Heteropoly blue

Chemical Analysis. The presence of siUcones in a sample can be ascertained quaUtatively by burning a small amount of the sample on the tip of a spatula. SiUcones bum with a characteristic sparkly flame and emit a white sooty smoke on combustion. A white ashen residue is often deposited as well. If this residue dissolves and becomes volatile when heated with hydrofluoric acid, it is most likely a siUceous residue (437). Quantitative measurement of total sihcon in a sample is often accompHshed indirectly, by converting the species to siUca or siUcate, followed by deterrnination of the heteropoly blue sihcomolybdate, which absorbs at 800 nm, using atomic spectroscopy or uv spectroscopy (438—443). Pyrolysis gc followed by mass spectroscopic detection of the pyrolysate is a particularly sensitive tool for identifying siUcones (442,443). This technique rehes on the pyrolytic conversion of siUcones to cycHcs, predominantly to [541-05-9] which is readily detected and quantified (eq. 37). 

Silica. The siUca content of natural waters is usually 10 to (5 x lO " ) M. Its presence is considered undesirable for some industrial purposes because of the formation of siUca and siUcate scales. The heteropoly-blue method is used for the measurement of siUca. The sample reacts with ammonium molybdate at pH 1.2, and oxaUc acid is added to reduce any molybdophosphoric acid produced. The yellow molybdosiUcic acid is then reduced with l-amino-2-naphthol-4-sulfoiiic acid and sodium sulfite to heteropoly blue. Color, turbidity, sulfide, and large amounts of iron are possible interferences. A digestion step involving NaHCO can be used to convert any molybdate-unreactive siUca to the reactive form. SiUca can also be deterrnined by atomic... 

Spectrophotometric methods based on an enhancement of the blue color produced on reduction of 12-molybdophosphate (arsenate) in the presence of antimony(III) are widely used for the determination of phosphoms(V) or arsenic(V). However, nature of heteropoly blue, their spectra, mechanism of the reaction are obscure. In addition, mixed POMs were shown as very efficient analytical forms for the determination of P(V) and As(V). 

It is shown that both Sb(III) and Bi(III) can speed reduction of 12-molybdophosphate (12-MPC) to the corresponding heteropoly blue (12-MPB) by ascorbic acid (AA). It is found that mixed polyoxometalates can be formed in solution which reduce considerably more rapidly than 12-MPC. Complete formation of mixed POM is observed only if significant excess of Me(III) ions is used in the reaction. POM responsible for blue color was synthesized by selective extraction. Chemical analysis of tetrabutyl-ammonium salt is in accordance with formula of (TBAl PMeflllfMo O j (Me = Sb, Bi). IR spectmm of mixed POM is identical to 12-MPC. 

Mild and reversible reduction of 1 12 and 2 18 heteropoly-molybdates and -tungstates produces characteristic and very intense blue colours ( heteropoly blues ) which find application in the quantitative determinations of Si, Ge, P and As, and commercially as dyes and pigments. The reductions are most commonly of 2 electron equivalents but may be of 1 and up to 6 electron equivalents. Many of the reduced anions can be isolated as solid salts in which the unreduced structure remains essentially unchanged and... 

Heterogenous reactions, Sh/Nu ratio, 27 64 Heteroligand complex, 32 260-262 Heteropolyacids defined, 41 117 heteroatoms, 41 118, 120, 121 Prins reaction, 41 156 supported, 41 149-150 Heteropolyanions, 41 113, 117, 119-121 Heteropoly blues, 41 191 Heteropoly compounds absorption, 41 179-180, 190-191 acid-catalyzed reactions heterogeneous, 41 161-178 liquid phase, 41 150-161 acidic properties in solid state, 41 141-150 in solution, 41 139—14] catalysis, 41 114, 116-117, 190-191 as catalyst, 41 113-116, 117, 223-232... 

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). 

The formation of intensely colored species ( heteropoly blues ) upon partial reduction of certain heteropolyanions forms the basis of widely used analytical procedures for the heteroelements in such anions (P, Si, etc.), or for the reductant (e.g. uric acid). Photochemical... 

Table 11 Intervalence Parameters for some Heteropoly Blues...

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... 

It can be only mentioned here that isopoly and betercpoly anions also give rise to highly colored mixed oxidation state species the tungsten bronzes64 and the heteropoly blues.63... 

An additional color development step may often be required to confirm the yellow color of silicomolybdic acid. The latter is reduced to a dark blue substance by treating with aminonaphthalo sulfonic acid. The color of heteropoly blue formed is more intense than the yellow color of silicomolybdic acid. The latter test is more sensitive and can give a detection limit of 50 pg silica/L when using a spectrophotometer. 

Traces of phosphorus, silicon, and arsenic can be determined colorimetrically by complexing with molybdate and reducing the heteropoly anion to a very deep blue color. Absorption spectra in solution of heteropoly blues show charge transfer bands in the visible region with molar extinction coefficients of the order 104 to 10s 123 ... 

Liu J., Zhang M.-L., Wang L.-J. and Wang E.-B., Synthesis, properties and antiviral of Keggin molybdogemnium heteropoly blues, Chinese J. Inorg. Chem., 21 (2000) pp. 1791 (in Chinese). 

Mo-heteropolyanions with a Keggin structure are readily reduced to give heteropoly blue in solution. A trend exists toward anions with X of higher charge (small value of n) being more easily reduced, while the reduction potential decreases in the order V > Mo > W. In the case of XMnWn04oH2", the Mn + in the anion is oxidized by O2. 

Heteropoly blue, heteropoly brown mixed-valence polyoxoanion formed by partial rednction of the addenda Isopolyanion polyoxoanion with one kind of positive-valent element... 

Chloride does not interfere in the procedure. Phosphorus interferes in the micro method by formation of a heteropoly blue similar to that formed by pentavalent arsenic. 

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