Hydrated molybdic acid

Normal isopoly- and peroxymolydates of ammonium and several metals are known. The normal or orthomolybdates may be considered as salts of molybdic acid having formulas H2Mo04 xH20 or M20 Mo03 xH20. They are either of monoclinic or scheelite type crystal structure and obtained as hydrated salts. 

When selenic acid and molybdic acid are heated together on a water-bath for several days a compound of composition Mo03.Se03 is formed, which is obtainable as a crystalline mass,4 and which with a little water yields a hydrate, having the formula Mo03.Se03.2H20 when dried at 110° C. This compound has the properties of a tetrabasic acid. 

Molybdenum trioxide is a white solid at room temperature but becomes yellow when hot and melts at 795°C to a deep yellow liquid. It is the anhydride of molybdic acid, but it does not form hydrates directly, although these are known (see later). One of its two polymorphs, the stable a-form, has a rare type of layer structure in which each molybdenum atom is surrounded by a distorted octahedron of oxygen atoms. 

It is considered that the transformation of the various hydrates of molybdic acid into one another involves the formation of intermediate polymerides. 

To summarize the mechanistic and structural studies with hex-2-uloses and 2-C-(hydroxymethyl)pentoses performed under the conditions of the Bflik reaction, the following conclusion can be made. Molybdic acid catalyzes two types of interconversions between the sugars shown in Scheme 13. D-Fructose (17), d-sorbose (21) and D-tagatose (29) when treated with the catalyst are subjected to highly stereospecific carbon-skeleton rearrangements to produce thermodynamic equilibrium mixtures with the respective 2-C-(hydroxymethyl)-D-ribose (D-hamamelose, 20), 2-C-(hydroxymethyl)-D-lyxose (30), and 2-C-(hydroxymethyl)-D-xylose (31). (For simplicity, all the sugars are represented in their acyclic non-hydrated forms in spite of which some of their interconversions proceed in the acyclic hydrated structures, while others proceed in cyclic hemiacetal ones. All of the interconversion relationships are schematically represented in the d series, despite the fact that some of them were experimentally performed with the L series.) Probably because of extensive formation of unproductive com-... 

Figures 11(a) and 11(b) [112] show the variation of Ni-Ge-P deposition rate and Ge content as a function of aspartic acid and Ge(IV) concentration, respectively. A relatively low P content, ca. 1-2 at%, was observed in the case of films exhibiting a high concentration of Ge (> 18 at%). Like other members of its class, which includes molybdate and tungstate, Ge(IY) behaves a soft base according to the hard and soft acids and bases theory (HSAB) originated by Pearson [113, 114], capable of strong adsorption, or displaying inhibitor-like behavior, on soft acid metal surfaces. In weakly acidic solution, uncomplexed Ge(IV) most probably exists as the hydrated oxide, or Ge(OH)4, which, due to acid-base reactions, may be more accurately represented as [Gc(OH)4 nO ] ".

Molybdic and Tungstic Acids. What hydrates do molybdenum and tungsten trioxides form ... 

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... 

Tn 1950 Weitz et al. (23) developed a sensitive colorimetric method to measure small amounts of oligomeric silicic acid in the presence of polymeric forms. Oligomeric acid in this sense comprises molecules up to hexameric chains of hydrated SiC>4 tetrahedrons, which react readily with ammonium molybdate. Hence, this fraction of a sol is frequently referred to as molybdate-active silicic acid. 

Molybdate diluting solution. Dissolve 0.5 g of Na2C03 and 0.5 g of Na2B407 in 100 ml of 0.39 M nitric acid. Add 100 ml of molybdate complexing solution and dilute to 500 ml with 0.126 M HN03. This solution can be used until a scale of hydrated molybdenum oxide develops on the container walls (several days to a week). Then a fresh solution should be prepared. The scale can be removed with ammonium hydroxide solution. 

The pentachloride is a little unstable in air when heated to about 1380° C. or less it leaves a residue of molybdenum when heated in hydrogen at 250° C. it is reduced to amorphous molybdenum trichloride. Its aqueous solution is unstable in air, especially on warming, when hydrogen chloride is more rapidly evolved and the blue oxide (p. 131) remains. Decomposition of its solution in hydrochloric acid also readily takes place. In alcohol and ether it dissolves to a green solution in sulphuric acid its solution is bluish green and in nitric acid colourless alkalies dissolve it with production of the hydrated dioxide and a molybdate. ... 

The blue oxide is best obtained by allowing powdered molybdenum to remain for a long time at ordinary temperatures in contact with an aqueous suspension of the trioxide, filtering, and then digesting with a further quantity of molybdenum. The solution is evaporated in vacuo. Cold water used for washing the solid should previously be rendered air-free. Another good method of preparation consists in precipitating in the cold, by means of excess of a solution of hydrated molybdenum tetrachloride, a solution of ammonium molybdate in hydrochloric acid the precipitate is w ashed with air-free w ater in an atmosphere of carbon dioxide, and is dried m vacuo. 

Manganese Molybdate, MnMoO, is obtained in the anhydrous condition by fusing together sodium molybdate, manganous chloride, and sodium chloride. By treating normal or acid molybdate solutions vith manganous salts, the manganese molybdate may be separated in the hydrated condition. Several hydrates have been described, but the existence of the monohydrate only appears to be established. This is a white crj stalline powder. 

Electrolytic Methods.—3Iolvbdenum mav be accurately deter-mined in solutions of molybdates by means of electrolysis. In presence of free sulphuric acid the metal is completely precipitated on the cathode as the hydrated sesquioxide, HyChoehlorie acid may be used instead of sulphuric acid to accelerate the deposition. The precipitate after washing may be converted by gentle ignition to molybdenum trioxide, and weighed as such. 

Various methods for the preparation of potassium octacyano-molybdate(IV) 2-hydrate from molybdenum(VI) oxide, molyb-dic acid, or molybdates have been reported. These methods, although in general rehable, have one unfavorable aspect in common low efficiencies due to conversions of less than 50%. 

When HCl is added to a hot solution of a tungstate the insoluble, yellow acid, H2WO4, separates. A cold solution yields soluble, colourless H2WO4.H2O. The material which separates when nitric acid is added to a cold solution of ammonium molybdate is the hydrate MoOg.HgO. It is yellow when cold, but becomes colourless when heated, being converted to H2M0O4. 

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