Molybdic arsenate

Molybdenum Arsenates and Molybdo-arsenates.—Molybdous Arsenate, Mo(HAs04)2.wH20( ), is said1 to be formed as a grey precipitate when molybdous chloride is treated with sodium monohydrogen arsenate the precipitate first redissolves, hut afterwards becomes permanent. Molybdic arsenate, obtained in a similar manner from molybdic chloride, has been described by Berzelius, who also considered that an acid salt was produced on dissolving the hydrate of molybdenum dioxide in excess of arsenic acid, since the solution turned blue on standing.1... 

The activity of the apoderivative can be fully restored by addition of vanadate [45], Addition of other metal ions did not restore bromoperoxidase activity [1,2], Vanadate is only fully incorporated in the absence of phosphate [45], Like phosphate, molybdate, arsenate, tetrafluoroaluminate (A1F4 ), and tetrafluorober-rylate (BeF42 ) are also reported to be competitive inhibitors of bromoperoxidase reactivation by vanadate [47],... 

For the determination of phosphate in the presence of arsenic, the arsenate ion is reduced with thiosulphate to arsenite which does not form complexes with molybdate. Arsenate up to 3 timoVL is quantitatively reduced within 15 min. The blue phosphomolybdic complex is developed using the reagents given in Section 10.2.5.4. Calibration must be performed as described because thiosulphate reduces the colour of the blue phosphorus complex by ca. 10 %. Range and precision are the same as for the main procedure. 

Tartrates and similar hgands should be destroyed with concentrated HNO3 before adding the molybdate. Arsenates shonld not interfere if precipitation is effected in the cold. It is better, however, to remove any As with H2S because a temperature of 30-40 °C promotes precipitation of the (NH4)3[PMoi204o]. (If the system becomes too hot, M0O3 will precipitate.) Hahdes should be removed with Ag before testing for phosphate. Aqueons [FefChOe] " forms a red-brown precipitate with the acidified molybdate, hence may canse some uncertainty. 

Figure 36. ENDOR spectra of uteroferrin in D2O buffer and in complex with molybdate, arsenate, and phosphate (top panel showing broad sweep and bottom panel showing narrow sweep). Reprinted with permission from [560]. Copyright 2002, American Chemical Society.

It is usually preferable to oxidise the compound directly as follows. Intimately mix 0 02-0 05 g. of the eompound with 3 g. of sodium peroxide and 2 g. of anhydrous sodium carbonate in a niekel erucible. Heat the crueible and its eontents with a small flame, gently at first, afterwards more strongly until the eontents are fused, and eontinue heating for a further 10 minutes. Allow to stand, extract the contents of the crucible with water, and filter. Add exeess of eoneentrated nitrie acid to the filtrate and test with ammonium molybdate reagent as above. A yellow preeipitate indicates the presenee of phosphorus. It must be borne in mind that the above treatment 1 eonvert any arsenie present into arsenate. 

Paris green, see Copper acetate arsenate(III) (1/3) Pawellite, see Calcium molybdate(VI)(2—)... 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

In an alternative industrial process, resorcinol [108-46-3] is autoclaved with ammonia for 2—6 h at 200—230°C under a pressurized nitrogen atmosphere, 2.2—3.5 MPa (22—35 atm). Diammonium phosphate, ammonium molybdate, ammonium sulfite, or arsenic pentoxide maybe used as a catalyst to give yields of 60—94% with 85—90% selectivity for 3-aminophenol (67,68). A vapor-phase system operating at 320°C using a siUcon dioxide catalyst impregnated with gallium sesquioxide gives a 26—31% conversion of resorcinol with a 96—99% selectivity for 3-aminophenol (69). 

Phosphate. Phosphoms occurs in water primarily as a result of natural weathering, municipal sewage, and agricultural mnoff The most common form in water is the phosphate ion. A sample containing phosphate can react with ammonium molybdate to form molybdophosphoric acid (H2P(Mo202q)4). This compound is reduced with stannous chloride in sulfuric acid to form a colored molybdenum-blue complex, which can be measured colorimetrically. SiUca and arsenic are the chief interferences. 

Fluoride < chloride < bromide = iodide = acetate < molybdate < phosphate < arsenate < nitrate < tartrate < citrate < chromate < sulfate < hydroxide. 

AMMO 2.5 EC , cypermetlu-in, 13 Ammonia, 13 Ammonium acetate, 13 Ammonium arsenate, 13 Ammonium benzoate, 13 Ammonium bicarbonate, 13 Ammonium bifluoride, 14 Ammonium bisulfite, 14 Ammonium carbamate, 14 Ammonium carbonate, 14 Ammonium chloride, 14 Ammonium chlorplatmate, 14 Ammonium clu omate, 14 Ammonium citrate, 14 Ammonium diclu omate, 14 Ammonium fluoride, 14 Ammonium fomiate, 15 Ammonium hexafluorosilicate, 15 Ammonium hydroxide, 15 Ammonium metavanadate, 15 Ammonium molybdate, 15 Ammonium nitrate, 15 Ammonium oxalate, 15 Ammonium perfluorooctanoate, 15 Ammonium persulfate, 15 Ammonium phosphate, 15 Ammonium picrate, 16 Ammonium salicylate, 16... 

