How this calculator works
List every dissolved ion and its molar concentration after dissociation, including counterions. Multiply each concentration by the square of its signed charge, sum the terms and divide by two. Multivalent ions contribute more strongly than monovalent ions at the same concentration. The net charge diagnostic should be near zero for a complete bulk-solution analysis; an imbalance can also mean the input intentionally lists only part of a solution. Ionic strength is not simply total dissolved concentration.
Formula
I = (1/2) sum(ci zi^2); net charge concentration = sum(ci zi). Lists use mol/L; the selectable ionic-strength output uses the engine concentration conversion.
Worked example
0.1 mol/L NaCl contributes 0.1 M sodium and 0.1 M chloride, giving I = 0.1 M. For 0.1 M CaCl2, use 0.1 M Ca2+ and 0.2 M Cl-, giving I = 0.3 M.
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Frequently asked questions
Do negative ions subtract from ionic strength?
No. The charge is squared, so both cations and anions add positive contributions.
Can I enter salt concentrations?
Convert each salt into the concentrations of its dissolved ion species first.
Is this activity or concentration?
It is concentration-based ionic strength. Activity coefficients require an additional solution model.