Methods for Determination of Transport Number


The transport number of an ion can be experimentally determined by means of  the following two methods




This method is based on the principle that the concentration changes occur around the electrodes due to the migration of ions. The apparatus used consists of a U-shaped tube, connected on one side to the “anode limb” and on the other side to the “cathode limb”. The anode is made of silver and cathode is made of platinum metal. The U-tube, the anode limb and cathode limb are filled with a standard solution of Ag NO3. The apparatus is connected in series to a silver voltmeter. A steady current of about 0.01 amperes is passed for about two to three hours. After that, the stopcocks S1 and S2 are closed. The whole of the solution from anode limb is taken out through stopcock S3 in a weighed flask. The silver content of this solution is determined by Volhard’s method. The weight of silver deposited in the silver voltameter is also determined.

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This method is based on the direct observation of the migration of ions under the influence of an applied electric field. Let us consider Hydrochloric acid and the transport numbers of H+ and CI ions present in HCI have to be determined. The hydrochloric acid is called the principal or experimental electrolyte. Another electrolyte containing a common ion (Cl-) called the indicator electrolyte is selected. The speed of the cation of the indicator electrolyte is selected. The speed of the cation of the indicator electrolyte should be less than that of the cation of the experimental electrolyte. For example: CdCl2 can be selected as the indicator electrolyte as the speed of Cd++ ion is less than that of H+ ions. A concentrated solution of CdCl2 is used, so that HCI solution can float on it.

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