Barium Peroxide, BaO₂

Thenard observed that barium oxide absorbs oxygen probably equal in amount to that which is already present, and forms a Barium Peroxide, BaO₂. The rate of reaction is practically zero at ordinary temperatures, but reaches an appreciable velocity above 400° C. At the same time, however, since combination with oxygen is accompanied by an evolution of 12.1 Cal. per gram-molecule, the tendency to dissociate again into barium oxide and oxygen is increased, and an increased pressure of oxygen is, therefore, necessary to bring about complete transformation into peroxide. Thus, by heating the oxide alternately at a lower temperature, and then at a higher one, in air, it was found possible to use it as a means of separating oxygen from the air. It was observed that the reactants must not be absolutely anhydrous, but that at the same time the amount of water present must be small. The process was modified by Brin, who kept the temperature constant and alternately raised and lowered the pressure. The air had to be previously purified from carbon dioxide and organic matter, and a porous oxide was necessary. Brin's process for obtaining oxygen has now been superseded by air-fraetionation methods.

Recent work on the catalytic acceleration of the reaction by various oxides, for example copper, zinc, cadmium, magnesium, calcium, silicon, antimony, bismuth, uranium, tungsten, and molybdenum oxides, has suggested the possible revival of the Brin process under more favourable conditions of temperature. Some of the oxides mentioned form stable salts with barium, and would, therefore, be unsuitable for commercial purposes, as they would no doubt soon put the barium oxide out of action.

Barium peroxide is a white powder, stable under ordinary conditions if anhydrous, and protected from carbon dioxide. It melts at bright red heat with the evolution of oxygen. When heated with hydrogen, water, sulphur, carbon, carbon monoxide, sulphur dioxide, or ammonia, it is reduced. By heating with chlorine, or iodine, the oxygen is replaced by the halogerf. With concentrated sulphuric acid at ordinary temperatures ozonised oxygen is given off. With dilute hydrochloric acid, hydrogen peroxide is formed, and oxygen with concentrated. Potassium ferricyanide is reduced by barium peroxide, forming a double potassium barium ferrocyanide and free oxygen, and this reaction may be used as a means of estimating barium peroxide. With formaldehyde, barium formate and hydrogen are produced. By heating to redness with precipitated metallic gold a bright green mass is obtained which, on treatment with water, gives a solution of barium aurate, Ba(AuO₂)₂.

The dissociation pressures of barium peroxide for different temperatures were first investigated by Le Chatelier. The difficulty of the determinations is much enhanced by the fact that a solid solution of barium oxide and peroxide is formed, so that the pressure at any one temperature will be influenced by the proportions of these two present, and, further, by the necessity for the presence of water vapour. The dissociation has, however, been carefully studied by Hildebrand. As already stated, a small amount of water is necessary as a catalyst. After some of the peroxide has been decomposed, barium hydroxide will therefore be present. Experiments on the vapour pressure of barium hydroxide do not indicate the formation of a solid solution with the oxide, and the same is probably true of the peroxide and hydroxide.

Starting from the peroxide, the system is at first divariant. There are three components, water, barium oxide, and oxygen; and three phases, barium hydroxide, an unsaturated solid solution of barium oxide in barium peroxide, and the gaseous phase composed of oxygen and water vapour. Therefore there are two degrees of freedom. At any one temperature the oxygen pressure will vary with the concentration of the solid solution, which will depend on the proportion of barium peroxide decomposed. When about one-third of the peroxide has been decomposed the barium peroxide is saturated with barium oxide, and the system become monovariant owing to the appearance of a second solid solution of the two, thus making a fourth phase. The composition of the two saturated solid solutions may vary slightly with varying temperature, but at constant temperature will remain constant, although the amount of the second solution will increase at the expense of the first as the proportion of barium peroxide is diminished. The point at which the system becomes monovariant depends on the amount of water present, and hence on the amount of barium hydroxide which will be formed. The system again becomes divariant when about 98 per cent, of the peroxide has been decomposed.

The conditions necessary to produce 100 per cent, barium peroxide from barium oxide were studied. Oxygen of definite moisture content, which was less than the vapour pressure of barium hydroxide at the temperature under consideration, was passed, under slightly more than one atmosphere pressure, over barium oxide heated to a definite temperature in an electric oven. Successive equal samples were acted upon for equal times and the product analysed. Similar experiments were carried out with air. The following results were obtained: -

Temperature, °C	200	300	350	400	500	600	700	750	800	900
Percentage BaO2, using oxygen.	3.7	17.3	71.1	96.4	100	100	96.9	61.9	18.2	0.9
Percentage BaO2, using air.		18.2		70.4	69.2	52.8	30.9		5.9

With sufficient water present to give the vapour pressure of barium hydroxide, and sufficient barium oxide to produce the monovariant system, the following values for the pressure were obtained and compared with the values calculated from the equation

log₁₀p = -6850/T + 1.75 log₁₀T + 3.807, where T is the absolute temperature and p is expressed in mm.

These are all lower than Le Chatelier's values. The heat of combination of barium oxide and oxygen calculated from these results is 18.71 Cal. per gram-molecule of peroxide. The disagreement with Berthelot's value, 12.1 Cal., seems too large to be accounted for by the formation of solid solutions.

Anhydrous barium peroxide combines with water with the evolution of 9.1 Cal. of heat per gram-molecule. The octahydrate, BaO₂.8H₂O, in pearly plates, is formed, but the formula BaO₂.10H₂O and 7H₂O have also been assigned to it.

When barium peroxide is obtained by precipitation of barium hydroxide solution with hydrogen peroxide, the octahydrate is always formed at ordinary temperatures if less than one molecule of hydrogen peroxide be present per molecule of hydroxide, and above 60° C. whatever the composition of the solution. Jaubert patented a process for obtaining hydrated barium peroxide by lixiviating barium sulphide with boiling water, cooling to 25°-30° C., and adding sodium peroxide in cold water. The barium peroxide separates out in scales which may be filtered, washed, and pressed.

The octahydrate effloresces on exposure to air, and carbonate is also formed.

Berthelot mentions a monohydrate, heat of hydration 1.4 Cal., but it is probably the peroxyhydrate.

Barium peroxide may be estimated iodometrically by adding potassium iodide to a hydrochloric acid solution of the compound and titrating the liberated iodine with thiosulphate, or by titrating with permanganate a hydrochloric acid solution to which a considerable quantity of manganous sulphate solution has afterwards been added. A sulphuric acid solution cannot be employed, because the precipitated sulphate carries with it a large quantity of peroxide which escapes reaction.

Barium peroxide may be used for bleaching purposes, but its chief use is in the production of hydrogen peroxide by decomposition with an acid.