Chemistry · Solutions
Ionic strength calculator.
Enter each fully dissociating salt with its ion counts and charges. Get the ionic strength in mol/L.
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Ionic strength formula
The molar ionic strength is I = 1/2 x sum of (c x z^2) over every ion in the solution, where c is the ion's molar concentration and z its charge number. A salt that gives a ions of charge za and b ions of charge zb per formula unit contributes 1/2 x C x (a x za^2 + b x zb^2).
Worked example. A solution of 0.050 M Na2SO4 and 0.020 M KCl, fully dissociated. Na2SO4: 1/2 x 0.050 x (2 x 1 + 1 x 4) = 0.150. KCl: 1/2 x 0.020 x (1 + 1) = 0.020. Total I = 0.17 mol/L. A 1:1 salt such as NaCl has I equal to its concentration, while a 2:2 salt such as MgSO4 has I = 4 x C.
The tool assumes complete dissociation, so weak electrolytes, ion pairing and partly dissociated buffer acids will give an overestimate. Sum all ions in the solution, including buffer salts and counter-ions from pH adjustment.
Frequently asked questions
What is the formula for ionic strength?
I = 1/2 x sum of c x z squared, summed over every ion, with c in mol/L and z the ion charge.
Is ionic strength the same as concentration?
Only for a 1:1 salt like NaCl. Multiply-charged ions count more because charge is squared, so MgSO4 has four times the ionic strength of NaCl at the same concentration.
Do I count every ion?
Yes. Include all salts, buffer components and counter-ions that are present as free ions.
Why does the tool report charges do not balance?
A salt is neutral overall, so cations x cation charge must equal anions x anion charge, for example Na2SO4 is 2 x 1 = 1 x 2.
Sources
Checked
- IUPAC Gold Book: ionic strength (I03180)
- Wikipedia: Ionic strength (I = 1/2 sum c z^2, cites the IUPAC Gold Book entry I03180)
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.