375
AUGUST,1942
COPPER MIRRORS
P. W. SELWOOD
Northwestern University, Evawton, ZUinois
IT IS common knowledge that the reduction of sulfur dioxide, from reduction of the sulfuric acid; and cupric oxide with hydrogen is often attended by the hydrogen chloride, by action of sulfuric acid spray on formation of a copper mirror on the walls of the con- the calcium chloride. tainer. The writer's attention was directed to this The next experiment was to introduce a drop of problem by experiences of students in the elementary hydrochloric acid into the hydrogen stream used for chemistry laboratory. These students were instructed reduction of the copper oxide. At once a mirror was to perform the familiar experiment of determining the formed. It seems therefore that the sequence of reequivalent weight of an unknown metal (copper) by actions leading to formation of the mirrors is: reduction of the oxide. A diagram of the apparatus 2Cu0 + HB CulO H20 used is shown in Figure 1. This diagram is given beCu10 + 2HCI-r 2CuC1 + H1O cause i t offers a clue to the formation of the mirrors. 2CuCl + Hn i=2Cu i 2HC1 The apparatus consists of a generator in which Conversion to the chloride may precede reduction of hydrogen is prepared by the action of zinc on dilute the divalent copper. sulfuric acid, a calcium chloride drying tube, and a pyrex test tube in which the actual reduction takes place. In the writer's experience a fairly large proportion of the students obtained copper mirrors on the inside of the pyrex test tube. These unfortunate students have, in the past, been instructed to repeat the experiment because of a mistaken belief that they had spilled copper oxide while introducing the porcelain boat into the test tube. Formation of a mirror under the circumstances described implies the presence of a compound of copper which is volatile a t a dull red heat or lower, and which is readily reduced by hydrogen. The oxides of copper are, of course, readily reducible, but they have no appreciable vapor pressure below 1000°C. The only volatile compound of copper appears to be cuprous FIGUREl . - A ~ p ~ n n r u saon D E T E ~ ~ms I ~EQUIVALENT G chloride, which may be readily sublimed a t a red heat. WEIGATOF A METAL The statement sometimes appears in the literature that Pure cuprous chloride was next substituted for cupric cupric chloride is reduced by hydrogen to cuprous chloride but that further reduction does not take place. oxide. A good mirror was produced almost as soon as the cuprous chloride melted. By appropriate heating, It is true that for the reaction the mirror could be produced a t will either adjacent to 2CuCI H2 + Cu 2HC1 the porcelain boat or several centimeters away. Somethe equilibrium lies very far to the left a t room tempera- times the mirror was so thin that i t was green by transture. But a t 400°C. the equilibrium constant ap- mitted light. No reduction to metallic copper took proaches unity, and for still higher temperatures reduc- place in a nitrogen atmosphere, proving that the reaction is actually reduction rather than decomposition. tion takes place readily. The first experiment tried in connection with this An effort was made to see if an analytical method for problem was to duplicate formation of the mirror. No copper could be devised. This would be analogous mirror could be produced when tank hydrogen was used, to the Marsh test for arsenic. The method proved, but mirrors were often formed when hydrogen, pre- however, to be rather insensitive, although a green pared by the action of zinc and dilute sulfuric acid, was color is imparted to the flame of burning excess hydrodried over calcium chloride. Formation of the mirror gen by as little as 0.1 mg. of cuprous chloride. was attended by a green color of the burning excess This semester, for the first time, the students in the hydrogen. This suggested that impurities in the hy- writer's classes were instructed to fill their calcium drogen were responsible for the mirrors. The impur- chloride tubes with a mixture of calcium chloride and ities to be expected are arsine, from arsenic in the zinc; quicklime. Not a single copper mirror was observed.
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