Page:Text-book of Electrochemistry.djvu/201

 ��1 86 THE DISSOCIATION THEORY. chap.

all osmotic forces, whereby the sodium would receive an acceleration, but the chlorine a retardation. As the unit of electromotive force is equal to 300 volts when expressed in electrostatic units, the 290 electix)static units mentioned

would possess a potential of =^-^- = 8700 volts. This

charge, when communicated to a solution in the form of a cube, the length of whose side is 10 cms., would be at a tension of the same order of magnitude, or, in round numbers, 10* volts.

Let us consider a column of 1 sq. cm. cross section and 1 cm. high, one end of which, A (Fig. 40), has a potential of

10* volts, whilst at the other end. By the potential is 0. Let the liquid between A and B contain dissolved sodium chloride, so arranged that at A the concentration is 0, at B 1-normal, i,e. in toto 0'5-normal. And, further, we assume that the sodium chloride is completely dissociated. The chlorine ions are acted on (compare p. 121)

by an electric force, e, where. is the

^ II

fall of potential per centimetre, in this case equal to 10* volts, and e denotes the number of coulombs with which the chlorine ions are charged, here equal to -2oVo^6>^^^=^^"^' since the cubic centimetre of the solution contains ^-

gram-ions. The force acting is therefore (see p. 6) —

482 X 10* volt-coulombs per cm. = 482 x 10^^ dynes.

The osmotic force acting on the same chlorine ions is given by the difference between the osmotic pressure of the normal solution at B, and that of concentration at A According to p. 26, this is for the temperature 18** = 29r abs.—

84,688 X 291 X ^.f;^^ = 242 x 10'^ dynes. The force is therefore 2 x lO*"' times smaller than the

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