How to Calculate the Ionic Strength of a Solution

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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

  2. 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.

  3. List Concentrations

  4. 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.

  5. Input Concentrations and Valences

  6. Input concentrations and valences into the Debye and Huckel equation.

    I = ½(2_3(Squared)+3_2(Squared)+1_2(Squared)+2_1(Squared)).

  7. Find Result

  8. 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.

    Things You'll Need

    • Concentrations of solutions
    • Periodic table with valences


    • A basic background in chemistry is useful in determining molar concentration and valences.

      You can use an ionic strength calculator to find ionic strength of a solution, which minimizes math errors. Select "ion" and input concentration of solution. For example if the concentration is 1.0 M, type 1 for concentration. Press "Calculate" or "Ionic Strength" to complete the calculation.


    • Consider all solutions potentially hazardous.


About the Author

Specializing in home and garden topics, Nina Stanley begin her freelance career in 2009. She received her Bachelor of Science in horticulture in 2007 from Sam Houston State University and Associate of Science in 2002 from Blinn College. Through her writings, she shares her knowledge of plants with others on various websites.