Ionic Strength Calculator

How it works

I = ½ Σ cᵢ zᵢ² · Davies equation (25 °C): log₁₀ γ = −0.509 z² ( √I ÷ (1 + √I) − 0.3 I ) · Debye length in water at 25 °C ≈ 0.304 ÷ √I nm (I in mol/L)

Ionic strength measures the total concentration of charge in a solution, weighting each ion by the square of its charge, so a 2+ ion counts four times as much as a 1+ ion. It controls activity coefficients, buffer pKa shifts, protein solubility, electrophoresis and the thickness of the electrical double layer (Debye length). Enter every ion separately: 0.1 M CaCl₂ is 0.1 M Ca²⁺ and 0.2 M Cl⁻. The Davies equation is a good activity-coefficient estimate up to about 0.5 mol/L.

Worked example

A 0.1 mol/L CaCl₂ solution has 0.1 mol/L Ca²⁺ (z = +2) and 0.2 mol/L Cl⁻ (z = −1). I = ½ (0.1 × 2² + 0.2 × 1²) = ½ (0.4 + 0.2) = 0.3000 mol/L. The charges balance (0.1 × 2 − 0.2 = 0), and the Davies equation gives activity coefficients of 0.2902 for Ca²⁺ and 0.7340 for Cl⁻.

Sources: IUPAC Gold Book — Ionic strength · Wikipedia — Davies equation

Frequently asked questions