Presentation on theme: "CHEM 213 Instrumental Analysis Lab Lecture – Ion Selective Electrode and HPLC."— Presentation transcript:
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. 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 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 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 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 (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 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 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 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 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
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 Integrator Output in Nitroaniline Lab p-nitroaniline o-nitroaniline Internal standard Identifies compound Proportional to concentration Write sample/standard #