Ionic Strength
I = ½Σcizi² measures total ion effect in solution. Debye-Hückel theory relates ionic strength to activity coefficients—essential for accurate equilibrium and pH calculations.
Why This Chemistry Calculation Matters
Why: Ionic strength affects activity coefficients, solubility, reaction rates, and buffer pH. Debye-Hückel corrects for non-ideal behavior in dilute solutions.
How: I = ½ Σ cᵢzᵢ². Use z² so charge sign doesn't matter. log₁₀(γ) = -A z² √I for activity coefficients.
- ●Divalent ions (z=2) contribute 4× more than monovalent (z=1).
- ●Debye-Hückel valid for I < 0.1 M; use Davies or Pitzer for higher I.
- ●Blood plasma I ≈ 0.15 M; seawater I ≈ 0.7 M.
- ●Activity coefficients < 1 at high I; correct equilibrium constants.
Compact Examples
Inputs
Ions
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
I = ½ Σ cᵢzᵢ². Charge squared weights the sum.
— IUPAC
log₁₀(γ) = -A z² √I — Debye-Hückel limiting law.
— Physical chem
0.1 M NaCl: I = 0.1 M. 0.05 M CaCl₂: I = 0.15 M.
— Electrochemistry
A = 0.509, B = 0.328 at 25°C in water.
— Debye-Hückel
📋 Key Takeaways
- • I = ½ Σ cᵢzᵢ² | Ionic strength from ion concentrations and charges
- • Charge squared | Divalent ions contribute 4× more than monovalent
- • Debye-Hückel | log₁₀(γ) = -A z² √I for activity coefficients
- • Valid I < 0.1 M | Extended models for higher ionic strength
Did You Know?
0.1 M NaCl has I = 0.1 M; 0.05 M CaCl₂ has I = 0.15 M.
Source: IUPAC
Blood plasma ionic strength ≈ 0.15 M.
Source: Physiology
Seawater I ≈ 0.7 M; Debye-Hückel less accurate.
Source: Oceanography
Buffer pH depends on ionic strength via activity coefficients.
Source: Analytical chem
Debye length = 3.04×10⁻¹⁰ / √I meters.
Source: Electrochemistry
Higher I reduces solubility of charged species.
Source: Equilibrium
How the Ionic Strength Calculator Works
Add ions with concentration (M) and charge. The calculator sums ½ Σ cᵢzᵢ² and estimates activity coefficients via Debye-Hückel.
Formula
I = ½ × Σ(cᵢ × zᵢ²)
Activity
log₁₀(γ) = -A z² √I
Expert Tips
Charge Sign
Use z² so sign doesn't matter.
Electroneutrality
Solution must be neutral: Σ cᵢzᵢ = 0.
Temperature
A and B constants vary with T.
High I
Use Davies or Pitzer for I > 0.1 M.
Common Ions
| Ion | Formula | Charge | Common [M] |
|---|---|---|---|
| Sodium | ext{Na}⁺ | +1 | 0.1 M |
| Potassium | K⁺ | +1 | 0.1 M |
| Calcium | ext{Ca}^{2}⁺ | +2 | 0.01 M |
| Magnesium | ext{Mg}^{2}⁺ | +2 | 0.01 M |
| Chloride | ext{Cl}⁻ | -1 | 0.1 M |
| Sulfate | ext{SO}₄^{2}⁻ | -2 | 0.05 M |
Frequently Asked Questions
What is ionic strength?
I = ½ Σ cᵢzᵢ² — measure of total ion effect, weighted by charge squared.
Why z²?
Higher charges have stronger electrostatic effect; z² reflects that.
What is activity coefficient?
Corrects concentration for non-ideal behavior. γ < 1 at high I.
When is Debye-Hückel valid?
I < 0.1 M. Use extended or Davies for higher I.
Blood plasma I?
≈ 0.15 M. Key for physiological buffers.
0.1 M NaCl I?
I = 0.1 M. Na⁺ and Cl⁻ each contribute 0.1; sum/2 = 0.1.
0.05 M CaCl₂ I?
I = 0.15 M. Ca²⁺: 0.05×4=0.2; Cl⁻: 0.1×1=0.1; ½×0.3=0.15.
Key Numbers
📚 Official Sources
⚠️ Disclaimer: Debye-Hückel is valid for I < 0.1 M. For concentrated solutions, use extended models. Consult IUPAC Gold Book for definitions.