Electrode Theory Flashcards
Redox Processes
1) Oxidation = Electron Release
2) Reduction = Electron uptake/absorption
3) Redox process: Electron Transfer
- -> Oxidant: Uptake of electrons causing Reduction –> ability to oxidize other substances
- -> Reductant: Release of electrons causing oxidation –> ability to reduce other substances
Reaction depends on electrochemical potential
Anode & Cathode
Anode: (base metal)
- Current flow Positive –> from metal into electrolyte
- oxidation at boundary surface: Metal –> Metal+ + e-
- reduction of electrode
Cathode: (noble metal)
- Current flow negative –> from electrolyte into metal
- reduction at boundary surface: Metal+ + e- –> Metal
- oxidation of electrode
Galvanic Cell
!!! Combination of 2 half cells
–> different electrodes and electrolytes
spatial separation of oxidation and reduction and electron flow through an electric conductor
–> use salt bridge for charge exchange without mixing the electrolytes
e.g. Daniell Cell
Helmoltz Double Layer
- forces of attraction between negative and positive charge at interface form a double layer
- -> between Anode and Cathode
- due to Nernst equation, an electric potential is formed
Polarization of Electrodes and Overpotentials
1) Transfer Overpotential (charge transport processes)
2) Diffusion Overpotential
3) Reaction Overpotential (chemical reactions)
- Ohmic polarization: resistive voltage drop (no overpotential!)
Transfer Overpotential
inhibition of the passage of the charge carriers through the electrolytic double layer requires energy and generates voltage drop
Diffusion Overpotential
occurs if the current flow of the advection of the consuming substances or the evacuation of produced substances is inhibited
Ion movements –> due to convection, electric potential, chemical potential (diffusion)
Reaction Overpotential
reaction inhibition may lead to different concentrations of the charge carriers near the electrode interface –> leads to charge separation –> reaction overpotential
chemical reactions may be tied to transfer reaction:
- solvation
- desolvation
- complex formation
Ohmic Voltage Drop
–> on what does it depend?
- Voltage Drop Outside the electrolytic double layer
- no influence over the electrochemical processes at the interface
- depends only on the conductivity of the electrolyte and geometrical factors
Electrodes - What kind of system?
What types of Electrodes exist?
Multiphase systems, where equilibria are formed between two conductive phases (metal, electrolyte)
1) Redox electrode: electrones are used as charge carriers through the interface
2) Ion Electrodes: ions travel through interface
Electrode Type 1
–> Ion Electrode
Polarizable Electrodes
- high R
- behave rather capacitive
–> potential depends directly on the concentration
of the electrolyte solution surrounding them
- metal/metal salt solution
Electrode Type 2
–> Ion Electrode
Unpolarizable Eletrodes
- small R
- behave rather conductive
- -> convenient for medical engineering
- 3 Phase system
e.g. Ag/AgCl electrode
Structure of Skin
1) Epidermes
2) Dermis
- elastic and collagen fibers
3) Subcutaneous adipose tissue
- thick
Tasks of Skin
- mechanical and chemical protective function
- protects body from drying up, heat and light
Bioimpedance Measurement
4 electrode Array –> determination of tissue resistance
Are electrons used as charge carriers through the interface in Electrodes of Type 2?
No, they use ions
Are common metals oxidants?
No
Is the capacitance of the Helmoltz Layer dependent on the current flow through the electrode?
Yes
Of what does the Daniell cell consist?
Anode: Zinc electrode (Oxidation)
Cathode: Copper electrode (Reduction)
typically 1 molar solutions
What are the advantages of Electrodes of Type 2 in comparsion to Electrodes of Type 1?
–> the electrode potential is almost insensitive to concentration fluctuations and is therefore very stable
- resulting electrode potential only depends on the concentration of chloride ions in the electrolyte solution
- -> almost independent of the concentration of silver cations
How is the advantage of Electrodes of Type 2 in comparsion to Electrodes of Type 1 achieved?
–> keep the concentration of the anion constant in order to achieve an almost constant potential
- electrode is coated with poorly soluble salt, which has the same cation as
the electrode itself (Ag+)
The electrolyte solution consists of a highly concentrated salt which has the same anion as the poorly soluble salt.
What is an overpotential?
Deviation of the electrode voltage from the resting potential
On what does the Electromotive Force depend?
on the concentration of the electrolyte solution
