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Chronoamperometry - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

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

Typology: Slides

2012/2013

Uploaded on 08/30/2013

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Chronoamperometry
Stationary electrode
Unstirred = mass transport by diffusion
Constant potential
Measure current vs time
Theory
assume Ox + n e- Red
- both Ox and Red are soluble
- reversible reaction (electrochemically)
- potential set so reduction goes to
completion at the electrode surface
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Chronoamperometry

Theory• Measure current vs time• Constant potential• Unstirred = mass transport by diffusion• Stationary electrode assume

Ox + n e



Red

  • potential set so reduction goes to- reversible reaction (electrochemically)- both Ox and Red are soluble

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

Chronoamperometry Applications

  • Can measure concentration by measuring I vs conc. at any fixed time
  • Can analyze the shape of the current-time reactionscurve in order to study coupled chemical
  • There are better ways to do both of these with more modern techniques
  • Chronoamperometry is important because techniques are basedit is a fundamental method on which other

Chronopotentiometry

Theory• Measure potential vs time• Constant current applied between electrodes• Unstirred = mass transport by diffusion• Stationary electrode assume

Ox + n e



Red

  • apply current and use up Ox at electrode- reversible reaction (electrochemically)- both Ox and Red are soluble

surface producing Red

Galvanostat

Theory of Chronopotentiometry

[Red]

E = E

o

  • ----------- log -----------

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

E

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



L

6DQG(TXDWLRQ

Coulometry

Methods based on counting coulombs (C), the basic unit of electrical charge (Q)

Q M

Faraday’s Law

W = ------------

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

  1. Controlled Potential Coulometry

t

Q =

i dt

  1. Constant Current Coulometry

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

Coulometric cell

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

  • ----- ln --------o^ nF

[Ox]

informationGives quantitative

informationquantitative

for stirred solutioninformationqualitative

Linear Scan Voltammetry (stirred)

I

E

Half wave potential (E

½ (^) )

is E when I = I

L (^) /

LI

CI

½E

Ox + e

 Red

Linear Scan Voltammetry (stirred)

I

E

L I I is proportional to [Red], represents the situation

and Ox is zero.where Red is maximum

LI

CI

½E

Ox + e

 Red