Molybdoarsenates

Molybdenum trisulfide, 3, 1431 Molybdoarsenates, 3, 1041 non-Keggin, 3, 1042 Molybdoarsenic acid liquid-liquid extraction, 1, 548 Molybdoborates, 3,1042 Molybdocerophosphoric add determination, 1, 548 Molybdoenzymes, 3, 1352 molybdenum site, 3,1391 Molybdogermanates, 3,1038 Molybdogermanic acid liquid-liquid extraction, 1, 548 Molybdoniobic acid... 

Johnson and Pilson [229] have described a spectrophotometric molybdenum blue method for the determination of phosphate, arsenate, and arsenite in estuary water and sea water. A reducing reagent is used to lower the oxidation state of any arsenic present to +3, which eliminates any absorbance caused by molybdoarsenate, since arsenite will not form the molybdenum complex. This results in an absorbance value for phosphate only. 

Aqueous solutions of molybdate and arsenate, pH 3-5, contain [AsMo903i(H20)3]3 and an As2Mo6 anion.71 The exact structure of the latter ion in solution is in some doubt, although the tetramethylammonium sodium salt contains the anion shown in Figure 15. Low angle X-ray scattering measurements of aqueous molybdoarsenate solutions are not compatible with this structure and the anion may be hydrated in a fashion similar to that observed for the analogous... 

Molybdogermanic, molybdoniobic, molybdoarsenic and molybdosilicophosphoric acids and other heteropolyacids have been used in liquid-liquid extraction procedures which also permit the resolution of mixtures.68 A comprehensive review has been published.69... 

The 12-molybdoarsenates are even less stable than the 12-molybdophosphates. 

The free acid has not been isolated, and the complex is very easily degraded. The only salts known are the slightly soluble K3[AsMoi204o] 6 H20 and the difficulty soluble (NH4)3[AsMo1204o] 6 H20. In solution, most polymolybdoarsenates are readily converted into dimeric 9-molybdoarsenate anions. With arsenic, these compound types are the most stable and best known heteropolymolybdates and tungstates. It has been shown recently that the 12-molybdoarsenate(V) anion is stable in 1 1 water dioxane solutions15. ... 

The method described by Merz involves burning the sample in an oxygen-filled flask, and the combustion products are absorbed in dilute iodine solution in which As(III) is quantitatively oxidized to As(V). The arsenic is determined by the molybdoarsenate blue reaction, with hydrazine sulphate as reductant the excess of iodine from the absorber does not interfere. Concentrations of arsenic greater than 10/rg are measured on a filter... 

Belcher and collaborators described a method for arsenic in which the sample (30-60 mg) is burnt by a modified oxygen fiask procedure and arsenic is determined by the precipitation as quinoline molybdoarsenate, which is then reduced with hydrazine sulphate and determined spectrophotometrically at 840 nm. The absolute accuracy is within + 1% for arsenic. Phosphorus interferes in this procedure. 

As(V) by the addition of aqueous iodine, followed by conversion to molybdoarsenate and reduction to molybdenum blue by boiling under reflux with a sulphuric acid solution of ammonium vanadate and hydrazine sulphate. The method is sufficiently sensitive to determine down to 3 fig of Lewisite per millilitre of sample. 

In the separation of caesium, the following inorganic cation exchangers are particularly useful potassium cobalt(II) hexacyanoferrate(II) [12,13], potassium copper(II) and nickel(II) hexacyanoferrates(II) [14], ammonium molybdophosphate [13,15], zirconium molybdoarsenate [16], thallium tungstophosphate [17], and tin molybdosilicate [18]. Rubidium has been selectively isolated on columns filled with titanium tungstoarsenate [19] or titanium ferricyanide [20]. Ammonium molybdoarsenate has been used to separate potassium from sodium [21]. 

As(V) has been separated from a number of elements by extraction into butanol from acidic medium as the molybdoarsenic heteropoly acid [11]. This acid has been extracted with solutions of some high molecular-weight amines in 1,2-dichloroethane [12]. 

The arsenomolybdenum blue method is the most widely used. The method using the reaction of AsHs with Ag-DDTC is less sensitive. In certain cases, the classical Gutzeit method is convenient. Methods based on ion-associates of molybdoarsenate with basic dyes are very sensitive. 

Arsenic, oxidised to As(V) (e.g. by evaporation to dryness with HNO3) is made to react in a suitably acid solution (0.1-0.3 M H2SO4) with ammonium molybdate to form the molybdoarsenic heteropoly acid. The molybdoarsenic acid is then reduced [the Mo(Vl) atoms linked with As(V) are reduced to a lower oxidation degree] and a blue reaction product, the arsenomolybdenum blue is formed. The absorbance is measured either in the aqueous solution, or after extraction into oxygen-containing organic solvent [29-33]. The reaction conditions are so selected that the unreacted molybdate ions are not reduced. 

