(1) The Chemical Effect of an Electric Current
280. Electroplating.—If two carbon rods (electric light carbons answer very well) are placed in a solution of copper sulphate (Fig. 260) and then connected by wires to the binding posts of an electric battery, one of the rods soon becomes covered with a coating of metallic copper while bubbles of gas may be seen upon the other carbon. If a solution of lead acetate is used in the same way a deposit of metallic lead is secured, while a solution of silver nitrate gives silver.
Fig. 260.—Two carbons placed in a solution of copper sulphate.
Fig. 261.—An electroplating bath.
This process of depositing metals upon the surface of solids by an electric current is called electroplating. Everyone has seen electroplated articles such as silver plated knives, forks, and spoons, and nickel-plated rods, handles, etc. Copper electrotype plates such as are used in printing school books are made by this process. In practical electroplating a solution of the metal to be deposited is placed in a tank; across the top of this tank are placed copper rods to act as conducting supports. From one of these rods, the cathode, objects to be plated are hung so as to be immersed in the liquid. From other rods, the anodes, are hung plates of the metal to be deposited. These are dissolved as the current deposits a coating upon the articles, thus keeping the solution up to its proper strength. (See Fig. 261.)
Fig. 262.—The current is carried through the solution by ions.
281. Electrolysis.—A solution from which a deposit is made by an electric current is called an electrolyte. The plates or other objects by which the current enters or leaves the electrolyte are called the electrodes. The electrode by which the current enters is called the anode (an = in) while the electrode by which it leaves is the cathode (cath = away). The process by which an electric current decomposes a solution and deposits a substance upon an electrode is called electrolysis. The current always flows within the cell from anode to the cathode. (See Fig. 262.) The metal goes with the current and is found deposited upon the cathode.
282. Theory of Electrolysis.—The action going on in an electrolytic cell has been carefully studied. The theory of electrolysis, which is supported by much experimental evidence, supposes that many of the molecules in a dilute solution of a substance "split up" into two parts called "ions," one ion having a positive, the other a negative charge. In a dilute solution of sulphuric acid, the positive ion is of hydrogen, while the negative ion is the (SO4) or sulphion. These ions bearing electric charges are believed to be the carriers of the electric current through the electrolyte.
The positive ions move with the current from the anode to the cathode, while the negative ions apparently are repelled by the cathode and appear upon the anode. Evidence of the accumulation of the two kinds of ions at the two electrodes is furnished by the electrolysis of water, described below.