Table 24.1 emf as a function of the titrant added in a potentiometric titration.
Vol of Ce4+ added
Excess ml of
[Fe2+] / [Fe3+]
[Ce3+] / [Ce4+]
E in volt
Fe2+
Ce4+
1
99
-
99/1
0.771- 0.59 log (99/1)
= 0.652
10
90
90 /10
0.771- 0.059 log (9)
= 0.715
50
0.771
91
9
9/ 91
0.771- 0.059 log (9/ 91)
= 0.830
1/ 99
0.771- 0.059 log (1/ 99)
= 0.889
99.9
0.1
0.1 / 99.9
0.771 – 0.059 log (1/999)
= 0.948
100.0
Equivalence point
1.19
100.1
100 /0.1
1.61-0.059 log (100/0.1)
1.433
101.0
1.0
100 /1
1.61 – 0.059 log (100/1)
1.492
110.0
10.0
100/10
1.61 – 0.059 log (10)
1.551
120.0
20
100/20
1.61- 0.059 log (5)
It will be seen that the change in potential within 0.1 ml of the equivalence point is quite high. i.e.. 0.485V (1.433 - 0.948)
