Bacterial oxidation is also employed industrially. Both bacterial and enzymatic methods may eventually prove to be powerful tools for selective oxidation in the laboratory. Alkenes react with peroxyacids to give epoxides. The addition of an alkene to osmium tetroxide in ether causes the rapid precipitation of a cyclic osmate ester. Ozone has therefore largely been displaced by the Lemieux reagents which consist of dilute aqueous solutions of sodium periodate with a catalytic quantity of potassium permanganate and of osmium tetroxide, respectively. The use of a partially aqueous medium favours the cleavage process, whereas an anhydrous medium such as glacial acetic acid favours allylic oxidation. The alkene is treated with iodine in aqueous acetic acid in the presence of silver acetate. Oxidation is carried out with chromium(vi) oxide in a mixture of acetic anhydride, acetic acid, and sulfuric acid at low temperature.