Note 39 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

To prepare ClO2 100 grams of sulphuric acid are cooled in a mixture of ice and salt, and 15 grams of powdered potassium chlorate are gradually added to the acid, which is then carefully distilled at 20° to 40°, the vapour given off being condensed in a freezing mixture. Potassium perchlorate is then formed: 3KClO3 + 2H2SO4 = 2KHSO4 + KClO4 + 2ClO2 + H2O. The reaction may result in an explosion. Calvert and Davies obtained chloric peroxide without the least danger by heating a mixture of oxalic acid and potassium chlorate in a test tube in a water-bath. In this case 2KClO3 + 3C2H2O4,2H2O = 2C2HKO4 + 2CO2 + 2ClO2 + 8H2O. The reaction is still further facilitated by the addition of a small quantity of sulphuric acid. If a solution of HCl acts upon KClO3 at the ordinary temperature, a mixture of Cl2 and ClO2 is formed, but if the temperature be raised to 80° the greater part of the ClO2 decomposes, and when passed through a hot solution of MnCl2 it oxidises it. Gooch and Kreider proposed (1894) to employ this method for preparing small quantities of chlorine in the laboratory.