Professor Chadwick: Thermodynamics of Electrochemical Methods

Question 1

Write down the Nernst Equation and define their terms.

Answer 1

delta E = Cell voltage

delta E₀ = standard cell voltage (1 molar concentration)

R = gas constant (8.314 J mol^-1 K^-1)

T = temperature (K)

n = number of electrons involved in reaction

F = Faraday constant

[X] = Concentration of X

Question 2

Why are electrochemical cells mainly used with Solutions?

Answer 2

1. Mainly used with aqueous solutions as cell voltage must reach equilibrium. The ions must flow and relatively fast, and so the ionic conductivity must be high. For most solids, ionic conductivity is low, hence why solutions are used.
2. Electrochemical cells are mostly used in measuring the concentrations of ions, and so they need to be in solutions.

Question 3

What is typical of the glass pH electrode?

Answer 3

• the glass pH electrode is a membrane electrode with the reference electrode stored inside of the glass.
• A potential difference is created across the membrane which is then measured. There is no change in solution, or actual contact.
• The inside of the electrode is usually filled with buffered solution of chlorides in which a silver wire covered with silver chloride is immersed.

Question 4

How does a glass pH electrode work?

Answer 4

• The surface of the glass is protonated by both internal and external solution until equilibrium is reached.
• Both sides of glass are charged by the adsorbed protons; resulting in a potential difference.
• This potential is described by the Nernst equation and is directly proportional to pH difference between solutions on both sides of the glass.

Question 5

Draw and label a diagram of a glass pH electrode.

Answer 5

Question 6

What is the alkaline error of glass pH electrodes?

Answer 6

• The electrode also responds to alkali metals (Na⁺, K⁺)
• It is at its worse when [Na⁺] > [H₃O⁺]
• This false response is the alkaline error as a common source of Na⁺ is NaOH
• The electrode shows no response to hydroxyl ions however.

Question 7

What is the Fluoride Ion Selective electrode?

Answer 7

• The fluoride ion selective electrode is based on a lanthanum fluoride membrane doped with EuF₂.
• The LaF₃ conducts electricity by motion of F^- ions, which move via F^- vacancies in the lattice.
• Adding EuF₂ increases the number of F^- vacancies, and thus increases the motion of F^- ions, increasing the conductivity and the response.
• Voltage is measured with a reference and the Nernst equation is used to calculate the unknown concentration.

Question 8

How does the Zirconium Oxygen Sensor work?

Answer 8

• The zirconium oxygen sensor is based on a zirconium dioxide membrane which is a good oxygen ion conductor.
• To improve conductivity and get a rapid response, the membrane has to be heated to around 400^oC
• The measured voltage is then converted to oxygen concentration via the Nernst equation.

Question 9

Why do zirconium oxygen monitors use cubic zirconia?

Answer 9

• 10% Y₂O₃ in ZrO₂ forms a tough ceramic
• It can be produced as single crystals.
• It is relatively cheap.
• It is a very good conductor of oxygen ions.
• Equilibrium is fast across the membrane.

Question 10

What are the uses of Semiconducting gas sensors?

Answer 10

1. Semiconducting gas senses are used to detect flammable gases.
2. They are based on semiconducting metal oxides like ZnO and SnO₂
3. They were developed by Japanese and were often called Tagushi sensors.

Question 11

What principles do semiconducting gas sensors work on?

Answer 11

1. ZnO and TiO₂ and SnO₂ are n-type semiconductors, meaning they have a conductivity due to the motion of electrons.
2. This is because these oxides are non-stoichiometic and contain excess metal

Question 12

How do semiconductors detect flammable gases?

Answer 12

1. When the metal oxide is in the presence of a flammable gas, the gas burns in with oxygen on the surface of the oxide.
2. This causes the electrons in the O2- ions to be released, increasing the conductivity, indicating the flammable gas.
3. The Oxide reacts faster at 500^oC

Question 13

What are the strengths and weaknesses of the semiconducting gas sensors?

Answer 13

Advantages:

• Highly sensitive
• Fast and cheap
• Very simple circuit.
• Not polluted.

Disadvantages

• Not selective; detects all flammable gases.

Question 14

How do catalytic gas sensors work?

Answer 14

• Catalytic gas sensors work on a platinum resistance thermometer.
• When flammable gases burn they give off heat which heats the platinum wire, increasing the resistance.
• The platinum would melt at the high temperatures, so they are coated in an alumina bead and are impregnated with a catalyst.
• The Wheatstone circuit bridge allows the resistance to be measured, allowing the concentration of gas to be calculated.