Ionic Strength Calculator

All chemistry tools

Calculate the total ionic strength of a solution from the concentrations and charges of all ions present.

Solution Components (Ions)

Concentration (c) (M)

Charge (z)

Concentration (c) (M)

Charge (z)

Calculation Results

Add ions with their concentrations (M) and charges to calculate the solution's ionic strength

About this calculator

Overview

Ionic strength (I) quantifies the total concentration of ions and the intensity of the electric field in a solution. It is the key to understanding how 'shielded' charges are in an electrolyte. High ionic strength can lead to 'salting out' of proteins, reduced solubility of salts, and changes in the speed of chemical reactions. The most important feature is the squared charge term (z²), which means a single Mg²⁺ ion contributes 4 times as much as a Na⁺ ion at the same concentration.

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Pro Tips

The formula is I = ½ ∑ (cᵢ × zᵢ²), where c is Molarity and z is the charge of the ion.
The ½ factor is there because the formula sums over all ions (cations and anions) in the salt.
Highly charged ions (like Al³⁺ or PO₄³⁻) have a massive impact on ionic strength because their charge is squared (3² = 9).
Common units are M (molarity) or m (molality). The calculator uses Molarity.
Activity vs Concentration: In solutions with high ionic strength, ions don't act like they are 'fully there' due to electrostatic drag.
The Debye-Hückel limiting law is used to calculate activity coefficients from ionic strength.

Fun Facts

"Lewis and Randall introduced 'ionic strength' in 1921 as a way to correct for non-ideal behavior in concentrated electrolytes."
"Seawater has an ionic strength of ~0.7 M, while typical human blood plasma is much lower, at ~0.15 M."
"At very high ionic strengths (e.g., in the Dead Sea), water molecules are so busy hydrating ions that they struggle to dissolve anything else."
"Ionic strength determines DNA stability; if it's too low, the negative charges on the DNA strands repel each other and the double helix 'unzips'."
"The 'Ionic Atmosphere' around an ion becomes smaller as the ionic strength increases, shielding the ion's charge more effectively."
"Protein crystallization often requires precisely tuned ionic transitions to encourage slow, orderly growth."

Formula: I = ½ ∑ cᵢ · zᵢ²

Ionic Strength of Common Solutions

Comparing the typical concentration of ions in various environments.