Ionic Strength Calculator
Why a 2+ Ion Disrupts a Solution Far More Than a 1+ Ion
Not all dissolved ions affect a solution's behavior equally — a doubly charged ion exerts four times the electrostatic influence of a singly charged one at the same concentration, because charge enters the calculation squared. Ionic strength captures that reality in a single number, and it's a required input for any calculation involving activity coefficients rather than raw concentrations.
The Formula
The sum runs over every ion in solution, where ci is each ion's molar concentration and zi is its charge (positive or negative — squaring removes the sign). The result is always in mol/L.
Where This Calculation Matters
- Activity coefficient corrections — the Debye-Hückel equation and its extensions require ionic strength to correct concentration-based equilibrium constants for real, non-ideal solutions.
- Buffer and electrolyte formulation — pharmaceutical and biological buffers are often matched to a target ionic strength to mimic physiological conditions.
- Protein solubility and stability — ionic strength influences protein folding and aggregation behavior in solution.
- Environmental water chemistry — ionic strength affects the solubility and speciation of trace metals in natural waters.
Ionic Strength for Common Electrolyte Types
| Electrolyte | Ions Present | Ionic Strength |
|---|---|---|
| 0.1 mol/L NaCl | Na+ (1+), Cl- (1-) | 0.10 mol/L |
| 0.1 mol/L MgCl2 | Mg2+ (2+), 2 Cl- (1-) | 0.30 mol/L |
| 0.1 mol/L CaSO4 | Ca2+ (2+), SO4 2- (2-) | 0.40 mol/L |
| 0.05 mol/L Na2SO4 | 2 Na+ (1+), SO4 2- (2-) | 0.15 mol/L |
A 1:1 electrolyte like NaCl produces an ionic strength numerically equal to its concentration; multivalent ions push ionic strength substantially higher at the same concentration.
How to Use This Calculator
- For each ion in solution, enter its Concentration (mol/L) and Charge (e.g. 1, -1, 2).
- Up to four ions can be entered; leave unused ion fields blank.
- Select Calculate to get the total ionic strength.
Related Calculations
Need the molar concentration of an ion from a measured mass first? Use the Molarity Calculator. For the osmotic effect of dissolved particles rather than their charge, see the Osmolarity Calculator.