Tungstates and molybdates

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). 

Discussion. The procedure is based on the formation of yellow tetraiodo-antimonate(III) acid (HSbI4) when antimony(III) in sulphuric acid solution is treated with excess of potassium iodide solution. Spectrophotometric measurements may be made at 425 nm in the visible region or, more precisely, at 330 nm in the ultraviolet region. Appreciable amounts of bismuth, copper, lead, nickel, tin, tungstate, and molybdate interfere. 

These are complexes formed between tungstates and molybdates, and silicate or phosphate, and have been used to generate hydroxyl radicals photochemically. The tungstates PWj204q and SiWj204o have been used most frequently. 

The rate of y -alumina island formation essentially depends on the nature of the electrolyte used. If outwards migrating (in the terms of Xu et al.102) anions, such as tungstates and molybdates, are used in the anodization process, y- alumina seed crystals are surrounded by pure alumina and crystallization occurs easily. In the case of inwards migrating anions (e.g., citrates, phosphates, tartrates), the oxide material surrounding the y-nuclei is enriched... 

In aqueous solutions, calcium chloride undergoes double decomposition reactions with a number of soluble salts of other metals to form precipitates of insoluble calcium salts. For example, mixing solutions of calcium chloride with sodium carbonate, sodium tungstate and sodium molybdate solutions precipitates the carbonates, tungstates, and molybdates of calcium, respectively. Similar precipitation reactions occur with carboxylic acids or their soluble salt solutions. CaCb forms calcium sulfide when H2S is passed through its solution. Reaction with sodium borohydride produces calcium borohydride, Ca(BH4)2. It forms several complexes with ammonia. The products may have compositions CaCl2 2NH3, CaCb dNHs, and CaCb SNHs. 

According to C. F. Barwald and A. Monheim (1835), the decomposition is accelerated by the presence of organic substances. J. Milbauer tried the effect of thirty-two metal chlorides of sodium tungstate and molybdate of uranyl sulphate and of sulphuric, selenic, arsenic, and boric acids on the photo-decomposition of chlorine water, and found. that none accelerated but that most retarded the action. Chlorine catalyzes the decomposition of bromine water and bromine, chlorine water while iodine does not accelerate, but rather retards the reaction, probably by forming relatively stable iodine compounds. A. Bcnrath and H. Tuchel found the temp, coeff. of the velocity of the reaction with chlorine water between 5° and 30° increases in the ratio 1 1 395 per 10°. 

Tungstate and Molybdate exchanged Layered Double Hydroxides (LDHs) as catalysts for selective oxidation of organics and for bleaching... 

In the Biuret reaction, a purple colour develops when the protein is treated with alkaline copper sulphate. This reaction is dependent on peptide bonds and not on the side chains of individual amino-acids present. In the Folin-Ciocalteu reaction, the protein is treated with tungstate and molybdate under alkaline conditions and the formation of a complex such phenylalanine and tyrosine gives rise to a blue colour. Lowry developed one of the most widely used protein assays in which a combination of the above reactions is involved07, l8). 

Defined as those containing only the simple MO ions, they can be obtained from solutions of M03 in aqueous alkali. The MO ions persist as such in basic solution. Although both molybdates and tungstates can be reduced in solution (see later), they lack the powerful oxidizing property so characteristic of chromates(VI). The normal tungstates and molybdates of many other metals can be prepared by meta-thetical reactions. The alkali metal, ammonium, magnesium, and thallous salts are soluble in water, whereas those of other metals are nearly all insoluble. 

Superacidic metal oxides prepared by calcination at a high temperature can be used at elevated temperatures and, thus, provide new trends for developing environmentally benign processes. Superacidity is generated on the oxides of Fe, Ti, Zr, Hf, Sn, Si, and A1 by treatment with sulfate, tungstate, and molybdate. Sulfated and tungstated zirconias have attracted much attention as potential catalysts the latter are thermally stable superacids and can be calcined at temperatures above 1000°C. 

Zirconium phosphate, tungstate and molybdate, ammonium molybdophosphate and tungstophos-phate, hydrous oxides... 

There are no doubts that, using variations of the U V ratio and chemical nature of the counter ions, many new compounds can be obtained with structures based upon six different building blocks described in the Introduction to this chapter. Other oxoanions with various geometries (tungstate and molybdate, for example) have already been used to build other frameworks. To compare these compounds with vanadates, we have initiated studies of uranyl niobates that should reveal new interesting topologies. 

Because the decompositions of many ammonium salts evolve NHj and HjO simultaneously (or conseeutively) in a 2 1 molar ratio, it is often convenient to represent the formula of such salts in the form m(NH4)20.n(metal oxide).xH20, where m, n and x are integers (see ammonium chromates, vanadates, tungstates and molybdates below). 

A. J. Bailey, W. P. Griffith, B. C. Parkin, Heteropolyperoxo- and isopolyperoxo-tungstates and -molybdates as catalysts for the oxidation of tertiary amines, alkenes and alcohols, J. Chem. Soc., Dalton Trans. 11 (1995) 1833. 

N. Melaine Gresley, W. P. Griffth, A. C. Lammel, H. 1. S. Noqueria, B. C. Parkin, Studies on polyoxo and poljqjeroxo-metalates part 5 Peroxide-catalysed oxidations with heteropolyperoxo-tungstates and -molybdates, /. Mol. Catal. 117 (1997) 185. 

In conclusion, it would seem that the chemistry of Re is very different from that of W and Mo. First, Re has a 7-valent form, while the other two do not exceed the 6-valent state. In addition, while the perrhenate is stable in strong acids, the tungstate and molybdate tend to hydrolyze and/or polymerize. Hence, it would be interesting to study further the mechanisms of electrodeposition of Re. 

Figure8.11 M radiidependenceoftheelectricalconductivityat 6OO C in an air atmosphere for tungstates ( ) and molybdates ( ) with the Sc2(WO4)3-type structure.

Figure 21. H NMR spectra of reduced uteroferrin and its tungstate and molybdate complexes. Reprinted with permission from R. C, Scarrow, J. W. Pyrz, and L. Que, Jr.,y. Am. Chem. Soc., 112, 657-665 (1990). Copyright (1990) American Chemical Society.

Tungstates and Molybdates Molybdenum Oxide as a Molecular Sieve... 

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