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Ionic Strength Calculator

Calculate ionic strength from ion concentrations and charges.

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Ionic Strength

Formula first

Overview

Ionic strength is a measure of the total concentration of ions in a solution, specifically accounting for the magnitude of their electric charges. It defines the electrical environment and significantly influences the activity coefficients of solutes and the thickness of the Debye-Hückel double layer.

Symbols

Variables

mol/dm³ = Ionic Strength, mol/dm³ = Σ(cᵢ zᵢ²)

Ionic Strength
Σ(cᵢ zᵢ²)

Apply it well

When To Use

When to use: Use this formula when calculating the activity coefficients of ions or the effects of electrolytes on chemical equilibria. It is most accurate in dilute solutions, typically below 0.1 M, where electrostatic interactions between ions dominate. It is essential when transitioning from molar concentrations to activities in non-ideal solutions.

Why it matters: In biochemistry, ionic strength determines the stability of proteins and the binding affinity of DNA-protein complexes. In industrial chemistry, it affects the solubility of salts through the salt effect and influences the rate of ionic reactions in solution. It is also a fundamental parameter in environmental science for modeling ion transport in groundwater.

Avoid these traps

Common Mistakes

  • Forgetting the 1/2 factor.
  • Not squaring the charge z.

One free problem

Practice Problem

Calculate the ionic strength of a 0.2 M aqueous solution of sodium chloride (NaCl).

Σ(cᵢ zᵢ²)0.4

Solve for:

Hint: Sodium chloride dissociates into Na⁺ and Cl⁻. Calculate the concentration of each ion and multiply by the square of their charges.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Atkins' Physical Chemistry
  2. IUPAC Gold Book: Ionic strength
  3. Wikipedia: Ionic strength
  4. Bird, Stewart, Lightfoot - Transport Phenomena
  5. IUPAC Gold Book: Molar concentration
  6. IUPAC Gold Book: Charge number
  7. Atkins, Peter, de Paula, Julio, and Keeler, James. Atkins' Physical Chemistry, 11th ed. Oxford University Press, 2018.
  8. Skoog, Douglas A., et al. Fundamentals of Analytical Chemistry, 9th ed. Brooks/Cole, Cengage Learning, 2014.