How To Calculate The Ionic Strength Of A Solution

Ionic strength is the total ion concentration in solution. Knowing ionic strength is important to chemists because ions have an electrical charge that attract or repel each other. This attraction and repulsion causes ions to behave in certain ways. Basically ionic strength represents interactions between the ions in water and the ions of a solution. Calculate ionic strength using a mathematical formula proposed in 1923 by Peter Debye and Erich Huckel.

1. Apply Formula

Use this formula to calculate ionic strength: I= 1/2 n∑i (CiZi )squared, where "I" represents ionic strength, "n" represents the number of ions in solution, "i" represents the specific ion in solution, "Ci" represents the concentration to the /th species, such as moles per liter, "Zi" represents the valence or oxidation number of the /th species and "∑" represents the summation of concentrations and valences of all ions. Remember positive and negative ions cannot separate, which needs to be considered as a factor in the equation.

For example, say you want to find the ionic strength of 1.0 M La2(SO4) plus 1.0 M CaCl2.

2. List Concentrations

List the concentrations. For example, La 3+= 2.0 M, SO4 2- =3.0 M, Ca2 1+= 1.0 M, Cl 1- = 2.0 M.

3. Input Concentrations and Valences

Input concentrations and valences into the Debye and Huckel equation.

I = ½(23(Squared)+32(Squared)+12(Squared)+21(Squared)).

4. Find Result

Calculate for result, understanding that the formula is the molar concentration of the ion multiplied by the valence squared. Take {2*32} in the formula above for example. The 2 is the molar concentration of La (Lanthanum), the 3 is the valence of electrons of La, and the valence is squared. The ionic strength is 18.0.