Page 183 - Analytical Electrochemistry 2d Ed - Jospeh Wang

168 POTENTIOMETRY

Example 5-2 The following potentials were observed for a calcium electrode
immersed in standard calcium solutions:

[Ca 2‡ M E (mV)

1 10 5 100
1 10 4 129
1 10 3 158

What potential is expected for a calcium concentration of 5 10 4 M? (Assume
activity coef®cient of 1.0.)

Solution Plotting E vs. log[Ca 2‡ ] gives a straight line with a slope of 29 mV=
decade and an intercept of 245 mV. A calcium concentration of 5 10 4 M thus
yields:

4
E ˆ K ‡ 29 log ‰5 10 Šˆ 245 95:7 ˆ 149:3mV

EXAMPLE 5-3 Calculate the error in millivolts that would occur if a solution
containing 5 10 5 MF (pH 10) were measured with a ¯uoride ISE.
k F;OH ˆ 0:1†

Solution The concentration of the interfering hydroxyl ion at pH ˆ 10 can be
obtained:

‡
‰OH Šˆ K =‰H Šˆ 10 14 =10 10 ˆ 10 4 M
w
From equation (5.4) we obtain:

4
E cell ˆ K 0:059 log 5 10 5 ‡ 0:1 10 †ˆ K ‡ 0:249

In the absence of hydroxyl ion:

5
E ˆ K 0:059 log 5 10 †ˆ K ‡ 0:254
cell
Therefore the error in mV is

Error ˆ K ‡ 249 K ‡ 254†ˆ 5mV