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Indicator Electrodes, Potentiometry, Electrochemistry, Electrode Kinetics, Electrogravimetry, Polarography, Square Wave Voltammetry, Wavelength Selection, Types of Spectroscopy, Detectors, Theory of Molecular Absorption, Absorption by Organic Molecules, Applying UV-vis Spectroscopy are major topics of this course. Main points from these slides are: Chronoamperometry, Constant Potential, Stationary Electrode, Measure Current Vs Time, Electrochemically, Components of Output Signal, Cottrell Equati
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Theory• Measure current vs time• Constant potential• Unstirred = mass transport by diffusion• Stationary electrode assume
Ox + n e
Red
completion at the electrode surface
Components of output signal in Chronoamperometry
I
(current)
t (time) Faradaic current (I
Far )
follows Cottrell equation
Capacitive current (I
cap )
constant applied potentialdecays exponentially for a
capI (^) is high as electrode capacitive layer charges up, then drops off
FarI (^) decreases because Ox used up at electrode surface
and Ox is only replenished by diffusion
Theory• Measure potential vs time• Constant current applied between electrodes• Unstirred = mass transport by diffusion• Stationary electrode assume
Ox + n e
Red
surface producing Red
Galvanostat
[Red]
o
n
[Ox]
A gradual change in E occurs as [Red] goes up and [Ox] goes down (transition region)
Ultimately the surface concentration of Ox make a new process goa rapid change to the value required tocurrent applied, electrode potential makesgoes to zero & to sustain the constant
Chronopotentiometry Output Wave
time Time (Transition
τ)
At start no Red is present so E is not defined
is used upPoint at which Ox
½ Q)$'
½ & 2[
½
Methods based on counting coulombs (C), the basic unit of electrical charge (Q)
Faraday’s Law
n F
Where: M = molecular weight (g/mole)
F = Faraday’s constant (96,500 C/mol)n = number of electrons (unitless)W = weight (g)
Fundamental assumption is that reaction is to oxidizing or reducing species of interest100 % current efficient i.e, all coulombs go
Kinds of coulometry
t
i dt
Q = i t
Can be referred to as chronocoulometryunder the curve in chronoamperometryNothing more than integrating area
stuff to carry the current at electrode surfaceCare must be taken so that there is enough
Rarely used anymore
2 Br (^) - (^) Br (^2) + 2 e
Br (^2) + C
(^6) H 10 (^) (^) C (^6) H 10 Br
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Define - Limiting Current as steady state such that all Ox is reduced at electrodei.e., applied potential is sufficiently cathodiccurrent when [Ox] = 0 at electrode surface
E
I
transport-limited current
limiting current
nFADC
bulk
I = -----------------
δ
Nernst controlled current
RT
[Red]
E = E
[Ox]
informationGives quantitative
informationquantitative
for stirred solutioninformationqualitative
Linear Scan Voltammetry (stirred)
Half wave potential (E
½ (^) )
is E when I = I
L (^) /
LI
CI
½E
Ox + e
Red
Linear Scan Voltammetry (stirred)
L I I is proportional to [Red], represents the situation
and Ox is zero.where Red is maximum
LI
CI
½E
Ox + e
Red
