Debye-Huckel: log(γ) = -A·z²·√I / (1 + B·a·√I)
In real solutions, ions interact electrostatically, causing their effective concentration (activity) to differ from their actual concentration. The activity coefficient γ quantifies this: activity = γ × concentration. The Debye-Huckel limiting law approximates γ for dilute solutions (I < 0.1 M). For a divalent cation (z=2) at I=0.1 M, γ ≈ 0.38 — meaning the ion behaves as if only 38% of its concentration is chemically "available." At infinite dilution, γ = 1. Higher ionic strength and higher charge both decrease γ. Extended Debye-Huckel adds the ion size parameter to improve accuracy up to I ≈ 0.1 M. For higher concentrations, the Pitzer equations or specific ion interaction theory (SIT) are needed.