Molybdates salts

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

Among molybdate salts, sodium and ammonium molybdates have commercial applications. The normal salt, sodium orthomolyhdate, Na2 M0O4, is used in pigments. It also is used as a corrosion inhibitor and as an additive to soil. Lead molybdate, Pb M0O4, occurs in nature as mineral ulfenite, from which molybdenum metal is recovered. 

Alternatively, the oxide may be prepared by heating a molybdate salt, such as ammonium molybdate, with a reducing agent such as zinc. The dioxide also may be obtained along with other oxides of molybdenum when molybdenum metal is heated in air. 

Molybdenum(VI) oxide is used in catalyst compositions to carry out desulfurization of petroleum feedstocks and to remove nitrogen-containing compounds from petroleum fractions. Other uses of this oxide include preparation of various molybdate salts and as reagents for chemical analyses. 

The density of several heteropoly molybdate salts determined pycnometricallys) is given in Table 9. 

Molybdenum compounds (molybdenum oxide/ molybdate salts)... 

Atomic Weight.—For reasons similar to those applying in the case of chromium (p. 16), the atomic %veight of molybdenum is three times the equivalent of the metal in the molybdic salts, or six times that in the molybdates. Molybdenum may be di-, tri-, or hexa-valent. 

The U-Mo-0 binaiy oxide system was first studied by Kovba and Trunov [14,15]. These authors reported existence of several U-Mo oxocompounds with different stoichiometries U2M0O8, UM0O5, UM02O8, UM07O22, and UM011O35. Later stractural studies demonstrated that, in their structures, either U or Mo coordination polyhedra polymerize to form extended stractural units and these compounds should be considered as complex oxides rather than uranium molybdate salts. Similar stractural features have also been observed for complex oxides in the U-W-0 and U-Mo-W-0 systems [16-21]. 

Different procedures may be used to prepare such samples as impregnation of silica with a molybdate salt or grafting molybdenum chloride or molybdenum based organo metallic compounds as Mo carbonyls on the hydroxyl groups of silica or at last solid-solid reaction between M0O3 and Si02 at temperatures near or above 500°C. 

Borates, molybdates, selenates, and arsenates occur in such trace quantities in soils that they probably exist only as impurities in major soil particles and on particle surfaces rather than as separate minerals. Soils in humid regions sometimes benefit from borate and, less often, molybdate additions. The range between deficient and excess is narrow, so spreading a few kg ha-1 (a few Ibs/acre) uniformly over the soil is difficult unless the borate and molybdate salts are mixed in with other fertilizers or inert materials. 

These results are unexpected, since it was previously found that molybdate salts were released when submitted to a usual electrochemical reduction and, moreover, their inhibitor properties towards iron were confirmed. XPS analysis of the defect after delamination showed that, in contrast to PPy-PMo, no molybdate could be detected with the PPy-Mo film, confirming that under these conditions no molybdate is released with this doping salt and thus self-heafing was absent. 

Mild steel (ferritic) OH NO 3, CNNH HjO, moist CO / COj - gas CO 3 /HC03, molybdates, salts from acetic acid, phosphates, saturated H2O vapour, acid SO 4 , SO4" + H2S. ... 

Flame retardants and smoke suppressants Sb Oj, M0O3, molybdate salts, zinc borate Chlorinated paraffins, brominated organic compormds Organophosphate esters A1(0H)3, Mg(OH) ... 

Broad Studies of Flame Retardants. Several broad studies of health, safety, and environmental factors of flame retardants have been published by public agencies. A critical review by a US government-appointed toxicology panel was conducted to facilitate CPSC regulations on flammability of furniture upholstery (145). The panel found ammonium polyphosphate, alumina trihydrate, zinc borate, hexabromocyclododecane, decabromodiphenyl ether (oxide), PYKOVATEX CP, and THPC to be usable with minimum risk on residential furniture even with worst-case assumptions. Antimony trioxide, several organophosphates, chlorinated paraffins, and molybdate salts were said to need more exposure studies. 

Numerous studies have attempted to elucidate the role of Mo in the passivity of stainless steel. It has been proposed from XPS studies that Mo forms a solid solution with CrOOH with the result tiiat Mo is inhibited from dissolving trans-passively [9]. Others have proposed that active sites are rapidly covered with molybdenum oxyhydroxide or molybdate salts, thereby inhibiting localized corrosion [10]. Yet another study proposed that molybdate is formed by oxidation of an Mo dissolution product [11]. The oxyanion is then precipitated preferentially at active sites, where repassivation follows. It has also been proposed that in an oxide lattice dominated by three-valent species of Cr and Fe, ferrous ions will be accompanied by point defects. These defects are conjectured to be canceled by the presence of four- and six-valent Mo species [1]. Hence, the more defect-free film will be less able to be penetrated by aggressive anions. A theoretical study proposed a solute vacancy interaction model in which Mo " is assumed to interact electrostatically with oppositely charged cation vacancies [ 12]. As a consequence, the cation vacancy flux is gradually reduced in the passive film from the solution side to the metal-film interface, thus hindering vacancy condensation at the metal-oxide interface, which the authors postulate acts as a precursor for localized film breakdown [12]. 

Evidence in support of several of these models has been reported. XPS studies of the passive and transpassive films formed on Mo in deaerated 0.1 M HCl [3] established that molybdate was absent from both surface films. In a later study the same authors used a twin potentiostat arrangement, with a second working electrode of either Fe, Cr, or Ni that was polarized near an Mo electrode at the same potential (-180mV vs. SCE) [18]. At this potential Mo and Cr are passive, while Ni and Fe are active. In this work it was shown that for the Fe-Mo and Ni-Mo electrode couples, iron or nickel molybdate was observed on the passive Mo surface. In the case of the Cr-Mo couple, molybdate was observed only on the passive film of Cr. This work was also able to show that transpassivity of Mo at 250 mV (SCE) was suppressed in the presence of Fe, which formed a molybdate salt on the surface of Mo. This indicated evidence of a possible mechanism by which Mo can remain passive in stainless steels at higher potentials than the transpassive potential of Mo. In addition, this work supported the idea that soluble molybdate anions can redeposit at active sites. 

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