Week 4 - Redox Chemistry

Question 1

What is the activity coefficient (γ) in geochemistry, why is it important

Answer 1

The activity coefficient (γ) corrects for non-ideal behaviour in aqueous solutions. It relates the activity (a) of a solute to its concentration (C):

a = γ x C

In ideal (very dilute) solutions,
𝛾 ≈1
So
𝑎 ≈ 𝐶

In non-ideal (more concentrated) solutions, ion-ion interactions reduce γ
γ<1

You cannot assume a = C in non-ideal solutions — accounting for γ is essential for accurate predictions.

Question 2

What is ionic strength?

Answer 2

Ionic strength (I) measures the total concentration of charged species in a solution, weighted by the square of their charges. It quantifies the extent of electrostatic interactions in a solution.

I= 1/2 x ∑ci x zi^2

Controls activity coefficients (γ) → affects solubility, reaction equilibria, speciation

Higher 𝐼 means more ion-ion interactions → more deviation from ideal behaviour

Very dilute solutions have ionic strength of <10^-5 mol, coupled with an activity coefficient value of unity

Question 3

What are the master variables that control species distribution in freshwater?

Answer 3

Master Variables are fundamental parameters that govern the distribution, speciation, and mobility of chemical species in natural waters. Two key master variables are:

1. pH – Acidity/Basicity
   -pH = –log[H⁺], measures hydrogen ion concentration
   -Low pH = acidic (more H⁺ from acids dissociating in water)
   -High pH = basic (fewer free H⁺ ions)

Controls:
-Speciation of acids/bases (e.g. CO₂/HCO₃⁻/CO₃²⁻)
-Metal solubility
-Adsorption/desorption behaviour

2. pE – Redox Conditions
   -pe is a measure of electron activity:
   pe = −log[e-]
   -Analogous to pH, but for oxidation-reduction potential
   -High pe = oxidizing conditions (electron loss)
   -Low pe = reducing conditions (electron gain)

Controls:
-Stability of redox-sensitive species (e.g. Fe²⁺/Fe³⁺, Mn²⁺/Mn⁴⁺, As³⁺/As⁵⁺)
-Precipitation of metal oxides
-Organic matter degradation pathways

-Relation to Eh (electrode potential):
pe = 16.9 x𝐸ℎ
(Ehinvolts)

Together, pH and pE define the chemical boundaries for aqueous geochemistry and guide system evolution toward thermodynamic equilibrium.

Question 4

How do pe and pH interact in redox equilibria?

Answer 4

Redox Equilibria often involve both:

-Electron transfer (controlled by pe)

-Proton transfer (controlled by pH)

Chemical speciation and the redox state of elements like Fe, Mn, S, As, N, etc. depend on both pe and pH.

pe–pH Diagrams (Eh–pH Diagrams):

-Show stability fields of different redox species

-Help visualise which forms of a substance are thermodynamically stable under given environmental conditions

-Used to predict redox-controlled transformations in natural waters

Water Stability Field:

-Natural waters are stable only within a specific pe–pH range

-Bounded by the oxygen (O₂) and hydrogen (H₂) gas lines:
High pe → water oxidises to O₂ gas
Low pe → water reduces to H₂ gas

Stable region corresponds to:
𝑃𝑂2 < 1 atm
𝑃𝐻2 < 1 atm