07-12-2012, 03:37 PM
Electroanalytical chemistry
[attachment=42953]
Electroanalysis
measure the variation of an electrical parameter (potential, current, charge, conductivity) and relate this to a chemical parameter (the analyte concentration)
Conductimetry, potentiometry (pH, ISE), coulometry, voltammetry
Potentiometry
Measure difference in potential
between two electrodes:
reference electrode (E constant)
indicator electrode (signal α analyte)
the measure of the cell potential to yield chemical information (conc., activity, charge)
Ion selective electrodes (ISEs)
A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potential difference being created across that membrane
Mass Transport or Mass Transfer
Migration – movement of a charged particle in a potential field
Diffusion – movement due to a concentration gradient. If electrochemical reaction depletes (or produces) some species at the electrode surface, then a concentration gradient develops and the electroactive species will tend to diffuse from the bulk solution to the electrode (or from the electrode out into the bulk solution)
Convection – mass transfer due to stirring. Achieved by some form of mechanical movement of the solution or the electrode i.e., stir solution, rotate or vibrate electrode
Difficult to get perfect reproducibility with stirring, better to move the electrode
Convection is considerably more efficient than diffusion or migration = higher currents for a given concentration = greater analytical sensitivity
Coulometric Titration
Due to concentration polarization it is very difficult to completely oxidize or reduce a chemical species at an electrode. Coulometry is therefore usually done with an intermediate reagent that quantitatively reacts with the analyte. The intermediate reagent is electrochemically generated from an excess of a precursor so that concentration polarization does not occur. An example is the electrochemical oxidation of I- (the precursor) to I2 (the intermediate reagent). I2 can then be used to chemically oxidize organic species such as ascorbic acid.
DPP
current measured twice during the lifetime of each drop
difference in current is plotted.
Results in a peak-shaped feature, where the top of the peak corresponds to E1/2, and the height gives concentration
This shape is the derivative of the regular DC data.
DPP has the advantage of sensitive detection limits and discrimination against background currents. Traditionally, metals in the ppm range can be determined with DPP.
Derivative improves contrast (resolution) between overlapping waves
[attachment=42953]
Electroanalysis
measure the variation of an electrical parameter (potential, current, charge, conductivity) and relate this to a chemical parameter (the analyte concentration)
Conductimetry, potentiometry (pH, ISE), coulometry, voltammetry
Potentiometry
Measure difference in potential
between two electrodes:
reference electrode (E constant)
indicator electrode (signal α analyte)
the measure of the cell potential to yield chemical information (conc., activity, charge)
Ion selective electrodes (ISEs)
A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potential difference being created across that membrane
Mass Transport or Mass Transfer
Migration – movement of a charged particle in a potential field
Diffusion – movement due to a concentration gradient. If electrochemical reaction depletes (or produces) some species at the electrode surface, then a concentration gradient develops and the electroactive species will tend to diffuse from the bulk solution to the electrode (or from the electrode out into the bulk solution)
Convection – mass transfer due to stirring. Achieved by some form of mechanical movement of the solution or the electrode i.e., stir solution, rotate or vibrate electrode
Difficult to get perfect reproducibility with stirring, better to move the electrode
Convection is considerably more efficient than diffusion or migration = higher currents for a given concentration = greater analytical sensitivity
Coulometric Titration
Due to concentration polarization it is very difficult to completely oxidize or reduce a chemical species at an electrode. Coulometry is therefore usually done with an intermediate reagent that quantitatively reacts with the analyte. The intermediate reagent is electrochemically generated from an excess of a precursor so that concentration polarization does not occur. An example is the electrochemical oxidation of I- (the precursor) to I2 (the intermediate reagent). I2 can then be used to chemically oxidize organic species such as ascorbic acid.
DPP
current measured twice during the lifetime of each drop
difference in current is plotted.
Results in a peak-shaped feature, where the top of the peak corresponds to E1/2, and the height gives concentration
This shape is the derivative of the regular DC data.
DPP has the advantage of sensitive detection limits and discrimination against background currents. Traditionally, metals in the ppm range can be determined with DPP.
Derivative improves contrast (resolution) between overlapping waves