CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 359
11.2.6 Hittorf method of determining transport numbers
The Hittorf method of determining transport numbers is based on the measurement of con-
centration changes. Let us consider an electrolyte dissociating into ions according to relation
(11.1). The vessel containing the electrolyte is divided into two compartments: the anode
compartment and the cathode compartment. We use the symbol ∆nanto denote the change
in the amount of substance of the electrolyte in the anode compartment (the amount after the
termination of the electrolysis minus the amount prior to its start). Similarly the symbol ∆ncat
will be used to denote the change in the amount of substance of the electrolyte in the cathode
compartment. Some of the following relations apply for transport numbers:
t+ = −z+ν+∆nan
Q/F, (11.18)
t+ =z+ν+∆ncat
Q/F, (11.19)
t− = −z−ν−∆ncat
Q/F, (11.20)
t− =z−ν−∆nan
Q/F. (11.21)
- Relation (11.18) is used when the cation is discharged at the cathode.
- Relation (11.19) is used when the cation reacts at the cathode and returns back to the
electrolyte. - Relation (11.20) is used when the anion is discharged at the anode.
- Relation (11.21) is used when the anion reacts at the anode and returns back to the
electrolyte.
Note:The Hittorf method cannot be used directly when ions are formed during reactions
at the electrodes which were not present in the initial electrolyte. During the electrolysis
of sodium chloride, e.g. the ions OH−are formed and start to participate in the charge
transport together with the initially present ions Na+and Cl−. In this case the Hittorf
method does not allow for determining what amount of charge has been transported by
the Cl−and OH−anions.