ConverterHub

Ionic Strength Calculator

Find a solution's ionic strength.

How it is calculated

Ionic strength measures how many charges a solution carries, weighting highly charged ions more: I = ½ × (c₁z₁² + c₂z₂²). Each c is the molar concentration of an ion in mol/L and each z its charge number (+2, −1, and so on). Because the charge is squared, its sign does not matter. This tool takes exactly two ion species, so a salt such as CaCl₂ is entered as its cation and anion, remembering that the anion concentration is twice the salt's. Concentrations cannot be negative. The result is in mol/L, to four decimals. For solutions with more ions, add their c·z² terms by hand.

Worked example

A 0.05 mol/L solution of CaCl₂ gives Ca²⁺ at 0.05 mol/L (z = +2) and Cl⁻ at 0.10 mol/L (z = −1). Ion 1: 0.05 × 2² = 0.20. Ion 2: 0.10 × (−1)² = 0.10. Sum = 0.30, half of that is 0.1500 mol/L. By comparison, 0.05 mol/L NaCl would only have I = 0.05.

Try this example →

Common mistakes

  • Entering the salt concentration for both ions; CaCl₂ gives two chloride ions per formula unit, so Cl⁻ is double.
  • Not squaring the charge in your head and expecting a divalent ion to count twice instead of four times.
  • Leaving out spectator ions or buffer components that also add to the strength.
  • Entering a negative concentration to represent an anion; concentrations are always positive, with the sign in z.
  • Confusing ionic strength with the concentration itself; for 1:1 salts they match, for others they do not.

About this calculator

This calculator finds the ionic strength (I) of a solution from its ions' molar concentrations and charges: I = ½·Σ(cᵢ·zᵢ²), summing over every ion, where cᵢ is concentration and zᵢ is the ion's charge. Because charge is squared, multivalent ions (like Ca²⁺ or SO₄²⁻) contribute disproportionately more to ionic strength than monovalent ones (like Na⁺ or Cl⁻) at the same concentration.

Ionic strength governs how electrostatic interactions between ions affect a solution's behavior, feeding directly into the Debye–Hückel equation for activity coefficients, which corrects ideal solution chemistry (based on concentration) to real behavior (based on activity) — essential in analytical chemistry, electrochemistry, and biochemistry, where high ionic strength buffers can shield charged biomolecules and affect enzyme activity or protein folding.

Analytical and physical chemists calculating activity coefficients, biochemists preparing buffers for protein or DNA work, and chemistry students working through Debye–Hückel problems all use this calculator, entering each ion's concentration and charge.

Was this helpful?

You can pick up to 3 reactions.

Comments

  • Be the first to comment.

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators