1. CHEM 213 Instrumental Analysis
Lab Lecture – Ion Selective Electrode and HPLC

2. Determination of Fluoride in Mouthwash by Direct Potentiometry with an Ion-Selective Electrode
Potentiometry (Ch 15) – analytical method in which an electric potential difference (a voltage) of an electrochemical cell is measured
Ion selective electrode (ISE) - electrical potential across a selective membrane is dependent on the activity of a specific analyte ion.
Fluoride electrode – uses LaF3 membrane to develop a potential that depends selectively on activity of F- (aq).
1000:1 preference for F- over Cl-, Br-, I-, NO3-, SO42-
OH- is an interference

3. Determination of Fluoride in Mouthwash by Direct Potentiometry with an Ion-Selective Electrode
Potentiometry (Ch 15) – analytical method in which an electric potential difference (a voltage) of an electrochemical cell is measured
Ion selective electrode (ISE) - electrical potential across a selective membrane is dependent on the activity of a specific analyte ion.
Reference electrode – electrode that maintains a constant potential
… against which another half cell can be measured
e.g. Ag-AgCl electrode

4. Analytical Response for Fluoride Ion-Selective Electrode
Ecell = EF- - Eref
Eref is a constant
Response = Nernst eqn
E = const - (0.0592V/z) log AF
AF = activity of F- (Sec. 8.2)
E = const V log AF
If ionic strength is constant, concentration is proportional to conc
E = const V log [F-]

5. Fluoride Electrode and Solution Conditions (Text: p317)
Issues: electrode responds to F- (not HF)
OH- is a significant interference
response is affected by ionic strength of sample
Solution: TISAB to control solution conditions
(a) Control pH
HF + H2O  F- + H3O Ka = 7 × 10-4
so... pH must not get too low (Sec 10-5, page 195)
OH- interferes, so… pH cannot be too high
Solutions buffered at pH = 5 (acetate buffer)
(b) Some metal ions (e.g., Cu2+, Fe3+) form fluoride complexes Cu F-  CuF+
- citrate added to complex metal ions

6. Fluoride Electrode and Solution Conditions (Text: p317)
(c) Ionic strength of the standards and samples must be the same and constant
Solution: prepare standards and samples in TISAB
TISAB (Total Ionic Strength Adjustment Buffer)
- contains: 1 M sodium chloride
1 M sodium acetate  to pH 5 with acetic acid
1 x 10-3 M sodium citrate.

7. Fluoride Expt. Serial Dilutions (LM Page 30-31)
Perform dilutions using TISAB solution prepared in step 1 (LM pg 30)
Prepare mouthwash sample using TISAB
Solution prep: hours

8. Calibration Curve (LM, pg 80-81)
2 min if drifting
Plot data while measuring
Rinse with dionized H2O
Sample
Reading
Blot sides of electrode
Linear
Slope  -59 mV
Sample bracketed by std?
Do repeat readings agree?
Log [Sample]
Measure standards and samples again
Low conc  High conc

9. Procedure Comments (Fluoride Kit)
Add TISAB to all solutions.
Serial dilution to prepare standards.
1-1.5 hours
Sign-up to use potentiometer (1 hour starting at 3 pm).
Calculations (Lab. Manual: Page 33 and 80-83).
- calibration curve: plot millivolts vs log[F-] CAREFULLY
- Appendix A (pg 73)
- Graphical method (line by eye)
- least squares fit (Monday Jan 12)

10. Nitroaniline isomers by Liquid Chromatography
Absorbance
Time
p-nitroaniline
o-nitroaniline
Internal standard
Pump
Detector
Integrator
Column
Injector
MeOH
Sample (20mL)
methanol
Nonpolar particles

12. Injector – introduces reproducible volume of solution onto column

13. Injector – introduces reproducible volume of solution onto column

14. 3. Internal standard method
NH2
NO2
NH2
NO2
OCH2CH3
More polar
Internal standard – known quantity of cpd added to sample
- conc of analyte measured relative to conc of IS
-corrects for systematic errors (e.g., losses)
Absorbance
Time
p-nitroaniline
o-nitroaniline
Internal standard

15. Integrator Output in Nitroaniline Lab
Write sample/standard #
p-nitroaniline
o-nitroaniline
Internal standard
Identifies compound
Proportional to concentration

16. Calibration Curves (Lab Manual, page 26-28)
Height p-nitroaniline
Height of internal std