Quant.txt

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.

• -- 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

• – 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)

• – 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

measuring the flow of charge

• • 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

• 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

• 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

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

• 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

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