and phospholungslic heteropoly acids to complex molybdenum-tungsten blue. detection wavelength 725 - 770 nm recommended for uniformity 765 nm complexes and reagent are unstable in alkaline solution, formation of precipitates, controlled sequence and timing of the addition of reagents (reproducibility ), deviation from Beer-Lambert law (high phenol contents), reaction is stoichiometrically predictable 105,106,110... 

P-P-acid reacts with the molybdenum(V)-molybdenum(VI) reagent under the conditions of Procedures I and II to form hypophosphoric heteropoly blue, the absorption spectrum of which has two maximums at... 

So, H. and Pope, M. (1972). Origin of Some Charge-Transfer Spectra. Oxo Compounds of Vanadium, Molybdenum, Tungsten, and Niobium Including Heteropoly Anions and Heteropoly Blues,/no/j. Chem., 11, pp. 1441—1443. 

A. Molybdenum blue method Discussion. Orthophosphate and molybdate ions condense in acidic solution to give molybdophosphoric acid (phosphomolybdic acid), which upon selective reduction (say, with hydrazinium sulphate) produces a blue colour, due to molybdenum blue of uncertain composition. The intensity of the blue colour is proportional to the amount of phosphate initially incorporated in the heteropoly acid. If the acidity at the time of reduction is 0.5M in sulphuric acid and hydrazinium sulphate is the reductant, the resulting blue complex exhibits maximum absorption at 820-830 nm. 

Under acid conditions, molybdate reacts with orthophosphate, P04 to form a blue heteropoly acid, molybdophosphoric acid. A similar reaction occurs with arsenate ion, As04. In the presence of vanadium, the product is yellow vanadomolybdophosphoric acid. These reactions are used for colorimetric analyses of phosphate, arsenate, and many other substances. Colloidal molybdenum blue has limited apphcations such as dyeing silk. It readily absorbs onto surface-active materials. 

Ingle and Crouch described a diflerential kinetic method for silicate and phosphate based on the faster rate of formation of heteropoly molybdenum blue from the yellow heteropoly acids in the presence of phosphate than in the presence of silicate. They found that silicon in the range of 1 to 10 ppm could be determined with 3% accuracy in the presence of 10 ppm of phosphorus, and phosphorus in the range of 1 to 10 ppm with 1% accuracy in the presence of 50 ppm of silicon. This system was also automated, with the analyses of mixtures being performed in less than 5 min. 

Heteropoly acids [oxygen compounds of Mo(VI), W(VI), Si, P(V), As(V), Ge, and other elements] and their reduction products (molybdenum blues) are extracted into oxygen-containing solvents by a mechanism similar to that above. 

Germanium(rV) forms heteropoly acids with molybdate and other ions. The method for determining germanium, based on yellow germanomolybdic acid [35-37] is insensitive (e = 2.0-10 at 430 nm), but reduction of the heteropoly acid to germano-molybdenum blue [38,39] considerably increases the sensitivity (e = 1.0-10 at 800 nm). 

Ions besides (P +) which may act as the central coordinating atom to form 12-fold heteropoly acids with molybdate include arsenic (As +), silicon (Si +), germanium (Ge +) and under some conditions molybdenum (Mo +) and boron (B +). Tungstate can also be coordinated about P as central atoms but with less avidity. The heteropolycomplexes, before reduction give a yellow hue to their water solution. With high P concentrations, a yellow precipitate is formed. In solution of low enough concentration to be suitable for determination by reduction to form the blue colour, the yellow colour is so faint that, it is not noticed and spectrophotometric measurements is done without any problem. The molybdenum blue colour is produced when either molybdate or its heteropolycomplexes are partially reduced. Some of the molybdenum ions are reduced from h-6 to a low valence state, probably h-3 and/or h-5, involving unpaired electrons due to which spectrophotometric resonance (blue colouration) would be expected. 

The inorganic phosphorus in a protein-free filtrate is reacted with ammonium molybdate [Mo(VI)] to form ammonium phosphomolybdate. This is reduced with a mild reducing agent to produce molybdenum blue, a heteropoly molybdenum(V) species. Molybdates are not reduced under these conditions. The blue color of the solution is measured spectrophotometrically. 

The heteropoly- and isopolyoxometalates of early transition metals have been known for more than a century and have been studied extensively. Their structures are characterized by networks of MOe octahedra in which the early transition metals M (typically M = V, Nb, Mo, W) appear in their highest oxidation states in which they have a a configuration. A characteristic of many, but not all, of such structures is their reducibility to highly colored mixed oxidation state derivatives often given the trivial names of molybdenum or tungsten blues. The reducibility of early transition metal polyoxometalates requires the presence of M06 octahedra in which only one of the six oxygen atoms is a terminal oxygen atom. Such an MO ... 

FIGURE 14.1 Absorption spectra of heteropoly molybdenum blues. 

The molybdenum content of the heteropoly acid formed can be reduced to molybdenum blue without reaction of the xmused iso-polyacid present. The colorimetric determination of soluble silicic acid depends on this principle. Polysilicic acids react more slowly in this analjrtical procedure because monosilicic acid must first be reformed from them [209, 746]. 

Silicate, arsenate, and germanate also form heteropoly acids, which on reduction yield molybdenum blue species with similar absorption maxima [97]. This positive interference in the determination of phosphate is particularly pronounced for silicate because of its relatively high concentration in many waters. However, the formation of silicomolyb-date may be suppressed by the addition of tartaric or oxalic acid to the molybdate reagent [98]. If, however, the organic acid is added after the formation of the heteropoly acid, the phosphomolybdate is destroyed, and this is used as the basis for determination of silicate in the presence of phosphate. Kinetic discrimination between phosphate and silicate, arsenate and germanate is also possible because of the faster rate of formation of phosphomolybdate. Thus, the widely adopted Murphy and Riley method employs a reagent mixture of acidic molybdate and antimonyl tartrate [83] at concentrations which are known to enhance the kinetics of phosphomolybdate and suppress the formation of silicomolybdate. 

Silicic acid may be detected by a non-specific method beginning by forming a stable heteropoly acid, yellow H4SiMoi204o The unknown is treated with a neutral solution of ammonium molybdate in a test tube, and the mixture is slightly acidified, forming the complex then a few drops of SnCb are added, producing a deep-blue Molybdenum Blue if more than 0.01-mM silicic acid is present. 

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