# 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 = t_{o} - A√C

where t & t_{o} 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-

CdI_{2} ⇌ Cd^{+2} + 2I^{−}

CdI_{2} + 2I^{−} ⇌ [CdI_{4}]^{−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 [CdCl_{4}]^{-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^{+}.

## Measurement of Transference Number by Moving Boundary Method

## Measurement of Transference Number by Hittorf Method

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