Ionic Strength Calculator
I = ½Σcz², where charge counts squared.
Work out Ionic Strength. I = ½Σcz², where charge counts squared. Written for the problem set you are stuck on.
One per ion, comma separated, in mol/L.
Signed whole numbers. Must line up with the concentrations.
Ionic strength (mol/L)
0.4
2 ions · the +2 ion dominates
I = ½ Σ cᵢzᵢ². The charge is squared, so a divalent ion contributes four times as much as a monovalent one at the same concentration and a trivalent ion nine times. That is why 0.1 M magnesium sulfate has an ionic strength of 0.4 while 0.1 M sodium chloride has 0.1. Ionic strength, not concentration, is what determines how much the ions in a solution interfere with each other — it feeds directly into activity coefficients, and through them into every equilibrium constant that matters at real concentrations. Include every ion present, not just the one you care about. A buffer's own ions and any added background electrolyte all count.
How the Ionic Strength Calculator works
Enter each ion's concentration and charge to get the ionic strength. The charge is squared, so a divalent ion contributes four times as much as a monovalent one at the same concentration — which is why 0.1 M magnesium sulfate is four times the ionic strength of 0.1 M sodium chloride.
Also known as: ionic strength formula · how to calculate ionic strength · ionic strength of a solution · debye huckel ionic strength
Frequently asked questions
How do I calculate ionic strength?
I = ½ Σ cᵢzᵢ² — half the sum of each ion's concentration times the square of its charge. Include every ion in solution, not only the one you are studying.
Why is the charge squared?
Because electrostatic interaction between ions scales with the product of their charges, and the ionic strength is designed to capture the total interaction environment. Squaring makes a 2+ ion count four times as much and a 3+ ion nine times.
What is ionic strength used for?
It is the input to activity coefficients through the Debye-Hückel equation, and through those it corrects every equilibrium constant that matters at real concentrations — solubility, pH, complexation.
Do neutral molecules count?
No. An uncharged species contributes zero because its charge squared is zero, so dissolved sugar or ethanol adds nothing to ionic strength however concentrated it is.
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