ACCtTKATE MEASUEEMENT OF IONIC VELOCITIES, ETC. 



455 



movement of the margin in the measuring tubes A due to this cause is infinitesimal 

 and can be neglected. In our earlier experiments, in which the apparatus was tested 

 with potassium chloride, the wide tubes B were open to the atmosphere, but as the 

 potassium and chlorine ions have practically the same velocity, the effect described 

 above did not manifest itself. But with n/10 sodium chloride the introduction of the 

 capillary tubes K changed the transport number for chlorine from 0'58 ( J to its correct 

 value, 0'614. 



The electrical connections and the method of illuminating the boundary are the 

 same as those described in_ previous papers.* 



Testing the Method. 



The only difficulty in the present method has been found to lie in the choice of 

 suitable indicators. The transport number of any ion in a given salt should be the 

 same at the same concentration, provided that the indicator used fulfils certain 

 conditions, which are briefly as follows : 



(1) The specific velocity of the indicator ion must be less than that of the ion 

 whose progress it indicates. 



(2) It must not react with any of the ions of the -salt being measured, nor must it 

 be hydrolysed or give rise to any other ion moving in the same direction as the 

 indicator ion. 



(3) The resistance of the indicator solution must not be too great, -i.e., the 

 concentration of ions in the indicator must not be too small, nor must there be too 

 great a difference between the specific ionic velocity of the indicator ion and that of 

 the ion whose velocity is being measured. 



The method was first tested by measuring the transport numbers of potassium and 

 sodium chlorides with different indicators, and the following results (Table 1.) were 



obtained : 



TABLE 1. 



* STEELE, loo. dt. DENISON, loc. at. 



t From KOHLRAUSCH and HOLBOKN'S ' Leitrermogen der Electrolyte,' p. 201. 



