50 



IONISATION 



1. Electrical Charge. It is obvious that if two electrodes from 

 a source of supply be dipped into a solution containing ions, 

 that in virtue of their charge the anions (negative ions) will be 

 attracted towards the anode (positive electrode) and the cations 

 (positive ions) will be drawn towards the cathode (negative 

 electrode). Such a solution will therefore conduct electricity, 

 and further, its efficiency as a conductor will depend on the 

 number of ions present, i.e. on the dissociation of the solute. 



This provides the basis on which the method for estimating 

 the concentration of ions has been devised. In doing this the 

 electrical resistance, in ohms, is measured. Conductivity is the 

 reciprocal of resistance (Part II. p. 415). 



Relative Speed. Another factor must be taken into account. 

 We have seen that ions do not all move at the same rate. The 

 rate depends on the atomic weight of the ions, the degree of 

 hydration and the influence of other ions. (Ions by their electric 

 charge influence one another.) Under similar conditions, each 

 ion moves at a constant rate. 



That the rate is slow is shown by passing an electric current 

 through a solution containing coloured ions at one electrode and 

 noting the time they take to reach a similar concentration at the 

 other electrode. Kohlrausch determined the relative speed of 

 ions at 18 C. and for a constant potential gradient found as 

 follows : 



TABLE VI. 



CATIONS + 



ANIONS - 



Bredig has pointed out that with organic ions the longer the 

 carbon chain, the less the speed. The decrease in speed is, how- 

 ever, comparatively slight. 



2. Hydration. Consideration of the numerical values of the 

 relative ionic rates gives a means for calculating the hydration 

 of the various ions. It may be taken for granted that the speed 



