Introduction to physical chemistry / by James Walker.

  • Walker, James, Sir, 1863-1935.
Date:
1903
Catalogue details

Licence: In copyright

Credit: Introduction to physical chemistry / by James Walker. Source: Wellcome Collection.

    349/388 (page 333)
    It is thus apparent that the displacement of one metal by another is often determined by the ratios of ionic concentrations, which may be themselves so small in absolute magnitude as to defy our ordinary means of detection. Polarisation and Electrolysis As long as we are dealing with reversible combinations, the passage of an electric current through the system in no way alters the nature of the electrode or of the liquid surrounding the electrode, except in so far as it may effect a change in the concentration of the electrolyte, which has little effect on electromotive force. The electromotive force therefore of the combination is practically constant, and if we apply to the system an opposed electromotive force of a magnitude greater than the direct electromotive force of the cell, a current will pass through the system in the reverse direction to that which the cell itself would generate. Since a metal immersed in the solution of one of its own salts is in general a reversible electrode, we can, as we have already seen, get a reversible combination of zero electromotive force by immersing two strips of the same metal, say copper in a solution of copper sulphate. The slightest electromotive force applied to this system will effect the passage of a current one way or other, copper being deposited on one electrode and dissolved from the other, and the current will continue to pass indefinitely if the concentration of the copper sulphate round the electrodes is maintained at the same value, say the saturation value at the experimental temperature. Suppose nowwe immerse platinum electrodes instead of copper electrodes in the copper sulphate solution. The electromotive force of the com- bination is still zero, but we. find that we can get no current of any magnitude to pass until the electromotive force applied to the system exceeds a certain value. The reason for this is obvious. As soon as a current passes through the system, copper is deposited on one electrode and sulphanion is discharged at the other with production of sulphuric acid and oxygen. The electrodes and solutions round the electrodes are no longer the same as they were originally, for in place of two platinum plates immersed in copper sulphate solution we have now one platinum plate (the kathode) covered with a thin coating of copper, and the other platinum plate (the anode) partially charged with oxygen and surrounded by a mixture of copper sulphate and sulphuric acid. A combination of this sort possesses an electromotive force of its own, which acts against the direct electromotive force and neutralises it if the direct force is of insufficient magnitude. The reverse electromotive force is called the electromotive force of polarisation, and is always encountered where the electrodes are non- reversible. When the difect electromotive force is of sufficient strength to overcome the electromotive force of polarisation a steady current may be made to pass through the system. Oxygen will then be