Transport Number

In-depth explanation of Transport Number

The transport number of an ion is the fraction of the total current that it carries through an electrolyte, and the transport numbers of the cation and anion sum to one. It depends on the relative mobilities of the two ions, so an ion with high mobility carries a larger share of the current. Transport numbers are determined experimentally by the Hittorf method or the moving boundary method. They are needed to interpret conductivity and electrode reactions in concentrated solutions. The concept links ion motion to electrical measurements.

Going deeper: Transport Number

Key ideas: The transport (transference) number t+ is the fraction of current carried by cations: t+ = u+/(u+ + u-), and t+ + t- = 1. Examples at 25 C: KCl has t+ = 0.49, HCl 0.82, NaCl 0.39. Measured by Hittorf (concentration changes near electrodes) or moving boundary methods. Individual ion conductivity is lambda+ = t+ Lambda0.

Common mistakes and connections: Mistakes: saying both ions carry equal current, or that t depends only on charge. Mobilities matter. KCl and KNO3 are chosen for salt bridges since their t values are near 0.5, reducing junction potential. Transport numbers vary with concentration and temperature.

Visualization of Transport Number

In hydrochloric acid, H+ is so mobile that it carries about 82 percent of the current, so the transport numbers are about 0.82 and 0.18. The interactive lesson shows this as a live animation or chart.

Worked example and use case

In hydrochloric acid, H+ carries about 82 percent of the current because it is much more mobile than Cl-.

Where Transport Number fits in Acids, Bases & Electrochemistry

Transport Number is topic 85 of 105 in Acids, Bases & Electrochemistry.

Related topics

• Kohlrausch's LawAdvanced
• Strong and Weak ElectrolytesBeginner
• Ionic MobilityAdvanced
• Debye-Huckel TheoryAdvanced
• Ionic StrengthAdvanced
• Activity and Activity CoefficientsAdvanced

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