1902.] 



Conductivity of Electrolytic Solutions. 



53 



lower conductivity zero of the solution ; but as this temperature is 

 approached the curve probably turns aside and becomes asymptotic to 

 the axis of temperature. 



Above the temperature of maximum conductivity the conductivity 

 also decreases, the decreasing viscosity being now more than counter- 

 balanced by the decreasing ionisation of the solution. The decay of 

 ionisation becomes more rapid as the temperature rises, and if the 

 solute is not an electrolyte per se, the curve EF runs steadily down and 

 cuts the axis at T 4 , the critical temperature of the solution, which is 

 thus the upper conductivity-zero of the solution. In the case of very 

 dilute solutions this would, of course, be identical with the critical 

 temperature of the solvent. If, however, the solute is capable of self- 

 ionisation, the curve EFG tends towards an upper conductivity zero 

 at T 5 a few degrees above the critical temperature, but as the critical 

 temperature is approached the conductivity falls abruptly along GH 

 to a value comparable with that which persists in the gaseous state HI. 



[Note added August 2, 1902. — The general scheme of fig. 3 serves to 

 bring into prominence at least one important point that has been very 

 generally overlooked, namely, that the normal form of the conductivity- 

 temperature curve for a composite electrolyte is one which contains a point of 

 inflexion. Two such points are shown in fig. 3, between B and G and 

 between E and F. Of these, the former should be frequently observed 

 in aqueous solutions, seeing that these give values lying on parts of 

 the curve both above and below the inflexion. A widespread impres- 

 sion exists, however, that an inflexion in the conductivity-temperature 

 curve indicates some abnormal change in the character of the solution. 

 This impression has been strengthened, if indeed it has not been 

 created, by the general adoption of a linear or parabolic formula to 

 express the influence of temperature on conductivity. Kohlrausch, 

 in his detailed review of the literature of the subject,* makes no 

 reference to the existence of inflected curves, and does not even hint at 

 the possibility of curves of this type. Trotsch, who observed inflexions 

 in the case of a number of sulphates and the chlorides of copper and 

 cobalt,! regarded them as due to the decomposition of hydrates 

 existing in the solution, whilst Donnan and Bassett, in a paper which 

 has only just appeared, % quote the inflexion observed by Trotsch as 

 evidence for the existence of a complex ion in solutions of cobalt 

 chloride. 



According to the views here put forward, the conductivity-tempera- 

 ture curves are all inflected. In the case of the acids, which owe their 

 conducting power entirely to the action of the solvent, the inflexion 



* ' Leitvermogen der Elektrolyte,' pp. 116-123 and 195-199. 

 + ' Ann. Phys. Chem.,' 1890 [iii], vol. 41, pp. 259-287. 

 X 'Jour. Chem. Soc.,' vol. 81, p. 953, August, 1902. 

 VOL. LXXI. F 



