Page:Text-book of Electrochemistry.djvu/151

 ��CONDUCTIVITY OF ELECTROLYTES. chap.

��this with dilution indicates that, on addition of water, more ions capable of transporting electricity are formed at the expense of the undissociated molecules. In this respect we may consider as types, ammonia and acetic acid. With increasing dilution, X» assumes greater and greater values, and it is difficult to find that X, approaches a certain limit, A„, which, nevertheless, can be ascertained in an indirect manner.

The Clausius hypothesis aids us in this determination. When the part of the electrolyte dissociated into ions is only a small fraction of the whole number of molecules present, the quantity of ions, and therefore also X,, must increase on dilution from 10 to 100 by about the same amount as on dilution from 100 to 1000, etc., which is actually the case for the types of weak bases and acids mentioned.

The other substances mentioned — H2SO4, HCl, MgS04, K2SO4. CHaCOOK, NaNOs, NaCl, and KCl, which may be regarded as types of good conductors — behave otherwise. At high concentrations the increase in X, for dilution to double the volume is tolerably great ; thus, e,g., for KCl the difference Xio - Xj = 407 ; for HCl, X^o - X^ = 9. As the following numbers show, this increase diminishes at higher dilution : —

��Sabetanoe.

�KCl HCl

��^1000—^000 ^2000—^1000 ^loooo-^aooo

1 I —

��0-3

��The increase of X evidently approaches the value zero with increasing dilution, when the concentration is always changed in the same ratio, or in other words, X converges with increasing volume {v) to a limiting value X^. The same conclusion is arrived at, but perhaps not quite so clearly, by considering the conductivities of sulphuric acid and magnesium sulphate.

The only possible cause of the fact that the decomposition into ions reaches a certain limit is that ultimately all the molecules are dissociated ; or we may say that at very great

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