Ionization of Electrolytes

Learn the basics of ionization

The degree of ionization is considered necessary in pharmacy because of its physicochemical properties and physiologic implications. We have learned that water is known as a polar molecule, and thus ionic substances get easily dissolved into it. When there is the presence of ions in water, it can conduct electricity, and the solution formed is known as an electrolytic solution. In an ionic equilibrium, the substances dissociate into ions in the polar solvents. Thus formed after the ionization process, the ions always stay in equilibrium with their undissociated solute in the solution.

Here is the representation for an ionic equilibrium:

XaYb ⇄ aXb+   +   bYa-

Definition of electrolyte

An electrolyte is a substance that has ions in it and usually behaves like an electrically conductive solution. Because of ions in the solution, the electrolytes are often tagged as ionic solutions. Conductivity measures the water’s ability to conduct an electric current through it. The more the number of ions in the solution, the higher the conductivity and the stronger the electrolyte.

Factors affecting the conductivity of electrolytes:

The following factors play a key role in deciding the conductivity of an electrolyte:

• Concentration of ions

The higher is the concentration of ions in the solutions, the higher is the level of conductivity present in the ionic solution.

• Substance type

Whether a substance is a weak electrolyte, a strong electrolyte or a nonelectrolyte, all will affect the conductivity of solution because there is a difference in the level of concentration of ions in the solution in every case. The strong electrolytes quickly form the ions, while weak electrolytes cannot quickly form the ions, and nonelectrolytes do not form the ions in the solutions.

• Temperature

The higher the temperature of the solution, the higher the solubility of the solute dissolved in it, and thus higher is the conductivity.

Categorization of electrolytes

There are two categories of electrolytes:

• Strong electrolytes

A strong electrolyte is a solution that contains a large percentage of dissolved solutes in the form of ions. Strong electrolytes are a good conductor of electric current. For instance, HCI dissolves into water to form hydrochloric acid. Here the ionic compounds and the polar compounds break down entirely into ions and thus become good conductors for current. Common examples include salt, mineral acids, and alkalis.

• Weak electrolytes

There are polar molecular compounds that get dissolved into water and become electrolytes. However, they do not get ionized to a great extent. Thus, a weak electrolyte is the one solution with a significantly less fraction of the dissolved solute in the form of ions. The weak electrolytes are considered to be a bad conductor of electricity. An example of a weak electrolyte is the gaseous nitrous acid that gets ionized into nitrate ions and hydrogen ions in the solution. But in a very weak manner.  Common examples include oxalic acid and acetic acid.

Writing equations for  ionization for a weak electrolyte

The aqueous nitrous acid comprises 95% of the intact nitrous acid molecules and just 5% of the ions. The equation that shows the ionization for a weak electrolyte uses a double arrow to indicate an equilibrium state between the products and reactants.

Here is how it goes:

HNO2 (aq) ⇄ H+(aq) + NO-2 (aq)

Ostwald’s dilution law

Ostwald’s Dilution Law is about applying the law of mass action to a solution’s weak electrolytes. 

Let’s consider a binary electrolyte AB dissociating into A+ and B- ions.

The equation for the ionization will be:

AB ⇄ A+ + B-

In a weak electrolyte, the degree of ionization is always inversely proportional to the square root of the molar concentration or is directly proportional to the volume’s square root that contains one mole of the solute. This is how we describe Ostwald’s Dilution Law.

For a weak electrolyte, α <<< 1, (1 – α) = 1

∴ K=Cα2

α= (K/C)1/2

The above law holds only in the study of  ionization of weak electrolytes. It fails in the case of strong electrolytes.

Ionic equilibrium 

It is necessary to study the percentage of a fraction of the initial amount of reactants converted into the product to attain the state of equilibrium. The fraction of the initial molecules so converted at equilibrium is the degree of ionization or dissociation.

Degree of ionization or dissociation= α (No. of reactant molecules dissociated or ionized) / ( No. of reactant molecules at the start)

Factors affecting the degree of dissociation

• Nature of electrolyte

The electrolyte that is strong, weak, or insoluble has a significant role in deciding the degree of ionization.

• Nature of solvent

The high dielectric solvents increase the level of ionization.

• Dilution

The more significant is the level of dilution, the more is the extent of ionization.

• Temperature

Higher temperature leads to higher ionization.

Conclusion

The ionization procedure of electrolytes is often used in the drugs and pharmaceutical industries. The percent ionized is the fraction of the original compound that has been ionized. The function of the electrolyte solution is to carry the charge in the liquid phase and close the circuit for maintaining the flux of the charge. The dilution, nature of electrolyte and solvent, and presence of solute affect the degree of ionization.