Ionic Strength

The concentration or quantity of the ions in a particular solution is termed as ionic strength of that solution. The ionic strength of an ionic compound can only be measured in the solution form and we know that an ionic compound dissociates into positive and negative ions in the water. The ionic strength of a solution is expressed using the symbol 'I'. Randall and Lewis were the first to give the concept of ionic strength in 1921. Some properties of ionic strength are as follows:

• Higher the concentration of the ionic compound or electrolyte in the solution, higher the dissociation constant of the solution.
• Higher the ionic strength, higher the electrical conductivity of the solution.
• Lower the ionic strength, lower will be the conductivity of the solution.
• The ionic strength also denotes the amount of interionic attractions and repulsions in the solution.

Ionic Strength Definition

Thus, we can say that the ionic strength of an ionic compound can be calculated as half of the sum of products of the molar concentration of the ions and the square of their respective charges. The factor of 1/2 comes from the fact that the ionic compound dissociates into cations and anions. We can see that the ionic strength is directly proportional to the square of the charge on the atom and thus the ionic strength of a solution increases rapidly with the increase in charge of the ion.

Ionic Strength Unit

The ionic strength is expressed in Moles per litre(molL^-1)or Moles per kg(mol kg^-1) depending upon the units taken for the concentration of the ions. 

Ionic Strength Formula

The ionic strength formula is as follows

where,
C is the molar concentration expressed in moles per litre
Z refers to the charge on the particular ion

Example: The charge on OH^- ion is -1, so the value of Z for hydroxide ion is -1.

Importance of Ionic Strength

Ionic strength is of great importance in chemistry such as:

• It is very important in Debye Huckle's theory. The deviations in the ionic strength of a solution from the ideal solution is clearly explained by the Debye-Huckle theory.
• Double layer theory, electroacoustic and electrokinetic phenomena are also explained with the help of ionic strength.
• The square root of ionic strength is inversely proportional to the Debye length.
• From the ionic strength formula, it is evident that if the concentration or charge of the ions increases then the electrical conductivity of the solution increases rapidly to a great extent.
• The ionic strength of a solution is impacted by the dissolved salts in a solution.
• The strength of a buffer solution can also be determined by using the ionic strength formula.

Read, More

• Ionic Equilibrium
• Ionic Bonds and Ionic Compounds
• Strength of Acids

Solved Examples on Ionic Strength Formula

Example 1: Calculate the ionic strength of NaOH solution with a concentration of 5 mol/L.

Solution:

NaOH ------> Na⁺ + OH^-

Given,

Concentration (C) = 5 (for both Na⁺ and OH^- ions)

For NaOH, 

Z= 1 for Na⁺
Z =-1 for OH^-

We know that Ionic strength I can be calculated using, 

= 10 / 2 = 5

Thus the Ionic strength of the given solution is 5

Example 2: Calculate the ionic strength of a CaCl₂ solution with a concentration of 2 mol/kg.

Answer:

CaCl₂ ------> Ca⁺² + Cl^- 

Given, 

C = 2 for both the ions,

Z = +2 for Ca⁺² ions

Z = -1 for Cl^- ions

Using ionic strength formula,

I =10 / 2 = 5

Thus, the Ionic strength of the solution is 5.

Example 3: Calculate the concentration of the solution of KCl if the ionic strength of the solution is 20 mol/L.

Answer:

KCl -------> K⁺ + Cl^-

Given, I = 20, 

Z = +1 for K⁺

Z= -1 for Cl^-

Using ionic strength formula,

20 = 2C / 2

20 = C

Thus the concentration of the solution of KCl is 20 moles per litre

Example 4: Compare the ionic strength of 6 mol/l of K₂SO₄ solution and 10 mol/l solutions of NaCl.

Answer:

For K₂SO₄

Given, C = 6, 

Z= +1 for K⁺ ions

Z= -2 for SO₄^-2 ions

Using ionic strength formula,

                   = 26 / 13 = 2

For Nacl, 

Given C = 10, 

Z= +1 for Na⁺ions

Z = -1 for Cl- ions

Using Ionic Strength Formula,

              = 20 / 2 =10

As 13>10, we can say that the  which means that the ionic strength of potassium sulphate solution is more than the sodium chloride solution in this case.

Example 5: Calculate the ionic strength of the solution containing 4N H₂SO₄ and 2M HCl.

Answer:

Here there are two acids dissolved in the solution. Thus the net ionic strength of the solution will be the sum of two ions.

For H₂SO₄

Given, C = 4N

Normality = Molarity × n-factor        (n = 2 for sulphuric acid)

C = 2M

                    =5

For HCl

Given, C = 2M

           =2

Thus net ionic strength is calculated as,