Characteristics. Voltammetry is based upon the measurement of a current that develops in an electrochemical cell under conditions of complete concentration polarization. Potentiometric measurements are made at currents that approach zero and where polarization is absent
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View current as a function of time or applied voltage
Current is transformed to voltage by electronics
Voltage is applied to analyte; appreciable current is measured
Current is a function of
II. Excitation process
generic redox species O
O = redox
double layer acts as a capacitor
B. Events that happen
1. supporting electrolyte forms an electrical double layer
cation movement to electrode causes an initial spike in current
Formation of double layer is good because it ensures that no electric field exists across whole sol’n (requires 100:1 conc ratio of supporting elyte:redox species).
1. Migration - movement in response to electric field. We add supporting electrolyte to make analyte’s migration nearly zero. (fraction of current carried by analyte zero)
In experiments relying upon diffusion, no
convection is desired, sol’n is quiescent.
1. Solutions: redox couple + solvent + supporting electrolyte
Cathode Working indicator electrode
O2(g) + 2H+ + 2e- < ==== > H2O2
H2O2 + 2H+ +2e- < ==== > 2H2 O
Id (average) = 6/7 (708) n D1/2m2/3t1/6C
A (not electroactive) + X Ox
A + X Ox + ne R
CH2O(H2O) CH2O + H2O
CH2O + 2H+ + 2e CH3OH
il = id + ik
Fe3+ + e Fe2+ + H2O2 Fe3+
Consider the following reversible equilibrium reaction at the electrode surface :
A + ne B
ld(unknown) = kCx
where k is a constant of proportionality. Let the concentration of standard solution be CS. When VS mL of standard solution is added to Vx mL of unknown, the diffusion current is the sum of diffusion currents due to the unknown and the standard.
ifar is much slower
The example above shows the simultaneous determination of Zn , Cd, Pb and Cu using standard addition
+ 0.3 V
Pb Pb2+ + 2e-
Anodic Stripping Voltammetry
DC Stripping Voltammetry
Adsorptive Stripping Voltammetry
Differential Pulse Polarography
The sensitivity of the instrument is comparable with AAS and in many cases it is even better.