Vanadates, alkali

I carbonates Co-Na, Co-K, Ni-Na, Ni-K nitrates Zn-alkali metals sulphates (schoenites) Mg-Na carbonates Al-K, Ce-Na nitrates rare earth-alkali metals sulphates (alums) Me111-alkali metals carbonates Ce-alkali metals nitrates Ce-alkali metals sulphates U-alkali metals alkali metals vanadates alkali metal chromates, Ag chromate, alkali metal molybdates and tungstates... 

Vanadium dioxide, VO2 is dark blue (V2O5 plus SO2) but is readily reduced further to Vo.i86-V,.6a. VO2 gives the (VO) ion with acids and vanadates(IV) with alkalis and as mixed metal oxides. 

It is extensively used industrially as a catalyst, notably in the oxidation of sulphur dioxide to the trioxide in sulphuric acid manufacture. It is an essentially acidic oxide, dissolving in alkalis to give vanadates however, addition of acid converts the anionic vanadate species to cationic species, by processes which are very complex, but which overall amount to the following ... 

Excess sodium hydroxide present can also be troublesome as the alkali reacts with the SO3 present in the gas stream to form a range of alkali sulfates which in themselves are highly corrosive to metallic components. In addition, the combination of alkali sulfate -l- V2O5 can result in compounds having melting points as low as 600°E. This situation is only encountered when alkali is present in amounts in excess of that which can react stoichiometrically with V2O5, since the formation of alkali vanadates is favored over that of alkali sulfates. 

Corrosion Influenced by Variables. Corrosion oi metals by V2O5 or alkali vanadates is influenced by several variables, some of which are independent. Those found to affect the corrosion rate are ... 

Reactions of contaminants in the fuel or air in the combustion zone can result in the formation of compounds which can condense as molten salts onto cooler components in the system. This type of process can occur when fuels containing sulphur or vanadium are burnt. In the case of sulphur contaminants, alkali sulphates form by reactions with sodium which may also be present in the fuel or in the combustion air, and for vanadium-containing fuels low-melting-point sodium vanadates or vanadium pentoxide are produced, particularly when burning residual oils high in vanadium. Attack by molten salts has many features in common which will be illustrated for the alkali-sulphate-induced attack, but which will be subsequently shown to be relevant to the case of vanadate attack. 

Phosphate, arsenate, and vanadate interfere. Borate, fluoride, and large amounts of aluminium, calcium, magnesium, and the alkali metals have no effect in the determination, but large amounts of iron (> 5 per cent) appear to produce slightly low results. 

Brown-yellow orthorhombic crystals density 3.35 g/cm melts at 670°C decomposes at 1,800°C slightly soluble in water, 0.8g/100 mL at 20°C soluble in concentrated acids forming an orange-yellow solution soluble in alkalies forming vanadates. 

The formation of ethyl radical in the reaction C2H6 + N2O has been confirmed by ESR using the matrix isolation method (3,18). The kinetic results and the product distribution suggest that this reaction route dominates in the oxidation of ethane on V2O5 (5) and alkali metal vanadates (6,8), and very probably in the present case too. 

Hypovanadous oxide resembles the metal in many of its properties. It is insoluble in water, but dissolves in acids without evolution of hydrogen to yield the lavender-coloured solutions which are characteristic of solutions of hypovanadous salts. These salts are, however, most conveniently prepared in solution by electrolytic reduction in an inert atmosphere of solutions of vanadium pentoxide in the various acids.7 Hypovanadous salts are isomorphous with salts of divalent iron, chromium, and manganese. On being treated with caustic alkalis, a brown precipitate of hypovanadous hydroxide, V(OH)a, is obtained, which rapidly oxidises to the greyish-green vanadous hydroxide, V(OH)s. 

Intermediate Oxides.—Oxides which are intermediate between liypovanadic oxide, V02, and vanadium pentoxide, V205, are known. By the partial reduction of vanadium pentoxide, or by the partial oxidation of one of the lower oxides, there have been prepared a number of oxides which are best considered as being formed by the combination of the acidic vanadium pentoxide with a lower basic oxide in varying molecular proportions. These oxides react yvith alkalis, and yield a series of salts called vanadyl vanadates, intermediate in composition between the vanadites and the vanadates. 

These salts are made by reducing hot solutions of alkali vanadates with sulphur dioxide after addition of acetic acid they can be salted out by addition of alkali acetate.5 The following are known —... 

Vanado-vanadates can also be obtained by fusing alkali vanadites with metavanadates, when no oxidation or loss of oxygen takes place. This process has furnished two salts,1 Na20.2V02.Va06 and K20. 2V02.V205. 

Vanadium pentoxide dissolves in acids, both organic and inorganic, to form vanadyl or unstable vanadic salts,7 and in alkalis to produce ortho-, pyro-, meta-, and poly-vanadates. The physico-chemical changes involved when vanadium pentoxide is heated with various basic oxides in the powder state have been investigated by Tammann.8 On being digested with liquid ammonia slow absorption of ammonia takes place the composition of the product has not been definitely established.9 The oxide also dissolves in alcohols to produce esters,10 and combines with methylamine and ethylamine to form compounds of the type 2(R.NHB).V205, where R represents the alkyl radical.11... 

All the vanadates are powerful oxidising agents and undergo reduction in add solution in the manner already described for vanadium pentoxide. The alkali vanadates are usually easily soluble in water, and are white or pale yellow, crystalline compounds, and frequently... 

Bismuth Orthovanadate, BiV04, is a bright yellow compound obtained by the double decomposition of bismuth nitrate with an alkali vanadate.4... 

Pyrovanadates, R 4V207 or 2R a0.Va05.—The alkali pyrovana-dates are prepared by dissolving the equivalent quantity of vanadium pentoxide in solutions of alkalis, or by the spontaneous decomposition in solution of the alkali orthovanadates. Pyrovanadates of other metals are obtained by fusing vanadium pentoxide with the salts or hydroxides of the metals in molecular proportions, or, when they are sufficiently insoluble, by double decomposition between an alkali pyro-vanadate and a salt of the metal required. 

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