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CHEM 213 Instrumental Analysis

CHEM 213 Instrumental Analysis. Lab Lecture – Ion Selective Electrode and HPLC. Determination of Fluoride in Mouthwash by Direct Potentiometry with an Ion-Selective Electrode.

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CHEM 213 Instrumental Analysis

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  1. CHEM 213Instrumental 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 - 0.0592V log AF If ionic strength is constant, concentration is proportional to conc E = const - 0.0592V 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 Cu2+ + 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: 1-1.5 hours

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

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

  11. Injector – introduces reproducible volume of solution onto column

  12. Injector – introduces reproducible volume of solution onto column

  13. Absorbance Time NH2 NH2 NO2 More polar OCH2CH3 NO2 3. Internal standard method 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) p-nitroaniline o-nitroaniline Internal standard

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

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

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