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to determine the concentration of analyte, we measure potential differences, with little or no current passed
to determine the concentration of analyte, we measure current differences, by applying a potential, to drive a redox reaction.
What can you do with electrolysis-based techniques?
- -- Determine concentration of analytes
- -- Identify analytes
- -- Characterize redox behavior of analytes (how much voltage does it take to drive the reaction)
- -- Sweep generators, potentiostats, cells, and data acquistion/computers make up most systems
the voltage needed to overcome the activation energy for a redox reaction to occur at the electrode. If you want the reaction to go fast (i.e. high current), then you apply high voltages.
the voltage needed to overcome the resistance of the solution (high resistance solutions do provide easy migration of the ions). Ohm’s Law: E = IR.
the concentration of ions at the surface of the electrode are less than they are in bulk solution.
the difference between the equilibrium potential and the actual potential
Sources of polarization in cells
- – Charge-transfer (kinetic) polarization:magnitude of current is limited by the rate of the electrode reaction(s) (the rate of electron transfer between the reactants and the electrodes)
- – Concentration polarization: rate of material transport to electrode isinsufficient to maintain current
- – Other effects (e.g. adsorption/desorption)
Some electrochemical cells have significant currents
- – Electricity within a cell is carried by moving ions – When small currents are involved, E = IR holds
- – R depends on the nature of the solution
When current in a cell is large, the actualpotential usually differs from that calculatedat equilibrium using the Nernst equation
measuring the flow of charge
Electroanalytical techniques are categorized by:
- • the excitation waveform:
- – Variation in Applied Potential (E)• Step, repeat step, etc.
- • Ramp (one way or cycled), etc
- .– Variation in Applied Current (I)
- • the response waveform
- – (usually in this chapt, I vers. E) Voltammetry
Potential Step Methods: apply voltage, then measure current or charge, before & after voltage is applied
- Chronoamperometry (CA)
- – Response:i (current) vs. t =time
- Chronocoulometry (CC)
- – Response:Q (accumulated charge) vs. t =time
- All in unstirred solution.
Before excitation, there is no current. After the excitation, the current starts high, And becomes smaller as material near the electrode gets used up.
To determine the charge, we integrate the current
Why would you use chronoamperometry or chronocoulometry????
- Determination of:
- – n (# of electrons)
- – A (surface area of electrode)
- – Do (diffusion coefficient of analyte)
- Kinetics/reaction mechanism
- Double potential step
- – Generate species, their probe fate
A current proportional to the analyte concentration is monitored, usually at a fixed potential.
A current proportional to the analyte concentration is monitored, at a variable, controlled potential.
measures current flowing through the dropping mercury electrode (DME) as a function ofapplied potential
Linear Sweep Voltammetry
performed by applying a linear potential ramp in the same manner as DCP
potential scan rate is usually much faster than with DCP (direct current polarography)
LSV asymmetric peak-shaped I-E curve
Applications of Linear Sweep Voltammetry
- Determination of:
- – n,A, Do, co
- Energy of reactioins
- Study of kinetics
- Study of adsorption
- Characterization of new materials
potential scans run from the starting potential to the end potential, then reverse from the end potential back to the starting potential
Hydrodynamic voltammetry is performed with rapid stirring in a cell
Light Interacts with Matter
a set of absorption spectra for a set of solutions, plotted on the same chart, in which the sum of the concentrations of two principal absoring components, A and B, is constant
The Scatchard Plot