# Quant.txt

 Potentiometry to determine the concentration of analyte, we measure potential differences, with little or no current passed Electrolysis 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 Over potential 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. Ohmic potential 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. Concentration polarization the concentration of ions at the surface of the electrode are less than they are in bulk solution. Overpotential 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 Coulometry 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 =timeChronocoulometry (CC) – Response:Q (accumulated charge) vs. t =timeAll in unstirred solution. Chronoamperometry (CA) Before excitation, there is no current. After the excitation, the current starts high, And becomes smaller as material near the electrode gets used up. Chronocoulometry (CC) 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 mechanismDouble potential step– Generate species, their probe fate Amperometry A current proportional to the analyte concentration is monitored, usually at a fixed potential. Voltammetry A current proportional to the analyte concentration is monitored, at a variable, controlled potential. DC Polarography 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, coEnergy of reactioinsStudy of kineticsStudy of adsorptionCharacterization of new materials Cyclic Voltammetry potential scans run from the starting potential to the end potential, then reverse from the end potential back to the starting potential Hydrodynamic Voltammetry Hydrodynamic voltammetry is performed with rapid stirring in a cell Light Interacts with Matter Isosbestic Point 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 Authorlilladi1022 ID82296 Card SetQuant.txt DescriptionQuant Analysis Updated2011-04-27T20:52:02Z Show Answers