Ionic Strength Calculator

Ionic strength I = ½ · Σ(c_i · z_i²) for all ions, where c_i is the molar concentration and z_i is the charge of each ion. It measures the total ion concentration weighted by the square of the charge. A 0.1 M NaCl solution has I = 0.1 M; a 0.1 M MgCl₂ solution has I = ½ · (0.1 · 4 + 0.2 · 1) = 0.3 M because Mg²⁺ has a higher charge.

Blood plasma has an ionic strength of approximately 0.15-0.16 mol/L, primarily from Na+, Cl-, K+, and HCO3-. Physiological saline (0.9% NaCl) has I = 0.154 mol/L. This ionic strength is used as the standard condition for many biochemical measurements and is important for maintaining the native structure of proteins and nucleic acids.

Higher ionic strength generally stabilizes charged species by screening electrostatic repulsions between ions of the same charge, and destabilizes interactions between oppositely charged species. This affects enzyme activity, DNA-protein binding, and precipitation reactions. The effect is captured quantitatively by substituting activities (a = γ · c) for concentrations in equilibrium constant expressions.

Add up every ionic species the buffer contains, each at its actual concentration and charge, then apply I = ½ · Σ(c_i · z_i²). For a phosphate buffer, include H₂PO₄^-, HPO₄^2-, and the counter-ions (Na⁺ or K⁺). Because the divalent HPO₄^2- contributes four times as much per mole as a monovalent ion, two buffers of the same molarity can have different ionic strengths depending on their pH and speciation.