Ion transport number

A transference number is the fraction of the total electric current is referred to, which is transported by a particular ion species. In honor of the German physical chemist Johann Wilhelm Hittorf this size is also called Hittorf transference number.

The transference number of the ion i can be defined mathematically as follows:

Transference number of the ionic species i

Electric current is carried by these ions

Total electric current

The transference number is dependent on the ion concentration and ion mobility. Both dependencies are easy to understand: If the concentration of ion species i high, of these ions, a large part of the electricity can be transported. Similarly, the connection with the ion mobility: Fast ions are able to carry a larger portion of the stream as a slow.

Determination of the transport number

The method Hittorf

To determine the transport number is a split into three rooms electrolysis apparatus is suitable. This is composed of the cathode compartment, the central space and the anode compartment. In the cathode and anode space platinum electrodes are immersed. The electrolysis rooms are filled with the electrolyte and connected by bridge. Looking at the two electrode chambers, it will be seen that the concentrations of the cations and anions vary differently.

In this experiment, we calculate the transport number as follows:

Transport number of the cations

Transference number of anions

The ratio between the difference of qA beginning and end of qE in the corresponding space charge present amount, and the amount of charge that has flowed through the electrolyte Qges.

The Endladungsmenge is composed as follows:

With the definition of the amount of charge and then dissolving the above equation to get the following result.

Definition quantity of charge:

F = Faraday constant; z = number of charges; c = concentration of the observed ion; V = volume of the electrolysis chamber

Based on this equation can be determined by a concentration, power and volume determination of an electrode space, the transference number.

From the known molar boundary conductivities of ions and the migration rates and thus the transport numbers of ions can be determined in electrolysis. At an electrolysis some ions migrate rapidly (eg, H , OH - ) other quite slow (Li , CH3COO - ).

For the transport numbers of cations: t = * /.

For the transport numbers of anions: t - = * /.

It is: t- t = 1

The method of moving boundary surface

The interface of two abutting electrolytes shifts under the influence of an electric field. If one uses a colored ion and manages to keep the interface during the experiment reasonably sharp, so you can determine the transference numbers of the speed of this shift.

In principle, it is sufficient even to measure the transfer speed of any ion. The ion mobilities of all other ions can then be determined from conductivity measurements by combination with this ion. The equivalent ionic mobilities of most ions in aqueous solution are enclosed with 3-10 · 10m / (val ). Only the proton and the hydroxide ion are much more mobile. This is due to the special motion mechanism of these ions: to the normal ion migration occurs here is a synchronous, abrupt change of place of many protons via intermolecular hydrogen bonds.

Application reference

When salt bridges is taken to ensure that the transport numbers of anions and cations are approximately equal. A salt bridge, KCl is therefore composed of two different ions, which have approximately the same ionic mobility.

• Electromagnetic Theory
• Electrochemistry