Other examples are the use of osmium(VIII) oxide (osmium tetroxide) as catalyst in the titration of solutions of arsenic(III) oxide with cerium(IV) sulphate solution, and the use of molybdate(VI) ions to catalyse the formation of iodine by the reaction of iodide ions with hydrogen peroxide. Certain reactions of various organic compounds are catalysed by several naturally occurring proteins known as enzymes. 

Arsenites may also be determined by this procedure but must first be oxidised by treatment with nitric acid. Small amounts of antimony and tin do not interfere, but chromates, phosphates, molybdates, tungstates, and vanadates, which precipitate as the silver salts, should be absent. An excessive amount of ammonium salts has a solvent action on the silver arsenate. 

Determination of phosphate as ammonium molybdophosphate. This may be readily effected by precipitation with excess of ammonium molybdate in warm nitric acid solution arsenic, vanadium, titanium, zirconium, silica and excessive amounts of ammonium salts interfere. The yellow precipitate obtained may be weighed as either ammonium molybdophosphate, (NH4)3[PMo12O40], after drying at 200-400 °C, or as P205,24Mo03, after heating at 800-825 °C for about 30 minutes. 

Molybdenum blue method. When arsenic, as arsenate, is treated with ammonium molybdate solution and the resulting heteropolymolybdoarsenate (arseno-molybdate) is reduced with hydrazinium sulphate or with tin(II) chloride, a blue soluble complex molybdenum blue is formed. The constitution is uncertain, but it is evident that the molybdenum is present in a lower oxidation state. The stable blue colour has a maximum absorption at about 840 nm and shows no appreciable change in 24 hours. Various techniques for carrying out the determination are available, but only one can be given here. Phosphate reacts in the same manner as arsenate (and with about the same sensitivity) and must be absent. 

Many heavy metals react with dithiol to give coloured precipitates, e.g. bismuth, iron(III), copper, nickel, cobalt, silver, mercury, lead, cadmium, arsenic, etc. molybdate and tungstate also react. Of the various interfering elements, only arsenic distils over with the tin when a mixture is distilled from a medium of concentrated sulphuric acid and concentrated hydrobromic acid in a current of carbon dioxide. If arsenic is present in quantities larger than that of the tin it should be removed. 

The effect of other inorganic anions (sulfate, molybdate, silicate), low molecular mass organic ligands (LMMOLs, such as oxalate, malate, citrate, tartrate and succinate), and fulvic or humic acid on the sorption of arsenate and arsenite onto variable charge minerals and soils has been studied (Roy et al. 1986 Grafe et al. 2001 Liu et al. 2001 Violante et al. 2005a,b). 

Sulfate poorly prevents arsenate sorption onto metal oxides and soils (Wu et al. 2001 Inskeep et al. 2002 Violante et al. 2005b). Violante et al. (2005b) found that high concentrations of sulfate (sulfate/arsenate molar ratio (rf) 4-10) retarded but not prevented arsenate sorption onto ferrihydrite (see their Fig. 15.10) or other metal oxides. Roy et al. (1986) showed that the sorption of arsenate by two soils (an Ultisol and a Typic Apludults) was reduced in the presence of molybdate. 

Roy WR, Hassett JJ, Griffin RA (1986) Competitive coefficient for the adsorption of arsenate, molybdate, and phosphate mixtures by soils. Soil Sci Soc Am J 50 1176-1182... 

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

Fujiwara et al. [94] found that, when present as a heteropolyacid complex, molybdenum(VI), germanium(IV), and silicon(IV) produced CL emission from the oxidation of luminol, and similar CL oxidation of luminol was observed for arsenic(V) and phosphorus(V) but with the addition of the metavanadate ion to the acid solution of molybdate. A hyphenated method was therefore proposed for the sensitive determination of arsenate, germanate, phosphate, and silicate, after separation by ion chromatography. The minimum detectable concentrations of arsenic(V), germanium(IV), phosphate, and silicon(IV) were 10, 50, 1, and 10... 

Dambies, L., Guibal, E., Rose, A., Arsenic (V) sorption on molybdate-impregnated chitosan beads, Colloids and Surfaces A, 170 (2000), 19-31. 

Complex D-mannitol borates are used in electrolytic condensers. Molybdic and arsenic acids also form association complexes with hexitols which increase the acidity of the inorganic acids. Rotational exaltation is obtained with the salts of these acids. 

Manning, B.A. Goldberg, S. (1996) Modeling competitive adsorption of arsenate with phosphate and molybdate on oxide minerals. Soil Sci. Soc. Am. J. 60 121-131 Manning, B.A. Fendorf S.E. Goldberg, S. (1998) Surface structures and stability of ar-senic(lll) on goethite spectroscopic evidence for inner-sphere complexes. Environ. Sci. Techn. 34 2383-2388... 

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. 

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