Conditions are:
(i)    High concentration of reactants.
(ii)    Low temperature. But an optimum temperature of 623-723 k must be maintained.
(iii)   High pressure: Normally a pressure of about two atmosphere is maintained.
(iv)   Presence of catalyst: In order to accelerate the reaction, the presence of catalyst is quite helpful. V₂O₅, is used as a catalyst.
(v)   Purity of gases: Gases must be completely free from dust and poisonous gases like arsenic oxide before they are passed through the catalyst.