chem 213 instrumental analysis
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CHEM 213 Instrumental Analysis. Lab Lecture – Ion Selective Electrode and HPLC. Determination of Fluoride in Mouthwash by Direct Potentiometry with an Ion-Selective Electrode. - PowerPoint PPT PresentationTRANSCRIPT
CHEM 213Instrumental Analysis
Lab Lecture – Ion Selective Electrode and HPLC
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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
Magnetic stirrer
Magnetic stir bar
Reference Electrode Fluoride
Electrode
mV meter
Sample or standard
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-, NO3-, SO4
2-
OH- is an interference
Fluoride electrode – uses LaF3 membrane to develop a potential that depends selectively on activity of F- (aq).
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Potentiometry (Ch 15) – analytical method in which an electric potential difference (a voltage) of an electrochemical cell is measured
Magnetic stirrer
Magnetic stir bar
Reference Electrode Fluoride
Electrode
mV meter
Sample or standard
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
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Analytical Response for Fluoride Ion-Selective Electrode
Response = Nernst eqn
E = const - (0.0592V/z) log AF
AF = activity of F- (Sec. 8.2)
Eref is a constant
Ecell = EF- - Eref
E = const - 0.0592V log AF
If ionic strength is constant, concentration is proportional to conc
E = const - 0.0592V log [F-]
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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
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(c) Ionic strength of the standards and samples must be the same and constant
Solution: prepare standards and samples in TISABTISAB (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.
Fluoride Electrode and Solution Conditions (Text: p317)
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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
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Calibration Curve (LM, pg 80-81)
Low conc High conc
Plot data while measuring2 min if drifting
Rinse with dionized H2O
Blot sides of electrode
LinearSlope -59 mV
SampleReading
Log [Sample]
Sample bracketed by std?
Measure standards and samples again
Do repeat readings agree?
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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)
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Pump Detector
Integrator
ColumnInjector
MeOH Sample (20mL)
Nitroaniline isomers by Liquid Chromatography
Nonpolar particles
methanol
Time
Absorbance
p-nitroanilineo-nitroaniline
Internal standard
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Injector – introduces reproducible volume of solution onto column
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Injector – introduces reproducible volume of solution onto column
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3. Internal standard method
NH2
NO2
NH2NO2
OCH2CH3More 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)
TimeAbsorbance
p-nitroanilineo-nitroaniline
Internal standard
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Integrator Output in Nitroaniline Lab
p-nitroanilineo-nitroaniline
Internal standard
Identifies compound Proportional to concentration
Write sample/standard #
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Calibration Curves (Lab Manual, page 26-28)
Height p-nitroanilineHeight of internal std