BLUE COPPER OXIDE BY H.
8. SCHENCK
Tommasi' showed many years ago that traces of a manganese salt stabilized the blue, hydrous copper oxide to such an extent that the latter could be boiled with water without turning black. Blucher and Rarnau2 showed that the same stabilization could be obtained with salts of nickel, cobalt, chromium, aluminum, mercury, magnesium, zinc, and manganese. Scheetz3 showed that by increasing the ratio of alumina to ferric hydroxide, it was possible to get a precipitate which would stay yellow a t temperatures at which i t seemed impossible that any hydrous ferric oxide could exist, thus confirming the conclusion drawn by Keane4 that sufficiently finely divided ferric oxide may be yellow. It was suggested by Professor Bancroft that some experiments be made with copper and aluminum sulphates, and this work was done as part of my senior research in the winter of 1916-1917. A mixture of copper and aluminum sulphates was made A1203 = 5 : IOO approximately. up such that CuO : CuO This mixture was dissolved in water and the hydrous oxides precipitated by a slight excess of NaOH. The precipitate was washed thoroughly on a filter and dried in an air-bath The color of the dried precipitate was a baby blue. a t 110'. The precipitate was ground to a fine powder and heated in a porcelain crucible, first with a Bunsen burner and then with a blast lamp. The color changed to a light grayish blue. The blue tint was quite distinct and there were no signs of blackening. Another mixture was made up containing about 107~ CuO instead of 570 and was treated as before. This mixture remained blue when heated in the Bunsen burner; but showed
+
1
4
Bull. SOC.chim. Paris, (2) 37, 197 (1882);Comptes rendus, 99, 37 (1884). Jour. Phys. Chem., 18, 629 (1914). Ibid., 21, 570 (1917). Ibid., 20, 734 (1916).
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signs of turning black when heated in the blast lamp. It is, therefore, clear that a mixture of copper and aluminum oxides blacken a t lower temperatures, the higher the percentage of copper oxide in the precipitate. Since the blue color persists in some precipitates when heated in the blast lamp, i t cannot be due to hydrous copper oxide, and this was confirmed by analyses which will be taken up later. The blue color cannot be due to a copper aluminate coexisting with alumina as a second phase because the color should then appear at the same temperature in all cases. It cannot be due to a copper aluminate dissolved in alumina because the blue color would then return if the precipitate were heated until it blackened and were then held a t a somewhat lower temperature until the dissociated copper oxide and alumina recombined. It is only fair to say that this possibility was not thought of a t the time and that the experiments do not exclude the possibility of a black precipitate turning blue again if heated sufficiently long at a suitable temperature. This should be tested carefully. One reason why this was not tried is that a blue precipitate is obtained in aqueous solutions where no alumina is present and it seemed probable that the change from blue to black was due to the same cause in all cases, the alumina merely retarding the change. If this is so and it seems to be, copper oxide itself must be blue when sufficiently finely divided. The change from blue, hydrous copper oxide to black oxide is then merely an agglomeration, the water acting as a rather inefficient stabilizer of the fine material. The alumina is more efficient and may keep the copper oxide finely divided a t relatively high temperatures. This is the more probable because, at low temperatures, so many gelatinous oxides stabilize the blue precipitate more or less. Since one is dealing with stabilization, it was possible to assume that the alumina stabilized the Cu02HZ,in case such a compound existed, and that the water had not all been driven off even after continued heating in the blast lamp. To disprove this assumption the grayish blue precipitate was analyzed
Blue Copper Oxide
Wt. of sample taken Wt. of copper found Wt. of AlaOafound CUO A1& Total oxides
Blue Pink Light yellow Bluish green White, blue tinge Green
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0.7296 g 0.0288 g. 0.6900 g 4.94% 94.56% 99. so%
Grayish blue Brown Darker and brown Green Deep blue Yellow
The general conclusion to be drawn from this investigation is that sufficiently finely divided copper oxide is blue and that the change of hydrous copper oxide from blue to black is due to an agglomeration of the particles. It is very desirable that some independent confirmation of this should be obtained. Cornell University
