Chemistry 65.2302*
Assignment 3
Ion Selective Electrode Potentiometry
1.
What is potentiometry? What is ion-selective electrode potentiometry?
2.
What regulatory level has been set for F- in drinking water? Is there any adverse health effect if
the F- level is too high?
3.
Does river water meet the regulatory F- level?
4.
How are ion-selective electrodes used in potentiometry? What does the set-up require?
5.
Is the instrumentation for ISE potentiometry complicated and expensive?
6.
Are ion-selective electrodes good for in-situ, continuous monitoring? Explain why or why not.
7.
Explain what it means by "ion-selective electrode potentiometry is a non-destructive method of
analysis". Can you name one destructive method?
8.
Is the operation of ion-selective electrodes affected by the colour or turbidity of a sample
solution? Explain.
9.
How many different classes of ion-selective electrodes are there?
10.
Can ion selective electrode potentiometry be applied for the determination of (a) NH3, (b) Cd2+,
(c) SO42-, (d) NO3-, (e) CO2 and (f) Cl-?
11.
How many different kinds of ion selective electrodes are commercially available from ElectroChemical Devices for in-line monitoring of industrial process streams?
12.
Draw a schematic diagram with labels to illustrate how a potentiometer can be used with an external
reference electrode and a F- ion selective electrode to monitor the F- concentration in the Rideau River
water.
13.
What kind of solid-state membrane is used in the fluoride ion selective electrode for continuous
monitoring of municipal water supplies?
14.
Why is it important to have 0.1 M NaF and 0.1 M NaCl in the internal filling solution of a
fluoride ion-selective electrode?
15.
What ionic species interfere badly with the proper operation of a fluoride ion-selective electrode?
How can you avoid such interference?
16.
Describe how F- ions can migrate through a LaF3 crystal doped with EuF2.
17.
Use the law of thermodynamics to derive an expression for the membrane potential.
18.
Bonus question How would you explain why the migration of F- ions (from the sample solution
towards the internal filling solution) can establish a membrane potential across the LaF 3 crystal?
19.
The membrane potential equation predicts that for every factor-of-ten difference in F- activity in
the sample solution, the cell potential (Ecell) at 25oC will change by 59.2 mV. How much will the
cell potential change if the F- activity in the sample solution drop by a factor of 25?
20.
21.
Bonus question
state membrane?
What can foul, poison and/or damage an ion-selective electrode with a solid-
A measurement of the potassium concentration difference between the interior and exterior of kidney
tubules (microscopic tubular structures involved with excretion) were made using a double
microelectrode. Two microelectrodes are fused together side-by-side. One electrode is a reference
electrode. The other is a K+ ion-selective electrode with a liquid membrane. The response of the ionselective electrode to changes in potassium activity is +50 mV per decade increase in aK+. The activity
coefficient for potassium at the solution ionic strength is 0.77. With the double electrode, the
measurement inside the kidney tubule produces a potassium ion-selective electrode potential of +68 mV
relative to the potential measured with the double electrode in the solution surrounding the outside of the
tubule. A spectrometric method was used to show the external potassium concentration to be 2.5 mM.
(a) What is the external potassium activity?
(b) What is the internal potassium activity?
(c) What is the internal potassium concentration?
22.
In the determination of F- by ion selective electrode potentiometry, how is the cell potential measured
and how is it determined by the activity of F- in the sample solution?
23.
Selectivity of Li+ ion-selective electrode: Problem 15-35 on p. 409 in Harris (5th edition).
Problem 15-36 on p. 345 in Harris (6th edition).
24.
The smaller the value of kXY, the more selective is the ISE for analyte ion X in the presence of
interferent ion Y. Does this mean a lower (or higher) degree of interference by Y in the determination
of X?
25.
The selectivity coefficient, kLi,Ca for a lithium ion-selective electrode is 5x10-5. When this electrode is
placed in 3.44x10-4 M Li+ solution, the potential is -0.333 V vs. SCE. What would the potential be if Ca2+
were added to give 0.100 M Ca2+?
26.
Bonus question An ISE used to measure the cation M2+ obeys the equation: Ecell = constant + 0.05968/2
log([M2+] + 0.0013 [Na+]2). When the electrode was immersed in 10.0 mL of unknown containing M2+ in
0.200 M NaNO3, the reading was -163.3 mV. When 1.00 mL of 1.07x10-3 M M2+ (in 0.200 M NaNO3)
was added to the unknown, the reading increased to -158.8 mV. Find the concentration of M2+ in the
original unknown.
27.
An analyst is using an ion selective electrode to determine [CN-] in water samples that vary greatly in
salinity -- all the way from freshwater to seawater. How can he overcome the effect of NaCl upon the
activity coefficient of CN- so that, using a calibration curve based upon standard NaCN solutions, he
can convert voltage measurements into cyanide concentrations?
28.
Write an equation for Ecell to show how you can quantitatively account for the presence of two
different interferent ions (Y1 and Y2) in the determination of X by ISE potentiometry.
29.
Bonus question
One way to determine the value of selectivity coefficient (kX,Y) is the mixed
solution method. In this method, the potentials of solutions containing a fixed concentration of the
interferent, Y, and varying concentrations of the analyte ion, X, are measured. A plot of Ecell versus log
[X] is prepared. As the concentration of X becomes low, the potential response levels off and
eventually becomes constant when interference is complete. Sketch the plot, label the Nernstian
response, indicate the interference, and show how kX,Y can be determined. What concentrations of the
interferent can one use in the mixed solution method?
30.
Sketch the response of an iodide ion selective electrode to [I-] in 0.1 M KNO3 in a graph. How would you
explain (a) the slope of -60.9 mV, and (b) a flat response at [I-] < 10-6 M?
31.
Are Eint. ref., Eext. ref.. and Ejunction constant, or do they vary with time and experimental conditions?
32.
What is the working principle of a Ca2+ ISE with a liquid ion-exchanger membrane?