Compute ionic strength and the dilute-solution activity coefficient from ion concentrations.
Ionic strength
How to use this tool
Enter concentration:charge pairs separated by semicolons.
Count each ion separately, with its stoichiometric amount.
The big number is in mol/L.
Valid only for dilute solutions; see the note.
Formula & worked example
0.1 M NaCl plus 0.05 M MgSO4 gives I = 0.5 x (0.1 + 0.1 + 0.2 + 0.2) = 0.3 mol/L.
Limitations — read this before you rely on it
Debye-Huckel limiting law is valid only for dilute aqueous solutions at about 25 C; concentrated or non-aqueous systems need other models.
Frequently asked questions
A measure of the total electric charge concentration in a solution, weighting each ion by the square of its charge.
Because both positive and negative ions shield charges, and the effect grows with charge squared, so a 2+ ion counts four times a 1+ ion.
A correction for non-ideal behaviour: effective concentration equals activity coefficient times concentration.
Above roughly 0.01-0.1 mol/L, where extended Debye-Huckel or Pitzer models are needed.
Split it into ions: 0.1 M MgCl2 becomes 0.1:2 and 0.2:-1, since each formula unit gives one Mg2+ and two Cl-.