Measurement of Transference Number: Moving boundary method:
Here briefly focus on Moving boundary method –
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 moving boundary method utilizes this principle. The arrangement is shown in Figure.
The solution of an electrolyte MA, which is to he studied, is placed between the solutions of two other salts MA’ and MꞌA such that MꞌA has the anion A common with MA and MA’ and MA have the common cation M. The salts are to be so chosen that the densities increase downwards. Also, the speed of the ion Mꞌ should be less than that of M while the speed of A’ should be less than that of A ion.
Figure: Moving boundary method
This is essential to maintain sharp boundaries between the three solutions of electrolytes. The initial sharp boundary between the solutions of MꞌA and MA is shown by the horizontal line P whereas the initial sharp boundary between MA’ and MA is shown by R. In passing current from the source B ionic migration starts and the boundary P moves downwards, say to P’, while the boundary R moves to R’. The distance travelled by the two boundaries in time t are PP’ and RRꞌ, which are directly proportional to the cationic and anionic velocities respectively. Therefore,
t+ = u+ / ( u– + u+) = PP’ / (PP’ + RRꞌ)
t– = u– / ( u+ + u–) = RR’ / (RR’ + PPꞌ) … …. … (14.23)
The transport numbers are thus measured. It should be noted that electrolytes MA, MA’ and M’A should he carefully chosen to get good results. In practice it is necessary to form one boundary and observe the rote of its movement to measure the transference number of one ion, that of the other ion may then be calculated as t+ + t– = 1.
Under this condition the transference number is calculated as follows: Suppose that the boundary of the moving cation is swept through a distance ‘l’ in a tube of cross section a, so that the volume swept out by the passage of Q coulombs of electricity is (l x a). If 1F of electricity flows through the solution t, equivalent mass of cation must pass through any given point. Let c, be the concentration of the solution in equivalent mass L-1; then the volume of solution contenting ‘l’ equivalent man of electrolyte is 1000/c. Hence during the passage of 1 F of electricity the canon boundary will sweep through a volume 1000/c t+. For the passage of Q coulombs, therefore, the same boundary will sweep out a volume of –
(1000 x t+ x Q) / (c x F)
Hence, l x a = (1000 x t+ x Q) / (c x F)
or, t+ = (l x a x c F) / 1000Q