Copper sesquioxide, or cupric anhydride, Cu2O3.
This compound is not known in the free state. M. Krüger noticed that when chlorine is passed into alkali-lye in which cupric hydroxide is suspended, gas is evolved, and the soln. reddens owing to the formation of a higher oxide of copper. Such an oxide, however, has not been isolated from these products. E. Müller and F. Spitzer2, L. Moser, and F. Mawrow, working at low temp., succeeded in separating cupric hydroxide from the red soln. A trace of active oxygen was present, and this L. Moser regarded as a sign that the primarily formed dioxide has been almost completely decomposed. J. Aldridge and M. P. Applebey said that the pink colour is produced by the formation of permanganates by traces of manganese present as impurity in the soln. By treating cupric hydroxide with potassium hypochlorite, E. Fremy noted the formation of a brown soln. which, on dilution, gave off oxygen and deposited cupric oxide. E. Müller also obtained the same product by the action of alkali hypochlorites or hypobromites on cupric hydroxide; during the reaction the colour changes from sky-blue to a greyish-blue, a brownish-blue, and a bluish-black; if the soln. is very strongly alkaline, the colour becomes carmine red, violet, and finally brownish-black. W. Crum, and also M. Krüger, treated cupric nitrate with bleaching powder. The oxygen evolved by the product corresponded with that required by the equation: 2Cu2O3= 4CuO+O2. The carmine-red precipitate which gradually becomes rose-coloured is assumed by W. Crum to be calcium cuprate, CaCu2O4, the calcium salt of cupric acid, H2Cu2O4, which has the constitutional formula, HO.Cu.O.Cu.O.OH. The evidence available is not sufficient to establish the formula with any degree of certainty. M. Kriiger prepared barium cuprate, BaCu2O4, in a similar manner; and T. B. Osborne made some observations on the cuprates of the alkaline earths. J. Aldridge and M. P. Applebey said that the compounds with the alkaline earths are chemical individuals, and not, like M. Krüger's alkali compounds, mixtures with impurities. The rose-red coloured liquid obtained by E. Fremy during the dissolution of brass (low in zinc) in nitric acid is supposed to contain a higher oxide of copper. G. Krüss dissolved cupric oxide in molten potassium hydroxide; when the mass is dissolved in water at 18°, a blue liquid is obtained which deposits a mixture of what is thought to be cupric oxide and a yellow higher oxide. The liquid is stable on boiling, but is decolorized by shaking it with the oxides of copper or lead, but not zinc or mercury oxide ; soln. of salts of the alkaline earths, magnesium, aluminium, chromium, iron, cobalt, or nickel. G. Kriiss obtained a similar product as a yellow precipitate by working at 0°. The same product was obtained by using alkali chlorides in place of the alkali hydroxide. G. N. Antonoff and B. V. Malysheff found that when copper or copper oxide is dissolved in melted potassium hydroxide above 360°, the resulting cake, when dissolved in water and neutralized, decolorizes permanganate solution. The soln., however, gives no reaction for peroxides, and continues to decolorize permanganate when a current of hydrogen is passed through it. This decolorizing action is due to the presence of cuprous oxide, Cu2O, formed by the dissociation of cupric oxide above 360°. When potassium or hydrogen peroxide is added to the soln. of copper oxide in potash, a red colour is first produced which vanishes quickly ; then a yellow precipitate separates, which also redissolves, quickly reproducing the original blue soln., which now contains no trace of either peroxide or cuprous oxide. The yellow precipitate is copper dioxide, CuO2•H2O, or CuO•H2O2, its production being a first stage in the decomposition of the peroxide by copper oxide. This reaction is vigorous only in alkaline soln. Commercial potash contains some peroxide, and whilst, when acting on iron, the latter is converted into ferrous oxide, at the expense probably of the water present in the potash, the action on copper and also on silver is due to the presence of these peroxides. Contrary to the statement of L. Moser (vide supra), G. Scagliarini and G. Torelli found that the action of potassium persulphate on cupric hydroxide in presence of barium hydroxide at temp, obtained by cooling with ice and salt results in various changes in the colour of the soln. and in the deposition of a tenuous amaranth-red precipitate, which may be purified by repeated washing with ice-water by decantation. The compound thus obtained yields oxygen when treated with sulphuric acid, and the amount of oxygen is in agreement with the formula Cu2O3; it oxidizes hydrochloric acid with liberation of chlorine, oxidizes ammonia in the cold with production of nitrogen, nitrous acid, and traces of nitric acid, decolorizes permanganate, and decomposes potassium iodide with liberation of iodine in quantity greater than that corresponding with the proportion of copper present. Since it does not yield hydrogen peroxide when treated with dil. acid, the compound lacks the grouping characteristic of peroxides, and is thus different from the orange-yellow copper peroxide obtained by means of hydrogen peroxide. B. Brainier and B. Kuzma obtained an intense red coloration by the oxidation of a soln. containing copper and tellurium with potassium persulphate—if an excess is used the colour is destroyed. The coloration is attributed to the formation of a derivative of tellurocupric acid. By processes of hydrolysis, unstable dark red precipitates of potassium tellurocuprates, 2K2O•Cu2O8•3TeO3•nH2O, and 2K2O•CuO•Cu2O3•3TeO3•nH2O. The last-named product has also been obtained by the electrolytic oxidation of telluric acid in an alkaline soln., with a copper anode. Silver telluroargentate has also been obtained, Ag2O•5Ag2O3•9TeO3. L. Moser thinks that cupric sesquioxide is possibly an intermediate product in the slow decomposition of copper dioxide; and J. Meyer obtained indications of the formation of a higher oxide by shaking ammoniacal cupric sulphate with water and air. The product obtained by A. V. Brochet by heating a cone. soln. of cupric chlorate with copper turnings is thought to be the sesquioxide. According to E. Müller and F. Spitzer, an orange-yellow to red coloured film is obtained by eleetrolyzing with a copper anode and a platinum cathode in a porous cell, a soln. 12 to 14 N NaOH, cooled by a freezing mixture, and with a current density less than 0,1 amp. per sq. cm. The product is stable at a low temp, under the soda-lye, but it gives off oxygen and becomes white when warmed; it oxidizes hydrochloric acid, forming chlorine ; converts nitrites to nitrates ; and oxidizes alcohol to aldehyde. A. C. Becquerel, and F. Schmiedt have also studied the formation of the higher copper oxides during the electrolysis of alkali-lye.