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.

    12/20 page 143
    caustic soda to (d) or O'OOl N hydrochloric acid to (c) caused them to turn permanently yellow and red respectively. The contents of beaker (a) was redder than that of (c). After the addition of 0'20 c.c. of O’OOl 1ST caustic soda to (a), they matched in colour ; but (a) slowly developed a pink tinge again. By adding a further 0T0 c.c., (A) remained yellower than (c) for some minutes. The titration of the acid liberated in each apparatus was, therefore, about 0T0 c.c., and certainly less than 0*15 c.c. of O'OOl hf solution. Many experi¬ ments were made to account for this small, but in proportion to the titration volumes very large, time change. It was not understood. The control experiments described above are evidence that it is due neither to atmospheric carbon dioxide nor the reaction inertia of the indicator. A special experi¬ ment was made, leaving the fluids in the apparatus for 24 hours without turning on the current. It showed that it could not be attributed to alkali from the glass. The titration of (b) to the same tint as (c) was certainly more sharp. It required about 0*10 c.c. of O'OOl N hydrochloric acid. The alkali liberation in each apparatus was therefore about 0*05, and certainly less than 0'08 c.c. of O'OOl 1ST solution. Theoretical Interpretation of the Results Observed. From the evidence available at this stage it is legitimate to draw the conclusion that these boundary phenomena cannot be attributed to any property of neutral salt solutions not common to them all. Since the most obvious electrochemical property common to all neutral salts in solution is that of enhancing enormously the specific conductivity of pure water when dissolved therein, it is not surprising that this, and the hypothesis of the dissociation of water into H* and OH' ions, should supply the greater part of the explanation sought. It may be noticed at once, therefore, that in either limb of the apparatus as filled for any one of these experiments there is a column of highly conducting solution {e.g. 5-per-cent, sodium chloride solution) with a column of very feebly conducting solution {e.g. 0'05-per-cent, sodium chloride solution or, even better, pure water) above it. The columns of liquid are placed in series and in electrical connection with two points at a different electrical potential. It follows, therefore, that the strength of the electro¬ static field, which is expressed as a change of potential per unit length, is great above the boundary and small below it. The field is downward on the left-hand side of the apparatus and upward on the right. Since the contents of the U-tube are at the commencement neutral throughout, the hydrogen and hydroxyl ions per cubic centimetre are equal in number and evenly distributed both in the dilute and in the concentrated
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