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XIII. On the Accurate Measurement of Ionic Velocities, with Applications to 



Various Ions. 



By R. B. DENISON, M.Sc., Ph.D., and B. D. STEELE, D.Sc, 

 Communicated by Sir WILLIAM RAMSAY, K.C.B., F.R.S. 



Received October 14, Read November 16, 1!)05. 



ACCORDING to ARRHENIUS' theory of electrolytic dissociation, the conduction of the 

 current in a salt solution is due to the presence of free ions, which, Tinder the 

 influence of an electromotive force, move towards the electrodes with a velocity 

 depending, other conditions being equal, upon the magnitude of the driving force, or 

 fall of potential. This "ionic velocity" can be determined by means of two quite 

 distinct methods, of which one may be termed the indirect and the other the direct 

 method. The former, or indirect method, was evolved by KOHLRAUSCH on his 

 recognition of the law of the independent migration of the ions, which he thus states : 

 " The molecular conductivity, p, of a solution is proportional to the sum of the 

 velocities of the anion and of the cation, p. = constant x (u + v)." 



The ratio of these velocities, u/v, had been determined many years previously by 

 HITTORF, whose " Uberfuhrungszahl," or transport number, p = u/(u + r), for any salt 

 represents the fraction of the total current that is carried by the anion. 



The knowledge of this ratio enabled KOHLRAUSCH to calculate the ionic velocities 

 from the molecular conductivity. In order to calculate the velocities of the ions 

 by the indirect method it is, therefore, necessary to know both the molecular 

 conductivity of the solution and its transport number. Although the determination 

 of the former is perfectly easy and straightforward, that of the latter by HITTORF'S 

 analytical method is both difficult and laborious, and the method suffers from the 

 great disadvantage that the success or failure of an experiment is known only after 

 the necessary chemical analyses have been completed. 



In recent years a method of measuring transport numbers and ionic velocities has 

 been worked out in which the actual rate of motion of the ions is read off by means 

 of a scale and telescope. The first steps in this direction were taken in 1886 by 

 LODGE (' British Assoc. Reports,' 1886, p. 389), whose idea was to make the invisible 

 ion indicate its presence by some characteristic physical or chemical property, such as 

 its colour or the formation of a precipitate with some other ion. For the gradual 



VOL. CCV. A 399. 3 M 12.3.06 



