In decomposing other compounds, the anode is made of the metal which is to be deposited at the cathode. If copper is to be deposited from a solution of copper sulphate the anode should be a copper plate; this keeps the solution at same strength, and avoids the opposing E. M. F. of polarization; that is, a very weak current will do the work (See [Exp. 149]), because the electrodes are of the same metal.
EXPERIMENT 148. To coat iron with copper.
Apparatus. Iron nail, solution of copper sulphate ([§ 283]).
374. Directions. (A) Clean the nail with sandpaper, then hold it in the copper solution for a few seconds. Machinists often cover iron or steel tools with a thin coating of copper in this way.
EXPERIMENT 149. To study the electrolysis of a solution of copper sulphate.
Apparatus. Galvanoscope, G V; two-fluid cell, 2-F C; a tumbler, T, containing about an inch of copper sulphate solution ([§ 283]); a wooden cross-piece to which is fastened a copper strip; carbon rod, C; wire 2 is held to C by a rubber band. Arrange as in [Fig. 114], so that Cu will be the anode ([§ 370]), the current passing as shown by arrow. A dry cell may be used for short experiments instead of the 2-F C.
375. Directions. (A) The carbon being clean, allow the current to pass, C and Cu being kept about ½ in. apart. Watch the surface of C, and note the beautiful color of the deposited copper. Save the coated rod for the next experiment. Has the Cu plate been acted upon?