Factors Affecting Transport Number
Factors Affecting Transport Number
Following factors govern the transport number of an ion in an electrolyte-
The transport number of cations depends upon the anions attached to it and vice-versa.
Effect of Concentration
The rate of migration of ions is affected by the concentration of electrolytic solution. The variation of transport number with concentration in dilute solution is shown as-
t = to - A√C
where t & to are the transport number of a given ion in a solution of concentration C and that extrapolated to infinite dilution respectively and A is a constant. The transport numbers change only slightly with increase in concentration unless complexation occurs. The transport number of Cd+2 decreases at low concentration to almost zero at higher concentration and becomes even negative at higher concentrations due to complexation-
CdI2 ⇌ Cd+2 + 2I−
CdI2 + 2I− ⇌ [CdI4]−2
Hence, some of the cadmiun forms a part of the anion and migrates towards the anode. So the fall of concentration around the anode is not as much as it should be. If [CdCl4]-2 moves faster than Cd+2, there will be actually an increase in concentration around the anode in place of decrease. This is the reason for the negative value of transport number at higher concentrations.
Effect of Temperature
Increase in temperature tends to gradually bring the two transport number closer to 0.05 i.e. ions have equal speeds at higher temperatures.
Effect of Hydration
Hydration of ions decreases transport numbers. Due to high ionic potential, Li+ ion is highly hydrated. Thus the highly hydrated Li+ is larger than the unhydrated Na+. So, the speed of Na+ in aquous solution is greater than that of Li+. Hence the transport number of Li+ ion is less than that of Na+.
