⚡ Ionic Strength Calculator
Calculate the ionic strength of a solution from the concentration and charge of each dissolved ionic species.
| Species | c (mol/L) | z | c·z² |
|---|---|---|---|
| Na⁺ | 0.1 | +1 | 0.1 |
| Cl⁻ | 0.1 | -1 | 0.1 |
Frequently asked questions
What is ionic strength used for?
Ionic strength quantifies the total concentration of ions in solution, weighted by the square of their charge. It's used in Debye-Huckel calculations of activity coefficients, in comparing buffer conditions between experiments, and in predicting electrophoretic and chromatographic behavior of charged molecules.
Do I need to include the buffer salts too?
Yes — ionic strength accounts for all ionic species in solution, not just your analyte. Include buffer components (e.g. phosphate, Tris-HCl counterions), salts (NaCl, KCl), and any other dissociated ionic species.
Why is charge squared in the formula?
Higher-charge ions contribute disproportionately more to a solution's ionic character and their effect on activity coefficients and screening. Squaring the charge (z²) reflects this — a divalent ion (z=2) contributes 4 times as much per mole as a monovalent ion.
Accuracy & how this is derived
Derivation: Standard Lewis-Randall ionic strength definition: I = 1/2 sum(ci * zi^2), summed across all dissociated ionic species entered.
Validated against: Classical physical chemistry formula (Lewis and Randall, 1921), used universally in electrolyte and buffer chemistry references.
⚠️ For educational and research support only — verify critical results independently before use in regulated, clinical, or publication-bound work.
✔ Last updated: July 2026 · Report an error
Understanding and calculating ionic strength
Ionic strength is defined by the Lewis-Randall formula as half the sum, over all ionic species in solution, of each species' molar concentration multiplied by the square of its charge. It was introduced to explain why activity coefficients of electrolytes deviate from ideality more strongly as charge increases, and remains central to the Debye-Huckel and extended Debye-Huckel equations used to estimate activity coefficients in dilute electrolyte solutions.