Paratungstates

The paratungstates are generally crystallised from slightly basic solutions. By far the most important salt is ammonium paratungstate [1311 -93-9] (NH 2o 12 40 known as the heavy form of commercial ammonium paratungstate. It is usually formed by crystallisation from a boiling... 

Traditionally, tungsten ore is chemically processed to ammonium paratungstate [1311-93-9] 5H20, and tungsten oxides, W O. These... 

Parasitic pressure drops, 15 823t, 824 Para- substituents, 9 371, 372 Paratacamite, 7 769 Parathion, 13 524-525 Paratungstates, 25 383 demand for, 24 276 Parcel charter, 25 330 Parenteral drug dosage forms, IS 713-716 Parenteral drug injections, prolonged action, 18 715-716 Parenteral formulations,... 

The protons in HvW,vO,vl i i are more accessible to water than in the dodecamer with Keggin structure [(H2)Wi204o]6, and a rapid exchange can be expected (145). Parenthesis are used in the Keggin formula to indicate the internal protons where necessary. The acceptance of a proton by paratungstate B, leads to the formation of three different species (141), its protonated form and the monoprotonated a- and /3-Keggin ions... 

Fig. 20. Polyhedral representation of the structure of [H2W12O42]10 (paratungstate B) showing the location of the protons in the center. The structure is built from two different types of trioctahedral subunits.

The main extra species that forms upon acidification of paratungstate was identified as a -[(H)Wi204o]7, which is internally protonated and forms at higher pH than a -[(H2)Wi204o]6 (141). This monoproto-nated species, a-[(H)Wi204o]7, was previously only obtained by reduction and reoxidation (154). It is slowly reprotonated to o -[(H2) Wi2O40]6-, the half-life of the reaction being 1 day at room tempera-... 

The structure of [H2Wi2O42]10 (paratungstate B) has been determined in at least eight different compounds (Fig. 20). This polyanion is not very soluble and is easily crystallized from aqueous solutions at pH 6-7 with cations such as Na+, K+, NH4, and Mg2+ (158). The role of hydrated cations in the stabilization and crystallization of this polyanion has been discussed (159). The position of the protons has been located by neutron diffraction as internally bound (159, 160) as predicted on the basis of crystallographic work (158b). 

Guo et al. reported that trace amoimts of aqueous organochlorine pesticides, such as hexachlorocyclohexane (HCH), could be totally degraded and mineralized into CO2 and HCl by near-UV irradiation of a suspension of Mg/Al LDH intercalated with paratungstate anions [108]. They demonstrated that photocatalytic degradation of the pesticide occurs in the interlayer galleries. It was also found that Zn/Al/W(Mn) mixed oxides, formed by calcination of POM-intercalated Zn/Al LDHs [109], exhibited higher photocatalytic activity in the degradation of HCH compared with the POM-LDH precursors. 

Mechanisms for the formation of [W60i9(0H)3] (paratungstate A) have been discussed. The relaxation spectra of aqueous molybdate have been determined by the temperature-jump technique at 25 °C(/i = 1.0 M, pH = 5.5— 6.8, and monomer concentration = 0.01—0.25 M). The results indicate two concentration-dependent relaxation effects, which are sensitive to heptamer, [Mo7 0 24] , and octamer, [Mog026]", formation. ... 

Preparation of Sodium Paratungstate, a. Pour 20 ml of a 50% sodium hydroxide solution into a 100-ml porcelain beaker provided with a mechanical mixer, put the beaker on a water bath, and introduce the calculated amount of a freshly precipitated wet suspension of tungstic acid in small portions while vigorously stirring. At the end of the reaction, the pH of the solution should be 6-6.5. Filter the solution, evaporate it a little, and let it stand for 24 hours. Separate the formed crystals by filtration and dry them in the air. 

Perform a similar experiment with sodium paratungstate, keeping the temperature within 600 °C. What properties do tungsten bronzes exhibit Where are they employed ... 

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