Chromic acid is liberated on adding to a concentrated solution of potassium anhydrochromate a sufficient excess of sulphuric acid : K₂Cr₂O₇. Aq -f H₂SO₄ = K₂SO₄.Aq + H₂O + 2CrO₃. The acid, in concentrated solution, loses water, and deposits the trioxide or anhydride in crystals of a deep red colour. Chromium trioxide is a powerful oxidising agent; hence it may not be brought into contact with filter-paper ; it must be filtered through a mat of asbestos or glass wool. The excess of sulphuric acid and potassium sulphate are washed out with concentrated nitric acid, in which the anhydride is almost insoluble ; the nitric acid is then volatilised by gentle heat. This anhydride dissolves in water, but it is doubtful whether the acid H₀CrO₄ is contained in the solution ; it is more probable that the ions are HH and =Cr₂O₇, from the colour, and other tests, such as the conductivity.

Oxidation by means of a solution of chromic anhydride is carried out either by boiling the substance to be oxidised with a mixture of bichrome and dilute sulphuric acid, or with a solution of chromic anhydride in pure acetic acid; the chromate ion, =CrO₄ or =Cr₂O₇, is changed into the chromic ion Cr ; the action is :-

If the sulphuric acid is hot and concentrated, oxygen is evolved as gas; if dilute, substances present in solution, if they are capable of being oxidised, are attacked by the oxygen. When chromic anhydride is heated, it is converted into chromium sesquioxide, Cr₂O₃, with evolution of oxygen.