Hermann's phenomenon / by G.S. Walpole.

  • Walpole, George Stanley.
Date:
[1914?]
Catalogue details

Licence: In copyright

Credit: Hermann's phenomenon / by G.S. Walpole. Source: Wellcome Collection.

    16/20 page 147
    at the boundary will occur: but that H* or OH' ions will always be found concentrated there, too, has escaped recognition. A simple experiment can be devised to demonstrate these facts in a striking manner. The apparatus is filled, say, for the experiment under the heading 6th Series, except that, before filling, a little dry azolitmin is dissolved in the middle fluid, and, in such quantities as to produce the same tint, into the side tube fluid as well. Shortly after turning on the current, the litmus will begin to collect at boundaries 1 and 3, and to move away from boundaries 2 and 4. A space above the left-hand tap 1 or 2 mm. deep will be left perfectly clearly defined and colourless in a quarter to half-an- liour. Moreover, though this requires care in the amount of azolitmin used, the changes of colour of the litmus at the boundaries demonstrate, in addition, that at each boundary either TI* or OH' ions concentrate there too. The red litmus anions in the middle tube will not move at all or show any change in their even distribution indefinitely. The analogy between the acid and alkali liberation at the boundaries of solutions of unequal conductivity (taken together with the supposed bleaching action of litmus in the regions becoming colourless in the experi¬ ment just described) led Hermann to consider the phenomena at these boundaries similar to those at metallic electrodes. He attributed them to the discharge of ions. In many experiments with potassium iodide, where, after the experiment, the boundary was tested with starch, and silver nitrate, where metallic silver was looked for, no evidence of this occurring could be found. Summary. At the boundary between two solutions of unequal specific conductivity a change of reaction is developed if a difference of potential be maintained between them. Alkali is liberated if the current passes from the better conducting solution to that not conducting so well: acid, if the current passes in the opposite direction. The amounts may be calculated from the potential gradients in the solutions on each side of the boundary, the time for which the difference of potential is maintained, the resistance constant of the vessel employed, the dissociation constant of water, and the known migration velocities of hydrogen and hydroxyl ions. Harrjsox and Sons. Printers in Ordinary to His Majesty, St. Martin’s Lane.
    digitised image 1
    digitised image 2
    digitised image 3