Page:Text-book of Electrochemistry.djvu/248

 electromotive forces which conduct negative electricity to the mercury, the surface tension of the metal rises until the charge of the double-layer becomes equal to 0. When this limit is exceeded, the mercury becomes negatively charged, whilst the sulphuric acid receives a positive charge. This occurs when P has a value of about 1 volt. Mercury in the ordinary condition exhibits, when in contact with sulphuric acid, a potential difference towards it of about 1 volt, the mercury being positively and the acid negatively charged.

A. Konig (3) arrived at practically the same result by examining the curvature of mercury drops in sulphuric acid ; this method also permits of the determination of the surface tension. It is, however, not easy to ascertain the maximum surface tension in this way.

Dropping Electrodes. — From these results, Helmholtz came to a conclusion which led to the construction of the so-called dropping electrodes. Let us suppose that we hav a quantity of mercury which can be allowed to flow out through a fine tube into an electrolyte. If the mercury is positively charged, the surface of contact will be vastly increased by the flowing out, and the charge must become smaller. The charge, however, will only diminish provided that no new mercury ions pass from the solution into the mercury, and thus recharge the mercury electrode; the solution in contact with mercury always contains some mercury salt. With an arrangement of this kind Helmholtz {i) found that the dropping mercury possesses the same potential as a drop of mercury vhich is polarised to the maximum surface tension. Ostwald repeated these experi- ments, but Paschen {5) was the first to successfully construct dropping electrodes, which he did by arranging the tube so that the stream of mercury is broken up into drops just at the surface of the electrolyte under examination. The opening of the tube should be from 002 to 0-05 mm. in diameter.

According to Nernst (^), the action of dropping elec- trodes can be explained as follows. Mercury is a " noble "

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