Electrode Theory

Question 1

Redox Processes

Answer 1

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

Question 2

Anode & Cathode

Answer 2

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

Question 3

Galvanic Cell

Answer 3

!!! 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

Question 4

Helmoltz Double Layer

Answer 4

• 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

Question 5

Polarization of Electrodes and Overpotentials

Answer 5

1) Transfer Overpotential (charge transport processes)
2) Diffusion Overpotential
3) Reaction Overpotential (chemical reactions)

• Ohmic polarization: resistive voltage drop (no overpotential!)

Question 6

Transfer Overpotential

Answer 6

inhibition of the passage of the charge carriers through the electrolytic double layer requires energy and generates voltage drop

Question 7

Diffusion Overpotential

Answer 7

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)

Question 8

Reaction Overpotential

Answer 8

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

Question 9

Ohmic Voltage Drop

–> on what does it depend?

Answer 9

• 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

Question 10

Electrodes - What kind of system?

What types of Electrodes exist?

Answer 10

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

Question 11

Electrode Type 1

–> Ion Electrode

Answer 11

Polarizable Electrodes

• high R
• behave rather capacitive

–> potential depends directly on the concentration
of the electrolyte solution surrounding them

• metal/metal salt solution

Question 12

Electrode Type 2

–> Ion Electrode

Answer 12

Unpolarizable Eletrodes

• small R
• behave rather conductive
• -> convenient for medical engineering
• 3 Phase system

e.g. Ag/AgCl electrode

Question 13

Structure of Skin

Answer 13

1) Epidermes
2) Dermis
- elastic and collagen fibers
3) Subcutaneous adipose tissue
- thick

Question 14

Tasks of Skin

Answer 14

• mechanical and chemical protective function

- protects body from drying up, heat and light

Question 15

Bioimpedance Measurement

Answer 15

4 electrode Array –> determination of tissue resistance

Question 16

Are electrons used as charge carriers through the interface in Electrodes of Type 2?

Answer 16

No, they use ions

Question 17

Are common metals oxidants?

Answer 17

No

Question 18

Is the capacitance of the Helmoltz Layer dependent on the current flow through the electrode?

Answer 18

Yes

Question 19

Of what does the Daniell cell consist?

Answer 19

Anode: Zinc electrode (Oxidation)

Cathode: Copper electrode (Reduction)

typically 1 molar solutions

Question 20

What are the advantages of Electrodes of Type 2 in comparsion to Electrodes of Type 1?

Answer 20

–> 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

Question 21

How is the advantage of Electrodes of Type 2 in comparsion to Electrodes of Type 1 achieved?

Answer 21

–> 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.

Question 22

What is an overpotential?

Answer 22

Deviation of the electrode voltage from the resting potential

Question 23

On what does the Electromotive Force depend?

Answer 23

on the concentration of the electrolyte solution