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The vanadium ii oxide formula is very complicated and varies with the pH of the solution. It is usually written as VO2+, but is more accurately [VO2(H2O)4]+.
The oxidation state of vanadium is +5, +4, +3 or +2 depending on the source of the vanadium and the way it is reduced. The way of changing between these oxidation states is called the contact process.
In the Contact Process, sulphur dioxide is passed over a solid vanadium oxide catalyst. The sulphur dioxide is converted to sulphuric acid (a strong reducing agent) and oxygen is removed, leaving the vanadium oxide.
Using a metal oxide as a catalyst for the contact process is very common in the manufacturing of sulphuric acid. The sulphuric acid is then used to make other chemicals, such as nitric acid and a number of other industrial products.
Zinc is used to reduce the vanadium in the set of reductions to give the vanadium ii oxide ion. The ion is then reduced by an acid – either hydrochloric or sulphuric acid, but a moderately concentrated one is normally used.
Another oxidising agent that will change the vanadium ii oxide to the oxidation state of vanadium +4 is nitric acid, although it is much less powerful than the other two. The nitric acid reacts with the vanadium ii oxide to produce blue VO2+ ions, which are then oxidised back to vanadium(IV) by the air.
This is because the vanadium ii oxide is very easily oxidised by the air. If you remove some cotton wool from a small flask and pour the solution into a test tube, it turns green as the air oxidises it.
