376. Electroplating is the name given to the process of coating substances with metal with the aid of the electric current. The copper sulphate, CuSO4, is broken up into Cu and SO4 by the current. The Cu goes to the cathode, and the SO4 attacks the anode, gradually dissolving it if it be copper; that is, the metal part of CuSO4 is carried in the direction of the current.
Most metals are coated with copper before they are silver or gold plated. A solution of silver is used for silver plating, silver being used as the anode.
EXPERIMENT 150. To study the chemistry of electroplating.
Apparatus. Same as in last experiment, but use two carbon rods for the electrodes. Arrange as in [Fig. 114], with the Cu replaced by another carbon. Two simple cells ([§ 275]) are also needed.
377. Directions. (A) Allow the current to pass as before. Is copper still deposited? Does anything occur now at the surface of the anode? Is the copper deposited as rapidly as before?
(B) Try the effect of the two simple cells joined in series, Instead of the two-fluid cell.
(C) After a fair coating of copper has been deposited upon the carbon cathode, reverse the direction of the current through the copper solution; that is, use the coated rod for the anode. Allow the current to pass until a change takes place in the anode.
378. Discussion. Ions are the names given to the parts into which an electrolyte is decomposed by the electric current. In the case of CuSO4, the ions are Cu and SO4, which is called an acid radical. This SO4 can not dissolve carbon or platinum, so these are used when water is to be electrolyzed. Where copper is used as the anode for copper plating, the SO4 attacks it, forming CuSO4 again, and this keeps the solution strong. If carbon were used instead, the SO4 would take H2 from the water around the anode and H2SO4 (sulphuric acid) would be formed, the oxygen of the water being set free at the anode. The amount of Cu dissolved from the copper anode equals nearly the amount deposited upon the cathode. [Exp. 150] shows that the metal is carried in the direction of the current. As hydrogen is produced at the cathode it is chemically considered a metal.
379. Electrotyping consists in making a copy in metal, of a woodcut, page of type, etc. A mould or impression of the type is first made in wax, or other suitable material (the pages of this book, for example, as set up by the printer). These moulds are, of course, the reverse of the type. They are coated with graphite to[155] make them conduct electricity, and hung as the cathode, in a bath of copper sulphate. After a thin coat of copper has been deposited by an electric current, the wax is removed and the thin copper backed with soft metal. The metal surface next to the wax will be just like the type, only made of copper. These plates or electrotypes can be printed from, the original type being used to set up another page. (See "Things a Boy Should Know About Electricity.")
380. Voltameters are cells used to measure the strength of an electric current. In the Water Voltameter the hydrogen and oxygen produced are measured. The H acts like a metal and goes to the cathode, two parts of H being formed to one of O.