Chemistry Formulas

Ionic Strength Formula

What is Ionic Strength?

This article deals with the ionic strength formula. Ionic strength refers to a measure of the concentration of ions present in a particular solution. Ionic compounds can certainly dissolve in water. On dissolving in water, they dissociate into ions.

Furthermore, the total electrolyte concentration present in the solution has an effect on the significant properties like the solubility of various salts or the dissociation constant. Moreover, ionic strength happens to be one of the main characteristics of a solution that has dissolved ions.

Ionic Strength Formula

This concept of ionic strength was formulated by Lewis and Randall in 1921. This concept is based on the dissociation that involves bases, acid, and salts in the presence of aqueous solution. Knowledge of ionic strength is important to chemists.

This is because the ions have an electrical charge that can attract or repel each other. Furthermore, this attraction and repulsion result in particular behavioral ways of ions. Moreover, ionic strength is representative of the interactions taking place between the ions of solutions and the ions in water.

Derivation of Ionic Strength Formula

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

\(I=\frac{1}{2}\sum ^{n}_{i=1}c_{i}z^{2}_{i}\)

Here, half the term ½ is because we consider both the ions as cation and anion. Furthermore, c refers to the concentration in molar units’ mol/L. Moreover, z refers to the charge of each ion. For example, ions contained in sulfate \(\left ( SO_{4}^{2-} \right )\) z = 2. Therefore, the multivalent ion has a bigger contribution.

Importance of Ionic Strength

The ionic strength plays a very important role in the Debye–Hückel theory. Furthermore, the Debye–Hückel theory describes the strong deviations from ideality that happen in ionic solutions.

Moreover, ionic strength is also important for the theory of double layer and related electroacoustic and electrokinetic phenomena in various heterogeneous systems and colloids. T

hat is, the Debye length is inversely proportional to the square root of the ionic strength. Also, the Debye length happens to be the inverse of the Debye parameter (κ).

Usage of both molar and molal ionic strength has taken place, often without explicit definition. Debye length happens to be the characteristic of the double layer thickness.

Furthermore, Increasing the valence or concentration of the counterions results in compression of the double layer and escalation of the electrical potential gradient.

Experts make use of the media of high ionic strength in stability constant determination. Furthermore, this stability constant determination helps in the minimization of changes during titration at lower concentrations in the activity quotient of solutes.

Natural waters such as seawater and mineral water often carry non-negligible ionic strength. This non-negligible ionic strength is because of the presence of dissolved salts which significantly impacts their characteristics.

The usage of ionic strength takes place in molecular biology and biochemistry. Moreover, ionic strength helps in the determination of the buffer solution’s strength. Also, these buffer strength must have concentrations similar to those existing in nature.

Considerations for Ionic Strength

Non-ideal solutions refer to those solutions that do not follow the ideal behavior. A good example can be solutions where one must consider the interaction forces. Above all, here the usage of a unit of molality (mol/kg of H2O) takes place rather than molarity.

Solved Question For You

Question- There is a solution of potassium chloride 3M. Calculate its ionic strength?

Answer- First of all, the dissociation will be \(KCL\rightarrow K^{+}+Cl^{-}\). Furthermore, each ion’s concentration is the same as the concentration of the salt, 3 mol/L.

Now, one can apply the equation:

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

Hence, the ionic strength is 3 M.

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