VOL.5, No. 2
COMBINING WEIGHTOR COPPER
223
THE COMBINING WEIGHT OF COPPER JESSE
E. DAY A?~DR. J. SUTMN.THE
OHIOSTATEUNIVERSITY,COLUMBUS, 0x10
This exercise is planned for those students entering the University with one unit in high-school chemistry or its equivalent. Quite a few of our students have already determined the combining weight of zinc (an element above hydrogen in the chemical activity series) in their entrance course, and by the time this exercise is assigned have also obtained the ratio of oxygen to hydrogen in water. Hence the equivalent of an element typical of those below hydrogen, such as copper, may be gotten in terms of zinc. In carrying out this determination, most laboratory manuals direct that an excess of zinc be added to a known weight of copper sulfate in aqueous solution, a procedure which involves the removal of the precipitated copper from the remaining zinc. If a known weight of zinc (either mossy or sheet, preferably the latter) is added to an excess of the copper sulfate in solution, the removal of the copper from the zinc is obviated. Three grams of zinc and 14 g. of copper sulfate pentahydrate constitute suitable quantities. The deposition of the copper is accomplished within a few minutes if the zinc is added to the hot (almost boiling) solution of the copper salt. If the solution is boiling the finely divided copper constantly comes to the surface and is appreciably oxidized. If mossy zinc is employed, the reaction is facilitated by frequently breaking up the copperzinc clumps. The time consumed from this point on in the manipulation depends upon which of the three following methods is employed. Method 1. The precipitate is filtered immediately on a common filter of known weight (or a tared filter), washed with hot water, and the filter containing the copper set in the desk to dry until the next period. The degree of oxidation is dependent upon the drying period. Method 2. The precipitate is transferred to a common filter, the ash content of which is given with sufficient accuracy on the package, washed with distilled water, wood alcohol (and also with ether if desired), and allowed to drain for a few minutes. The filter is folded over the copper and the roll carefully dried in a heated and weighed crucible. When the drying is completed, the filter is incinerated and the crucible and contents are cooled and weighed in the customary manner. The amount of oxidation is proportional to the intensity and duration of the drying and incineration. It is interesting to note that the alcohol, ether, and filter afford a reducing atmosphere and cover for the copper. A flter of a high silica content is more satisfactory than one of the ashless variety as the siliceous material remaining after incineration also protects the copper from the air.
Method 3 . While still in the beaker, the copper is washed by several decantations using hot water. The beaker is covered with an inverted watch glass, placed on a wire gauze and the water cautiously driven off over a low flame. The effluent steam keeps most of the air away from the copper. The beaker and contents may be allowed to cool either in the desiccator or the open air for a few moments and are then weighed. This method gives the best results of the three described here, but requires a little care in the avoidance of the destruction of the beaker near the end of the drying.
