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

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Presentation on theme: "CHEM 213 Instrumental Analysis Lab Lecture – Ion Selective Electrode and HPLC."— Presentation transcript:

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

2 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. 1000:1 preference for F - over Cl -, Br -, I -, NO 3 -, SO 4 2- OH - is an interference Fluoride electrode – uses LaF 3 membrane to develop a potential that depends selectively on activity of F - (aq).

3 3 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 e.g. Ag-AgCl electrode … against which another half cell can be measured Determination of Fluoride in Mouthwash by Direct Potentiometry with an Ion-Selective Electrode

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

5 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 + H 2 O  F - + H 3 O + K a = 7 × 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., Cu 2+, Fe 3+ ) form fluoride complexes Cu 2+ + F -  CuF + - citrate added to complex metal ions

6 6 (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 M sodium citrate. Fluoride Electrode and Solution Conditions (Text: p317)

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

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

9 9 Procedure Comments (Fluoride Kit) Add TISAB to all solutions. Serial dilution to prepare standards 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 10 Pump Detector Integrator Column Injector MeOH Sample (20  L) Nitroaniline isomers by Liquid Chromatography Nonpolar particles methanol Time Absorbance p-nitroaniline o-nitroaniline Internal standard

11 11

12 12 Injector – introduces reproducible volume of solution onto column

13 13 Injector – introduces reproducible volume of solution onto column

14 14 3. Internal standard method NH 2 NO 2 NH 2 NO 2 OCH 2 CH 3 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) Time Absorbance p-nitroaniline o-nitroaniline Internal standard

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

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


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