Electrochem Flashcards
Potentiometry
Measures potential at electrode where no current is flowing.
No oxidation/Reduction
Voltammetry
Measure Current as a function of applied voltage.
Requries oxidation/reduction
Three Electrode Cell
- Working electrode
- electrode at which redox occurs - Auxillary Electrode
- provides potential to cell - Reference electrode
- material of a known constant potential
Helmholtz Model
- Couterions balance charge of electrode
- Acts as a capacitor
- When potential changes the ions move to balance the new charge causing non-faradaic current
Bockris Model
Splits inner layer into 2 plance
- inner helmholtz plane
- 1 molecule thick
- specifically absorbed - Outer helmholtz
- tightly bound couter inos. - Diffuse layer
- predominately counter ions - bulk solution
- equal co and counter ions
Movement of ions
- Diffusion
- Migration
- Convection
Diffusion limited current
- t = 0 all reagent in contact with electrode reacts
- Strong diffusion gradient towards electrode
- Over time gradient declines as ions pulled from further away
- eventually reaches steady state where current is limited by diffusion
Macro vs Micro Electrodes
1 Macro
> 100 um
follow full cottrell equation
- Micro
<100 um
-spherical diffusion
quickly reaches diffusion limited current
Voltage Drop
Drop in voltage between auxillary and working electrodes due to resistance in electrolye V=IR. Results in an overpotential (i.e. potential above that of theoretical)
Minimise Voltage Drop
- Low current
- Low resistance
- decrease electrode separation
Cyclic Voltammetry
current measured as potenital is scanned between two switch potentials.
- characterisation
- determining reaction potential
- determining reversibility
Stipping Voltammetry
High sensitivity measurements.
- concentrate analyte on electrode
- reverse voltage stripping it back in to solution
- measure current during stripping to determine conc to ppt
anodic stripping
- stripped by anodic oxidation
- deposited by reduction
e. g metals
cathodic stripping
- stripped by cathodic reduction
- deposited by oxidation
adsorptive stripping
- analyte physically adsorbed onto mercury film (dissolved)
- stripped by application of voltage
hanging mercury drop electrode benefits
- reproducible clean surface
- large range of operating voltages
thin film is more sensitive as forms concentrated amalgam but deteriorates. Hanging drop is very reproducible.
Gas Sensor
- gas passes through selective membrane
- dissoves in solution as ion
- is detected directly (CN-) or indirectly (pH)
Gas sensor limit
other gases may pass through membrane interferring with measurement
Enzyme based sensor
- enzyme interacts specifically with analyte
- detect product of reaction
- calculate analyte conc by reaction stoichiometry
- requires specific enzyme
Microfabricated Sensor Array
- single ion selective electrodes are often have interference from other ions
- array uses multiple ion selective electrodes
- analyse all simultaneously to identify ion
- more reliable
O2 sensor
- measures dissolved oxygen
Clark Cell
- O2 diffuses through selective membrane
- reduced to OH- at specific voltage
- current proportional to O2 conc
Glucose Sensor membranse
- 3 layer membrane
1. polycarbonate film permeable to glucose and filters blood
2. glucose oxidase membrane selectively converts glucose to hydrogen peroxide
3. cellulose acetate membrane permeable to peroxide
glucose oxidase detection
- peroxide oxidised to oxygen at electrode
- current is proportion to peroxide concentration
- can be related to glucose concentration by oxidase reaction
Linear Waveform
voltage linearly increased over time
constant charging current
Normal Pulse
Pulses of increasing magnitude on constant baseline
current measured at end
reduced charging effects
Differential Pulse
pulses of same magnitude on increasing baseline
current measured just before start and end and difference plotted
highest sensitivity, lowest charging effects
peak size proportional to concentration
peak voltage equal to standard potential
Cyclic Voltammagram
From Point b —> Point d: Cathodic current begins due to the reduction of Fe(III) to Fe(II), ie Fe(CN)63- + e- —> Fe(CN)64-
At Point d: The Cathodic Peak has two contributions: the current required to adjust [Fe3+] at the electrode from its initial concentration to its equilibrium concentration (which approaches zero) and the diffusion controlled current for reduction of fresh Fe3+ diffusing from the bulk solution.
Point d —> Point e: Current drops as the diffusion layer gets thicker (due to depletion of Fe3+ close to the electrode by reduction).
O2 Sensor Reduction
O2 + 2H2O + 4e- → 4OH-
Glucose Sensor Reactions
Glucose + O2 →Glucose oxidase→ H2O2
H2O2 + OH- → H2O + O2 + 2e-
