Transport Number or Transference Number or Hittorf Number
Transport Number or Transference Number or Hittorf Number
The fraction of the total current carried by an ion in an electrolytic solution is known as transport number. It is denoted by letter 't'. For cation it is represented as t+ and for anion it resepresented as t–.
Sum of transport numbers of different ions is always one. It is unitless quantity.
t+ + t− = 1
We know that the value of ionic conductivities or mobilities are proportional to speeds of the ions
i.e. λ+ ∝ U and λ− ∝ V
or, λ+ = F.U
and λ− = F.V
Transport number of Cation (t+) = U/(U + V)
or, t+ = λ+/(λ+ + λ-)
or, t+ = λ+/Λ
or, Λ = t+/λ+
Similarly,
Transport number of Cation (t−) = V/(U + V)
or, t− = λ−/(λ+ + λ-)
or, t− = λ−/Λ
or, Λ = t−/λ−
where U and V are speed of cations and anions respectively.
In some conditions, transport number is either greater or less than its theoretical value called abnornal transport number.
Transport number can be determined by Hittorf's method, moving boundary method, emf method and from ionic mobility.
Factors Affecting Transport Number
Temperature
On increasing temperature, the transport number of cation and anion approches closely to 0.5. It means that the transport number of an ion, if less than 0.5 at the room temperature increases and if greater than 0.5 at room temperature decreases with rise in temperature.
Concentration of the electrolyte
Transport number generally varies with the concentration of the electrolyte.
Nature of the other ions present in solution
The transport number of anion depends upon the speed of the anion and the cation and vice versa.
Hydration of ion
Transport number increases when degree of hydration decreases.
Factors Affecting Transport Number in details