Alternatively, arsenic may be extracted as molybdoarsenic acid, and the reducing agent may then be added to the organic phase [25]. 

Sensitive methods for determining arsenic are based on ion-associates formed by molybdoarsenate and basic dyes. Flotation-spectrophotometric methods with the use of Crystal Violet (mixture of cyclohexane and toluene as flotation solvent, e=3.210 ) [51] and Malachite Green (flotation with diethyl ether, e=3.2-10 ) [52] have been proposed. The ion-associate with Ethyl Violet is extracted with a mixture of cyclohexane and 4-methylpentanone (1+3) (8=2.810 ) [53]. 

Methods based on extractable associates of the reduced form of molybdoarsenic acid with Crystal Violet (e=3.1 10 ). Methyl Violet, Brilliant Green, or Malachite Green have also been applied [54]. 

The amplification method for arsenic is based on its extractive separation as molybdoarsenic acid and determination of Mo with Sulphonitrophenol S (e = 4.5 10 ) or zinc dithiol [56]. The molybdoarsenic acid, after extraction into a mixture of butyl acetate and ethanol, is decomposed, then Mo(VI) is reduced to Mo(Ul) in a Jones reducer, and finally Mo is oxidised to Mo(Vl) be means of Fe(Itl). The Fe(ll) produced in this reaction gives a coloured complex with ferrozine. The amplification factor is 36, and e=9.4 10 [57]. 

The separation of P(V) from other elements, in particular from Si, is often achieved by extracting phosphorus as a heteropoly acid from a slightly acidic solution of pH -1.4. Higher alcohols, esters, and ethers are suitable extractants [3-5]. By extraction with a mixture of butanol and chloroform, molybdophosphoric acid can be separated from molybdoarsenic acid [6]. Isobutyl acetate extracts molybdophosphoric acid, but not molybdosilicic acid, from a solution at pH 0.3-1.0. 

Phosphorus(V), Ge(IV), and As(V) which give yellow heteropoly acids, interfere. Before silicon is determined, Ge and As may be separated by volatilization or extraction of GeCU and AsCli. Molybdophosphoric and molybdoarsenic acids are separated from molybdosilicic acid by extraction with butyl acetate at pH 0.3-1.0. The extractive separation of silicon from P(V) and As(V) has been discussed [5,21]. Molybdosilicic acid has been extracted with TOA in toluene [22]. Ion-associates with basic dyes, e.g., Chrompyrazole have been floated with a mixture of toluene and acetone [23]. 

Molybdcnum(V) complexes magnetic behavior, 271 spectra, 251 Molybdoarsenic acid liquid-liquid extraction, 548 Molybdocerophosphoric acid determination, 548 Molybdogermanic acid liquid-liquid extraction, 548 Molybdoniobic acid liquid-liquid extraction, 548 Molybdophosphates ammonium salt in gravimetry, 534 Molybdosilicophosphoric acid liquid-liquid extraction, 548 Mononuclear compounds neutral... 

Solvent extraction, chromatographic separation, distillation and precipitation of heavy metals as sulphides are among the techniques employed for this purpose. Isobutyl acetate, for example, extracts molybdophosphate but not molybdosilicate at pH = 0.3-1.0. Butanol and chloroform can be used to separate molybdophosphate from molybdoarsenate. Under the appropriate circumstances. Si, As and Ge can be distilled off from the sample as volatile halides, and B removed as McjB. 

Other more or less specific but less important reagents are for unsaturated compoimds molybdoarsenate [111], tetranitromethane [111, 113], osmium tetroxide [111, 113), p-anfinodimethylaniline-SOg [111, 113] for phenols, the nitroso reaction (Boute reagent) and Feigl reagent [113] for o-diphenols, phloroglucinol and vanadic acid [113] for reactions of the side chain, the Porter-Silber reaction [111], methyl ketones and formaldehydogens [111] and further modified reactions usual in PC [129]. 

An automated method reported for the determination of cellulase activity is based on stopping the enzyme reaction at alkaline pH, dialysis, and determination of the reducing power of the diffusate by the spectrophotometric ferricyanide-molybdoarsenic acid reaction. ... 

Ammonium molybdate, (NH4)2Mo04, when warmed to 60-70 °C with an arsenate (but not As ) in HNO3, precipitates yellow (NH4)3[AsMoi204o], arrunonium 12-molybdoarsenate. In appearance and properties it resembles the phosphate compound, except that the latter is precipitated in the cold. The limit of sensitivity is claimed to be one in 100 milhon. 

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