Ionic Strength
A measure of the total concentration of ionic charge in a solution.
Formula / equation
I = ½ Σ c_i z_i²
Unit
mol/L
In depth
Because each concentration is weighted by the square of the charge, multiply charged ions contribute disproportionately. Ionic strength controls activity coefficients, so analytical procedures often add an inert salt to fix it at a constant level and make calibration reliable.
Examples in the real world
0.10 M NaCl has I = 0.10 M, while 0.10 M MgSO₄ has I = 0.40 M.