CLEAPSS Recipe Book 64 Oxygen gas Oxygen is prepared by adding hydrogen peroxide solution onto a catalyst, manganese(IV) oxide. The reaction is very vigorous and exothermic and chemicals have been known to shoot up the thistle funnel or the bung has been forced out of the flask. The rate of the reaction depends on the surface area of the catalyst. It is better to use granules rather than a fine powder. If only powder is available, then only a small amount is required. Potassium iodide (1 g) produces oxygen at a steadier rate. The solution first saturates itself with oxygen and then the gas is released. Hydrogen peroxide decomposes slowly during storage. Suppliers add an inhibitor to slow the process, but once this is used up, decomposition accelerates. If more hydrogen peroxide than expected is required, then this is a sure sign that the hydrogen peroxide is ‘old’ and new stock is required. 5 ml of 100 vol solution of hydrogen peroxide produces 500 ml of oxygen, ie, 5 x 100 ml. General Hazards • • • • • • • • See Hazcards 50, 60 & 69. Wear eye protection. In the 250 ml Büchner flask, add 0.1 g of manganese(IV) oxide (HARMFUL) powder or 1 g of granules. Fill the measuring cylinder with water ready to collect the first 250 ml of gas produced. The measuring cylinder allows you to be confident that apparatus is well flushed out before oxygen is collected. Pour at least 25 ml of cold water, enough to cover the bottom, into the thistle funnel. Now add 5 ml of 100 vol hydrogen peroxide (HARMFUL). Once the measuring cylinder is full of gas, remove it and replace with a gas jar full of water. Keep collecting gas jars of gas. When full of gas, place a glass cover on the gas jar. If the rate of gas being produced slows down, add further 5 ml portions of 100 vol hydrogen peroxide. Add 5 ml of 100 vol hydrogen peroxide 250-ml Büchner Flask 250 ml measuring cylinder 72 © CLEAPSS 2011