Page:Text-book of Electrochemistry.djvu/261

 2+6 OXIDATION AND REDUCTION ELEMENTS, chap.

so that we obtain —

CuOu ] SO4X I SOiZn 1 Zn,

Avhei-e X is divalent, hydrogen (Ha) or zinc in the Daniell element, magnesium in Meidinger's element. On account I

of the passage of the current, the chemical reaction —

between zinc and copper sulphate takes place in the element through the medium of the ions, although the two reacting substances are spatially separated from each other. On account of the charges on the ions, electricity is transported during the reaction, and so the ion may be regarded as a sort of machine for transforming chemical into electrical energy.

We can imagine Bancroft's measurements carried out as follows. Several platinum wires, each surrounded by its oxidising or reducing agent (A, B, C, 2>, etc.), are immersed in a conducting liquid. The potential difference between A and D will then be equal to the sum of the differences between A and J5, B and C, and G and JD, All the substances examined may be arranged in a series, starting with the strongest reducing agent, stannous chloride in potassium hydroxide solution, and ending with the most energetic oxidising agent, potassium permanganate in sul- phuric acid. The numbers in this series (see the following table) give the potential differences between the compound indicated and the last in the series, potassium permanganate in sulphuric acid. It may be noticed that quite considerable potential differences exist between stannous chloride in potassium hydroxide and stannous chloride in hydrochloric acid, between chlorine in potassium hydroxide and chlorine in potassium chloride, etc. In the first case, stannic chloride is formed, which decomposes into 4HC1 and Sn(0H)4, and the hydrochloric acid is neutralised by the potassium hydroxide present. Consequently, in presence of potassium hydroxide more chemical energy is capable of being

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