Tungstate salts

In contrast to the preceding reagents, oxidation with H202 catalyzed by molybdate and tungstate salts or Hg(OAc)2,722,723 and the Hg(OAc)2-promoted oxidation with a molybdenum peroxo complex724 can be applied to transform acetylene and terminal acetylenes to glyoxal and a-ketoaldehydes, respectively, in fair to good yields. 

Reaction of this W04 - with H2O2 produces peroxocomplexes, which in an aqueous methanohc medium epoxidize allyHc alcohols. The reactivity of our system agrees well with that of tungstate salts, dissolved in a single polar Liquid phase. The alkaline nature of the LDH support seems however to prevent solvolysis reactions. In the epoxidation of (homo)allyhc alcohols, selectivities are therefore better with the W04 -LDH A than with the homogeneous W salts [2,31. However, for some of the simple olefins, allyhc oxidation is not neghgible. 

Nonsulfide Ore Flotation. Nonsulfide minerals recovered by flotation include native elements such as graphite, diamonds, copper, gold, and numerous oxides as well as salts such as carbonates, phosphates, tungstates, and the like. Examples of value-bearing nonsulfide, noncoal minerals include... 

Anionic Complexes. Compounds of tungsten with acid anions other than haUdes and oxyhaUdes are relatively few in number, and are known only in the form of complex salts. A number of salts containing hexavalent tungsten are known. Potassium octafluorotungstate [57300-87-5] K WFg, can be prepared by the action of KI on W(CO)g in an IF medium. The addition of tungstates to aqueous hydrofluoric acid gives salts that are mostly of the type M(I)2(W2F. Similarly, double salts of tungsten oxydichloride are known. 

Salts containing pentavalent tungsten may be obtained by the reduction of alkaU tungstate in concentrated hydrochloric acid. Salts of types M(I)2(W0C1 ) (green), M(I)(WOCl (brown-yeUow), and M(I)(WOCl 2 ) h ve been isolated. Thiocyanato and bromo salts are also known. 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

In 1826 J. J. Berzelius found that acidification of solutions containing both molybdate and phosphate produced a yellow crystalline precipitate. This was the first example of a heteropolyanion and it actually contains the phos-phomolybdate ion, [PMoi204o] , which can be used in the quantitative estimation of phosphate. Since its discovery a host of other heteropolyanions have been prepared, mostly with molybdenum and tungsten but with more than 50 different heteroatoms, which include many non-metals and most transition metals — often in more than one oxidation state. Unless the heteroatom contributes to the colour, the heteropoly-molybdates and -tungstates are generally of varying shades of yellow. The free acids and the salts of small cations are extremely soluble in water but the salts of large cations such as Cs, Ba" and Pb" are usually insoluble. The solid salts are noticeably more stable thermally than are the salts of isopolyanions. Heteropoly compounds have been applied extensively as catalysts in the petrochemicals industry, as precipitants for numerous dyes with which they form lakes and, in the case of the Mo compounds, as flame retardants. 

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... 

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. 

The constancy of the quotient in the last line of Table 7-2 is greatly improved over that in the line above the last, proving that Equation 7-5 holds. So far as we know, this is the first case in which the absorption effects for a series of solutions have been obtained so precisely. Examples of this kind place on a firmer basis the calculation of semiquantitative analytical results from measured intensities when the composition of the matrix (all of S but E, the element sought) in a sample is approximately known. For example, tungsten contents could be estimated from measurements of L7I intensity for sodium tungstate solutions even when other salts are present in the absence of such salts, tungsten contents... 

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