Page:Text-book of Electrochemistry.djvu/263

 248 OXIDATION AND REDUCTION ELEMENTS, chap.

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KOH + HNOg into KNOg + HaO. On the basis of older experiments with platinum instead of palladium, Ostwald (4) assumes that the electromotive force is about 0*74 volt. This electromotive force (E) is governed by the formula —

£ = 0-0002 r log ^ = 0-0002 T log - "

where pa and Ca are the osmotic pressure and the concen- tration of hydrogen ions in the acid, pi, and C6 the corresponding values for the hydrogen ions in the solution of potassium hydroxide. Since Ca is known, the value of Cb can be calcu- lated. If the concentration of the hydroxyl (OH) ions in the alkali, which is known, be denoted by C^b, the equation of equilibrium (see p. 87) is —

where C^^o is the concentration of the water in the solution,

and it may be regarded as constant (55*5 gram-molecules per litre). From this, K, the dissociation constant of water, may be calculated.

For water, in which the number of hydrogen ions is equal to the number of hydroxyl ions (Co), we have the equation, CI = KCb^o

However, in the element cited, electromotive forces appear at the surfaces of separation of KOH and ENOg, and at that between KNOs and HNOs, and, according to Planck's formula, the combined value for these is 0065 volt, which must be subtracted from the total electromotive force in order to give that due to the neutralisation. From the data obtained in this way we arrive at the result that the number of gram-ions of hydrogen in a litre of water is 0*8 x lO"'', a value which agrees excellently with that found by Kohlrausch, 0-8 X 10-7 at 18° (see p. 194).

Irreversible Elements. — If we construct an element according to the scheme Zn | H2SO4 | Pt, we find that it gives rise to a current which, however, soon ceases because Ha is

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