Calculate ionic strength from ion concentration and charge, or from up to four-ion mixtures and salt presets directly in mol/L or mol/m³.

Concentration basis, not molality. Single-ion and incomplete-mixture results are contributions to ionic strength; include all ions for a solution total. Salt presets assume ideal complete dissociation, without hydrolysis or ion pairing.

For example, +2 for Ca²⁺ or −1 for Cl⁻.

Result unit

Ionic Strength Formula

The following formula is used to calculate the ionic strength:

I = 0.5 × ∑(cᵢ × zᵢ²)

Variables:

  • I is the concentration-based ionic strength (mol/L when concentrations are mol/L)
  • c_i is the concentration of each ionic species in the final solution (mol/L)
  • z_i is the signed, dimensionless integer charge number of that species

To calculate the ionic strength, multiply the concentration of each ion by the square of its charge. Sum up the results for all ions, and multiply the sum by 0.5.

What is an Ionic Strength?

Ionic strength is a measure used in chemistry to quantify the concentration of ions in a solution. It is denoted by the symbol 'I' and is calculated as half the sum of each ionic species’ concentration (in moles per liter) multiplied by the square of its charge number. The concept of ionic strength is important because it influences several properties of a solution, such as the solubility of salts, the pH, and the speed of chemical reactions. It is particularly significant in solutions with high concentrations of ions, such as seawater or mineral-rich groundwater, where it can affect the behavior of the ions and their interactions with other substances.

How to Calculate Ionic Strength?

The following steps outline how to calculate the Ionic Strength:

  1. First, determine the concentration of each ion in the solution (in mol/L).
  2. Next, determine the charge of each ion.
  3. Next, square the charge of each ion.
  4. Next, multiply the concentration of each ion by the square of its charge.
  5. Next, sum up the products obtained in the previous step.
  6. Finally, divide the sum by 2 to calculate the Ionic Strength.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Concentration of ion A (mol/L) = 0.5

Charge of ion A = +2

Concentration of ion B (mol/L) = 0.3

Charge of ion B = -1

Concentration of ion C (mol/L) = 0.2

Charge of ion C = +3

Concentration of ion D (mol/L) = 0.4

Charge of ion D = -2

For these listed ions, I = 0.5(0.5 × 2² + 0.3 × 1² + 0.2 × 3² + 0.4 × 2²) = 2.85 mol/L. This hypothetical list has a net signed charge concentration of +0.5 mol/L, so it is incomplete as a neutral bulk solution. Additional counterions would contribute to the solution’s total ionic strength.