Vanadium compounds, General

Magnesium vanadates, as vanadium compounds in general, are known irritants of the respiratory tract and conjunctiva. The threshold limit value (TLV) for vanadium compounds in air recommended by the National Institute of Occupational Safety and Health is 0.05 mg/m based on a typical 8-h workday and 40-h workweek (7,147). Chronic inhalation can lead to lung diseases such as bronchitis, bronchopneumonia, and lobar pneumonia. These dust-related effects can be avoided by use of individual respirators in areas where exposure is likely. 

Vanadium, a typical transition element, displays weU-cliaractetized valence states of 2—5 in solid compounds and in solutions. Valence states of —1 and 0 may occur in solid compounds, eg, the carbonyl and certain complexes. In oxidation state 5, vanadium is diamagnetic and forms colorless, pale yeUow, or red compounds. In lower oxidation states, the presence of one or more 3d electrons, usually unpaired, results in paramagnetic and colored compounds. All compounds of vanadium having unpaired electrons are colored, but because the absorption spectra may be complex, a specific color does not necessarily correspond to a particular oxidation state. As an illustration, vanadium(IV) oxy salts are generally blue, whereas vanadium(IV) chloride is deep red. Differences over the valence range of 2—5 are shown in Table 2. The stmcture of vanadium compounds has been discussed (6,7). 

In general, the direct-oxidation processes employ a redox couple that has sufficient oxidation potential to convert H2S into elemental sulfur but insufficient potential to oxidize sulfur to higher states. Examples of materials that have this redox potential are vanadium compounds, arsenic compounds, iron compounds, and certain organic species. Typically, the redox materials, dissolved in a hot potassium carbonate solution with the species in its oxidized form, contacts the I S-laden gas and the H2S dissolves as the hydrosulfide. This sulfur reacts with the redox couple, forming elemental sulfur and the reduced state of the couple. Airblowing of the solution reoxidizes the couple and removes the elemental sulfur from solution as a product froth. 

In addition to the reactions that produce elemental sulfur, competing reactions also occur that produce undesirable by-products such as sodium thiosulfate. This is detrimental, because the thiosulfate remains in solution, and its concentration can generally be reduced only by bleeding off a portion of the solution inventory. This solution purge waste stream is hazardous, largely because it also contains vanadium compounds. The key to reducing the metal content of the waste stream is to reduce the rate of thiosulfate formation. 

The general characteristics of vanadium compounds are outlined in Chapter I. 

The several oxides of vanadium have already been referred to in the section describing the general properties of vanadium compounds (see p. 80). They are set out in the table on p. 38. The thermal changes involved in their formation are discussed collectively on p. 32. 

In order to fully understand the antidiabetic effects of vanadium, or any other drug, cellular studies are extended to mammalian models of diabetes, frequently in rodents. Models exist and have been used for vanadium studies of both type 1 and type 2 diabetes. Below, general results observed with vanadium compounds in the various model systems are described. More detailed descriptions of the molecular signal transduction systems affected are given in references [12,13,100,133],... 

These observations on the sulfuric acid catalyst arc full in line with the general thermodynamic behaviour of fused catalyst systems. The mctastablc solid in Figure 2 has to be replaced in this case by a cascade of the partly reduced vanadium ternary sulfates. The processes sketched above occur under thermodynamic control in a quaternary phase diagram, vanadium-oxygen-sulfur-alkali, as illustrated by the reversibility of the exsolution of the partly reduced vanadium compounds under suitable partial pressures of oxygen... 

This section describes the organovanadium compounds containing jj -cyclopentadienyl (Cp) or substituted (Cp ) ligands, which are classified into half-sandwich (see Halfsandwich Complexes), sandwich (see Sandwich Compound), bent sandwich (see Bent Metallocenes), and pseudo-triple decker derivatives. Cyclopentadienyl vanadium compounds are nsed for the synthesis of varions vanadium compounds as mentioned above. Generally, the oxidation state of the cyclopentadienyl vanadium complexes is Irom — 1 to -1-5. 

One common type of vanadium trap contains a basic species to react with and neutralize the acidic vanadium compounds. The vanadic acid can react with the basic component of the trap according to the general reaction scheme ... 

Tetra- and pentavalent vanadium compounds such as VCI4 and VOCI3 are generally used for phenolic oxidation. Phenol was oxidized with VCI4 in CCI4 to afford two ortho-para and para-para coupled biphenyls 622 and 19 in 18 and 34% yields. 

Vanadium is a white to gray metal with compounds widely distributed at low concentrations in the earth s crust. The average concentration of vanadium compounds in the earth s crust is 150 mg/kg. Elemental vanadium does not occur in nature, but its compounds exist in over 50 different mineral ores and in association with fossil fuels. It has six oxidation states (1-, 0, 2+, 3+, 4+, and 5+) of which 3+, 4+, and 5+ are the most common. The ion is generally bound to oxygen. 

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