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