How to Calculate the Ionic Strength of a Solution

Written by nina stanley
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How to Calculate the Ionic Strength of a Solution
Ionic strength helps determine ion behaviour. (Beaker With Blue Liquid image by explicitly from

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 against 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. Although ionic strength has been looked at for over the past 100 years, it is still not fully understood. Ionic strength is calculated by using a mathematical formula proposed in 1923 by Peter Debye and Erich Huckel.

Skill level:

Things you need

  • Concentrations of solutions
  • Periodic table with valences

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  1. 1

    Use this formula to calculate ionic strength: I= 1/2 n'i (CiZi )squared.

  2. 2

    Represent ionic strength with a capital I in the formula.

  3. 3

    Let "n" represent the number of ions in solution. Since positive and negative ions can not separate, this needs to be considered as a factor in the equation.

  4. 4

    Let "i" represent the specific ion in solution. "i" equal's 1.

  5. 5

    Allow "Ci" to represent the concentration to the /th species, such as mole per litre.

  6. 6

    Let "Zi" represent the valence or oxidation number of the /th species. This is also known as the electrical charge of the ion.

  7. 7

    Represent the summation of concentrations and valences of all ions by ' .

  1. 1

    Determine the ionic strength of 1.0 M La2(SO4) plus 1.0 M CaCl2.

  2. 2

    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. 3

    Input concentrations and valences into the Debye and Huckel equation.

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

  4. 4

    Understand 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.

  5. 5

    Calculate for result. The ionic strength equals 18.0.

  1. 1

    Use an ionic strength calculator to determine the ionic strength of a solution. These calculators can be found on the Internet or in a software program.

  2. 2

    Select ion. Most calculators will already have the ions ready to choose from along with valences and weight considered.

  3. 3

    Input concentration of solution. If the concentration is 1.0 M, type in 1 for concentration.

  4. 4

    Press "Calculate" or "Ionic Strength" button for the ion strength. Using a calculator helps minimise human math errors.

Tips and warnings

  • Having a basic background in chemistry is useful in determining molar concentration and valences.
  • Consider all solutions potentially hazardous.

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