Page 184 - Analytical Electrochemistry 2d Ed - Jospeh Wang

QUESTIONS 169

Example 5-4 A student calibrated a Mg 2‡ ion-selective electrode using two

standard solutions at 25 C and constant ionic strength. He obtained the following
results:
[Mg ‡ 2 ] (M) E(mV)

1 10 3 142
1 10 4 113

What is the concentration of the test solution that gave a potential reading of 125 mV
under the same conditions?

Solution Plotting E vs. log[Mg 2‡ ] gives a straight line; the magnesium concen-
tration (2.6 10 4 M) corresponding to the 125 mV reading can be read directly
from the axis.

EXAMPLE 5-5 Calculate the error caused by sodium ion, a Na ˆ 0:01, in the
measurement of lithium, a ˆ 0:01, using a lithium ion-selective electrode
Li
k Li;Na ˆ 0:06
Solution From equation (5.6), we obtain

E ˆ K ‡ 0:059 log‰0:001 ‡ 0:06 0:01†Š ˆ K 0:165 V

Without sodium the potential is

E ˆ K ‡ 0:059 log 0:001†ˆ K 0:177 V

Thus

Error ˆ 0:012=0:177† 100 ˆ 6:8%

QUESTIONS

1. Discuss clearly the structural requirements for designing selective ionophores
for ISE work. Give examples of such structures.
2. Explain clearly (using equations) why a highly selective ISE is not always
suf®cient for accurate potentiometric measurements.
3. Explain how the ion-recognition event is translated to the response of the
corresponding ISE.

4. Explain clearly (using equations) why the sodium ISE is more sensitive than the
calcium one.
5. Describe the source of errors in pH measurements using the glass pH electrode.