Electrochem Eng L02-20 Introduction to Pourbaix diagram

Summary

The video introduces the concept of Pourbaix diagrams, also known as potential-pH diagrams, which map the stability regions of redox species in aqueous solutions. Using iron as an example, it explains how different species like Fe²⁺, Fe³⁺, and various iron oxides are stable under specific combinations of potential and pH. The diagram’s features, such as solid lines representing equilibria between species and inclined lines indicating combined redox and acid-base reactions, are detailed. It also highlights water-splitting half-reactions and how horizontal or vertical lines correspond to reactions involving only electron or proton transfers. The video sets the stage for deeper exploration in subsequent slides.

Takeaways

• 😀 Pourbaix diagrams, also known as potential–pH diagrams, map the stability of redox species in aqueous solutions based on electrochemical potential and pH.
• 😀 The vertical axis of a Pourbaix diagram represents electrode potential, typically measured versus the Standard Hydrogen Electrode (SHE).
• 😀 The horizontal axis represents pH, indicating the acidity or basicity of the solution.
• 😀 Each region in the diagram shows where a particular species, such as an ion or solid, is thermodynamically stable.
• 😀 For iron at 1 mM concentration, Fe³⁺ is stable at high potential and low pH, while Fe²⁺ is stable in a mid-range potential and pH region.
• 😀 Metallic iron (Fe⁰) is stable in broad regions of negative potential, and solid oxides like Fe₃O₄ or Fe₂O₃ have specific stable regions denoted as (s).
• 😀 Solid lines on the diagram represent boundaries where two species are in equilibrium: horizontal for redox-only reactions, vertical for acid–base-only reactions, and sloped for combined redox and acid–base reactions.
• 😀 Dashed blue lines indicate water-splitting half-reactions: the lower line represents hydrogen evolution and the upper line represents oxygen evolution.
• 😀 The slope of lines indicates whether the reaction involves proton transfer, hydroxyl groups, or purely electron transfer.
• 😀 Pourbaix diagrams are practical tools for predicting corrosion, identifying stable species, and understanding electrochemical behavior in aqueous solutions.

Q & A

What is a Pourbaix diagram and what does it represent?

A Pourbaix diagram, also called a potential-pH diagram, maps the regions where different redox species are stable in an aqueous solution. It shows stability regions in terms of electrochemical potential (usually versus the standard hydrogen electrode, SHE) and pH, indicating the acidity or basicity of the solution.

What are the axes on a typical Pourbaix diagram?

The vertical axis represents the electrochemical potential (E) versus the standard hydrogen electrode (SHE), while the horizontal axis represents the pH of the solution.

How does concentration affect the Pourbaix diagram for iron?

The Pourbaix diagram shown is for iron at a concentration of 1 millimolar for ions like Fe²⁺ and Fe³⁺. This concentration affects the boundaries and stability regions of the various species in the diagram.

What does a solid line in a Pourbaix diagram represent?

Solid lines indicate boundaries where two species coexist in equilibrium. For example, a vertical line may show Fe³⁺ coexisting with Fe₂O₃, while a horizontal line may show Fe³⁺ coexisting with Fe²⁺.

What is indicated by a horizontal line in a Pourbaix diagram?

Horizontal lines represent redox reactions that involve a change in oxidation state without involving protons or hydroxide ions. They indicate charge transfer reactions without acid-base involvement.

What does a vertical line in a Pourbaix diagram represent?

Vertical lines represent acid-base reactions without any charge transfer. For instance, they can show protonation or deprotonation processes between species without redox reactions.

What do inclined or sloped lines in a Pourbaix diagram indicate?

Sloped lines represent general electrochemical half-cell reactions that involve both redox (electron transfer) and acid-base (proton transfer) processes.

What is the significance of the blue dashed lines in the diagram?

The blue dashed lines represent the water stability limits. The bottom line corresponds to the reduction of protons to hydrogen gas, and the top line corresponds to the oxidation of water to oxygen gas.

What do the large solid regions in a Pourbaix diagram represent?

These regions indicate the stability of a single species. For example, areas may show where Fe³⁺, Fe²⁺, solid Fe₃O₄, or metallic Fe are thermodynamically stable under certain potential and pH conditions.

Why are Pourbaix diagrams useful in practical applications like corrosion studies?

They help predict the stable form of metals under different environmental conditions, indicating where metals may corrode, form protective oxides, or remain in a stable ionic state. This is crucial for designing materials and preventing corrosion.