PROCEEDINGS 
OF THE 
Cambridge Philosophical Society. 
The Conductivity of Extremely Dilute Acid and Alkali 
Solutions. By H. H. Paine, M.A., B.Sc., and G. T. R. Evans, 
BSc. University College of Wales, Aberystwyth. 
[Received 18 August 1914.] 
I. 
The conductivity of water distilled with the usual precautions 
employing block tin or Jena glass apparatus, is of the order of 
1 x 10~ reciprocal ohm (1 gemmho) at 18° C. Distilling in vacuo, 
Kohlrausch has obtained water with a conductivity as low as 
0043 x10. This fact alone suggests that most of the contami- 
nation in the former is dissolved from the air. Direct evidence to 
this effect has recently been obtained by Bourdillon*, who has 
prepared water with a conductivity 0-086 x 10 by maintaining 
an atmosphere of pure air in the distilling apparatus. 
This residual impurity has often been regarded as the cause of 
the final drop, at extreme dilution, in the equivalent conductivity 
urves for acid and alkali solutions. It has been shown, however, 
that the presence of carbonic acid alone will not account for the 
observed phenomenon}. It has been suggested that a substance 
of the nature of ammonium carbonate must be present. 
Let us suppose that the distilled water contains as an impurity 
any substance (e.g. KOH or Na,CO,) which leads to an association 
of ions on the addition of a small amount of strong acid 
eg. H,SO,). The result will be that, as far as the conductivity 
of the solution is concerned, the first traces of the acid are used 
up and removed from activity—the conductivity of the new 
Solution being less than the sum of the conductivities of the 
* Journ. Chem. Soc., May 1913, p. 791. 
t Whetham and Paine, Proc. Roy. Soc., 81 a. p- 58 (1908). 
VOL. XVIII. PT, I. 1 
