Definition

The ionic strength of a solution is the amount of ion concentration in it. It’s written in the form of I. It has an impact on ion activity. The ion interaction with water and other ions in the solution is marked. The ionic strength formula is used to calculate half of each ionic species’ total concentration.

The formula for ionic strength is as follows:

where, C_i – ionic concentration

Z_i – ion charges

Table of Contents

• Ionic Strength: Definition and Unit
• The General Formula of Ionic Strength
• Ionic Strength Examples
• Importance and Uses of Ionic Strength
• Frequently Asked Questions

Ionic Strength: Definition and Unit

Different textbooks and monographs define the word ionic strength differently. Some use the SI unit mol kg^-1 to determine it, whereas others use the non-SI unit mol L^-1. Computing means ionic activity coefficients are complicated by the conversion to the SI unit mol m³.

There’s also a dimensionless ionic strength unit. Students find the idea of ionic strength particularly perplexing due to the multiple definitions, especially when utilising it to determine mean activity coefficients using the Debye-Hückel theory. To clear up this ambiguity, it is suggested that ionic strength be defined as a dimensionless quantity.

The General Formula of Ionic Strength

The formula for calculating ionic strength is the sum of each ion’s molar concentration multiplied by the valence squared.

where 1/2 is because both ions (cation and anion) are taken into account, C is the concentration in molar units (mol/L), and Z is the charge of each ion. For example, if the ions are sulphate (SO₄^2-), for example, Z = 2. As can be seen, the multivalent ion makes a more significant contribution.

Ionic Strength Examples

Example 1: Calculate the ionic strength of a 3M potassium chloride solution.

To begin, the dissociation must be drawn: KCl → K⁺ + Cl^–

As a result, each ion has the same concentration as the salt, 3 mol/L.

The equation can then be used:

I = 1/2 [(3 mol/L)(+1)² + (3 mol/L)(-1)²] = 3M

Hence, the ionic strength is 3M.

Example 2: Calculate the ionic strength of a potassium chloride solution of 1M and magnesium sulphate of 0.2M.

First, the dissociation must be drawn: KCl → K⁺ + Cl^– and MgSO₄ → Mg²⁺ + SO₄^2-.

Now applying the equation,

I = 1/2 [(1 mol/L)(+1)² + (1 mol/L)(-1)² + (0.2 mol/L)(+2)² + (0.2 mol/L)(-2)² ] = 3.6 M

Thus, the ionic strength is 3.6 M.

Ionic Strength Considerations

Non-ideal solutions are those that do not conform to the ideal behaviour. Solutions that need consideration of interaction forces are a good example. Above all, the unit of molality (mol/kg of H2O) is used here rather than molarity.

Importance and Uses of Ionic Strength

The Debye–Hückel theory, which describes the severe departures from ideality common in ionic solutions, relies heavily on ionic strength. It’s also crucial for the theory of double layers in colloids and other heterogeneous systems and related electrokinetic and electroacoustic phenomena.

To reduce changes in the activity quotient of solutes at lower concentrations throughout a titration, high-ionic-strength media are utilised in stability constant determination. Due to the presence of dissolved salts, natural waters such as mineral water and seawater typically have a non-negligible ionic strength, which has a substantial impact on their properties.

In theoretical chemistry, ionic strength is used to calculate salt dissociation in heterogeneous systems like colloids. It’s also used in biochemistry and molecular biology to determine the strength of buffer solutions with concentrations that should be close to those found in nature.