Page:Text-book of Electrochemistry.djvu/200

 explained, it is quite certain that the degree of dissociation found from the osmotic method generally agrees closely with that found by the electrical method Such an agreement was essential, in order that the idea of electrolytic dissociation might be valid.

The most important objection which has been raised by chemists is that salts in solution show a higher osmotic pressure than corresponds with their chemical formulae, and this seems to correspond with the phenomenon that a gas — as, for instance, ammonium chloride — shows a higher gas pressure than would be expected from the composition of the mole- cule NH4C1. In this latter case it was admitted that the deviation from the gas laws was only apparent and due to the decomposition of the molecule into simpler constituents (NHg and HCl). However at that time it could be shown that the products of dissociation might be separated by diffusion, and the question arose why it is not possible to effect a similar separation by diffusion of the products of dissociation of sodium chloride (sodium and chlorine).

The explanation of our inability to effect this separation

lies in the extraordinarily high chaige of 96,500 coulombs

per equivalent which the products of electrolytic dissociation,

i,€, the ions, receive, whilst the products from an ordinary

dissociation remain unelectrified. If we had a layer of pure

water over a solution of sodium chloride, then if this charge

had no influence, the chlorine, which is appreciably more

mobile than the sodium (in the ititio 68 : 45), would be found

to be in excess in the upper layer. Let us assume that

10"^'^ gram-equivalents more of chlorine than of sodium have

diffused into the pure- water layer, then this would have a negative charge of --' ^— coulombs, or 290 electrostatic

units, a quantity of electricity which, if brought on to a sphere of 10 cms. radius, would be able to give a spark 0*2 cm. long. Now, it can easily be shown that the electric (electromotive) forces which would be exercised by even so small a quantity (10"^'^ gram-equivalent) would far exceed

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