In the contact process the catalyst acts by the steps V2O5 + SO2 -> V2O4 + SO3 and 2V2O4 + O2 -> 2V2O5. The oxidation state of vanadium in the intermediate is

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In the contact process the catalyst acts by the steps $V X_{2} O X_{5} + SO X_{2} V X_{2} O X_{4} + SO X_{3}$ and $2 V X_{2} O X_{4} + O X_{2} 2 V X_{2} O X_{5}$ . The oxidation state of vanadium in the intermediate is

V X_{2} O X_{4}

2 x + 4 (- 2) = 0

gives

x = + 4

. Vanadium goes from +5 to +4 and back, which is a catalytic cycle of variable oxidation states. Both equations balance (V, O) as written.