vanadate n : a salt or ester of vanadic acid; an anion containing pentavalent vanadium

English

Noun

1. any salt of vanadic acid
2. any of the corresponding anions, VO3, VO4, or V2O7

Derived terms

• ammonium vanadate
• cadmium vanadate
• lead vanadate
• potassium vanadate
• strontic vanadate
• yttrium vanadate

Vanadium(V) oxide (vanadia) is the chemical compound with the formula V2O5. Commonly known as vanadium pentoxide, this orange solid is the most important compound of vanadium. Upon heating it reversibly loses oxygen. Related to this ability, V2O5 catalyses several useful aerobic oxidation reactions, the largest scale of which underpins the production of sulfuric acid from sulfur dioxide. It is a poisonous orange solid which, because of its high oxidation state, is both an amphoteric oxide and an oxidising agent. Unlike most metal oxides, it dissolves slightly in water due to hydrolysis.

Vanadium(V) indicates that vanadium is in the +5 oxidation state. The oxygen atoms in the compound are in the -2 oxidation state.

Chemical properties

Acid-base reactions

V2O5 is an amphoteric oxide. Thus it reacts with strong non-reducing acids to form solutions containing the pale yellow salts containing dioxovanadium(V) centers:

V2O5 + 2 HNO3 → 2 "VO2(NO3)" + H2O

It also reacts with strong alkali to form polyoxovanadates, which have a complex structure that depends on pH. If excess aqueous sodium hydroxide is used, the product is a colourless salt, sodium orthovanadate, Na3VO4. If acid is slowly added to a solution of Na3VO4, the colour gradually deepens through orange to red before brown hydrated V2O5 precipitates around pH 2. These solutions contain mainly the ions HVO42− and V2O74− between pH 9 and 13, but below pH 9 more exotic species such as V4O124− and HV10O285− predominate.

Thionyl chloride converts it to VOCl3:

V2O5(s) + 3 SOCl2(l) → 2 VOCl3(l) + 3 SO2(g)

Redox reactions

V2O5 is easily reduced in acidic media to the stable vanadium(IV) species, the blue vanadyl ion (VO(H2O)52+). This conversion illustrates the redox properties of V2O5. For example, hydrochloric acid and hydrobromic acid are oxidised to the corresponding halogen, e.g.,

V2O5(s) + 6 HCl + 7 H2O → 2 [VO(H2O)5]2+ + 4 Cl− + Cl2

Solid V2O5 is reduced by oxalic acid, CO, and SO2 to give vanadium(IV) oxide, VO2 as a deep-blue solid. Further reduction using hydrogen or excess CO can lead to complex mixtures of oxides such as V4O7 and V5O9 before black V2O3 is reached. Vanadates or vanadyl(V) compounds in acid solution are reduced by zinc amalgam through the interestingly colorful pathway -

colorless "VO3-" → yellow "VO2+" → blue "VO2+" → green "V3+" → purple "V2+" The ions are of course hydrated to varying degrees.

Preparation

Technical grade V2O5 is produced as a black powder used for the production of vanadium metal and ferrovanadium. Vanadium(V) oxide can catalyse its production from a variety of organic starting materials such as n-butane, furfural and benzene, the last of which is the usual commercial method. In a related process, phthalic anhydride, used for making plasticisers for PVC manufacture, may be produced by V2O5 catalysed oxidation of ortho-xylene or naphthalene at 350-400°C.

Other applications

In terms of quantity, the major use for vanadium(V) oxide is in the production of ferrovanadium (see above). The oxide is heated with scrap iron and aluminium, producing the iron-vanadium alloy along with alumina as a by-product. In 2005 a shortage of V2O5 caused a price rise to around $40/kg, which in turn caused a rise in the price of ferrovanadium.

Due to its high thermal coefficient of resistance, vanadium(V) oxide finds use as a detector material in bolometers and microbolometer arrays for thermal imaging.

Possible new uses include the preparation of bismuth vanadate ceramics for use in solid oxide fuel cells.

Biological activity

Despite being highly toxic in humans, vanadium occurs in some organisms, notably the Ascidiacea (sea squirts). These organisms contain the protein vanabins, the role of which is unclear. Vanadate (VO43−), formed when V2O5 by hydrolysis of V2O5 at high pH, appears to inhibit enzymes that process phosphate (PO43−). However the exact mode of action remains elusive.

References