I = ½ Σ cᵢzᵢ²
Ionic strength (I = ½Σcᵢzᵢ²) quantifies the total electrostatic environment of a solution. It accounts for both the concentration and charge of every ion. For 0.1 M NaCl: I = ½(0.1×1² + 0.1×1²) = 0.1 M. For 0.1 M CaCl₂: I = ½(0.1×4 + 0.2×1) = 0.3 M — the divalent Ca²⁺ contributes much more. Ionic strength is central to the Debye-Hückel theory, which predicts activity coefficients: log γ± = −A|z₊z₋|√I. It affects: solubility (salting-in/out effects), reaction rates in ionic solutions (primary kinetic salt effect), protein stability (Hofmeister series), and electrode potentials. The Debye length (κ⁻¹ = 0.304/√I nm at 25°C) describes the electrostatic screening distance in solution.