Phosphomolybdate heteropoly acid

All methods for the determination of inorganic phosphate in seawater are based on the reaction of the ions with an acidified molybdate reagent to yield a phosphomolybdate heteropoly acid, which is then reduced to a highly coloured blue compound. In early work, tin(//) chloride was used as the reductant in flow-analysis (Hager et al, 1968). However, this reductant has several disadvantages, including the appreciable temperature dependence of the reduction rate and the pronounced salt error. 

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. 

Chloramine-T also functions as a nitrene source in the presence of heteropoly acids (HPAs) such as phosphomolybdic and phosphotungstic acids. The aziridination of alkenes by treatment with the combination of HPA and chloramine-T is... 

A standard Lowry-based protein assay has been adjusted to the special conditions encountered with skin [126], Basically, proteins reduce an alkaline solution of Cu(II)-tartrate to Cu(I) in a concentration-dependent manner. Then, the formation of a blue complex between Folin-Ciocalteau reagent (a solution of complex polymeric ions formed from phosphomolybdic and phosphotungstic heteropoly acids) and Cu(I) can be measured spectrophotometrically at 750 nm. A calibration curve can be obtained by dissolving known amounts of stratum corneum in 1 M sodium hydroxide. A piece of tape that has not been in contact with skin is subjected to an identical procedure and serves as negative control. The method was recently adapted to a 96-well plate format, notably reducing analysis times [129],... 

Chromatographic method The determination of silicate based upon the adsorption of preconcentrated phosphomolybdic and silicomo-lybdic heteropoly acid (HPA) in the dynamic mode on a microcolumn packed with an Amberlite XAD-8 polyacrylate adsorbent is reported [49]. The method is based on the adsorption of HPAs followed by desorption with acetonitrile and determination by reversed-phase HPLC. Another highly sensitive HPLC method for the simultaneous determination of soluble silicate and phosphate in environmental waters is used in ion-pair liquid... 

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... 

Heteropoly acids, a class of coordination complexes in which the central ion is complex rather than monatomic, as with phosphomolybdic acid, H3PO3I2M0O3. 

Since the heteropoly-acids crystallize in the same system, it would be expected that dried phosphomolybdic acid would also form body-centered cubes. Examination of a sample of phosphodecimolybdic acid verified this assumption, the unit cell in this case being 14.31 A.U. on each side. The same technique for obtaining the large spacings as mentioned under silicotungstic acids was employed with similar success in this case. 

Electrolyte is formed from a poly (isopoly or heteropoly) acid. Specific examples are phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) for which conductivities of 17 and 18 mS/ cm, respectively, were reported. 

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. 

Heteropoly acids (HPAs) such as 12-phosphomolybdic acid (PMA) and 12-phosphotungstic acid (PWA) function as a catalyst in the aziridination of alkenes with chloramine-T (Scheme 2.37) [56]. PMA catalyzed the reaction most efficiently in the presence of a quaternary ammonium cocatalyst and molecular sieves, affording aziridine products in good yields. The low stereospecificity of the reaction indicates the stepwise mechanism. The authors proposed the involvement of metallo-oxaziridine 22 as an intermediate. 

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