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TRANSFORT NUMBER OR TRANSFERENCE NUMBER
WHAT IS TRANSPORT NUMBER ?
“The fraction of the current carried by a given ionic
species in the solution is called the transport number
of Hittorf’s number or transference number of the ion
”.If v+ represents the speed of migration of the cation and v– that
of the anion,
the transport number of cation =
v+
v- + v+
the transport number of anion =
v-
v- + v+
And t++t-=1
4. MECHANISM OF IONIC TRANSPORT
The following cases may
now be considered:
• (i) Let the anions alone be
capable of movement.
• (ii) Let the anions and cations
move at the same
• (iii) Let the cations move at
twice the speed of the anions
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FIGURE: Mechanism of ionic
transport.
5. METHODS OF DETERMINATION OF TRANSFORT
NUMBER
1. Hittorf,s Method.
2. Moving boundary method.
ABNORMAL TRANSPORT NUMBERS
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Exhibits a marked deviation in transport number with
concentration.
Example:Cadmium iodide.
Explanation: In conc. Solution-
CdI2 + 2I CdI4
2-
6. FACTORS AFFECTING TRANSFORT NUMBERS
1. Variation of transport number with concentration
The variation of the transport numbers with the concentration of
the electrolyte was proposed by onsager,i,e.,
t=t0 -A √C
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For 1,1-electrolytes.
1.Cause of conc. Dependence:
2.The results lead to generalization that-
• When t+ is close to 0.5
• When t+ <0.5.
• when t+ >0.5.
8. 2.Variation of transport number with
temperature
Effects:
•t+ increases with the increase of temperature.
•t- decreases with the increase of temperature.
•Transport number greater than 0.5 decreases with the increase
in temperature.
•Transport number less than 0.5 increases with rise in
temperature.
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9. 3.Variation of transport number with electrolyte.
ELECTROLYTE t+
ELECTROLYTE t-
KCl 0.490 HCl 0.177
KBr 0.488 LiCl 0.662
KNO3 0.508 NaCl 0.605
KCl 0.510
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