Ionic Strength
Background
Ionic strength describes the overall electrostatic intensity of an electrolyte solution.
Ionic strength:
I = 0.5 × Σ(cᵢ × zᵢ²)
I = 0.5 × Σ(cᵢ × zᵢ²)
Debye-Hückel (limiting law):
log₁₀(γ) = -A × z² × √I
log₁₀(γ) = -A × z² × √I
- Activity instead of concentration
- Electrolyte equilibria
- Salt and buffer solutions
Formulas
I = 0.5 × Σ(cᵢ × zᵢ²)
log₁₀(γ) = -A × z² × √I
inverse: I = ( log₁₀(γ) / (-A×z²) )²
Examples
NaCl 0.10 M
I = 0.5×(0.1×1² + 0.1×1²)=0.10
I = 0.5×(0.1×1² + 0.1×1²)=0.10
γ for z=1
I=0.10; A=0.509
γ≈0.69
I=0.10; A=0.509
γ≈0.69
Multivalent ions
z² strongly increases contribution (z=2 → factor 4).
z² strongly increases contribution (z=2 → factor 4).
Debye-Hückel range
Useful at low to moderate ionic strength.
Useful at low to moderate ionic strength.
Technical Background
Why activity coefficients?
In electrolyte solutions, activities deviate from concentrations. γ corrects this behavior.
Applications
- Chemical equilibrium
- pH and buffer calculations in real solutions
- Geochemistry and environmental chemistry
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