Weak Bases

In the world of Chemistry, that we're introduced to in our middle and high school, acids and bases are two important class of compounds. Before we move on to know more about weak bases, you must understand that there are numerous theories in Chemistry that define acids and bases.

What are Weak Bases?

While the Bronsted - Lowry definition defines a base as a substance that accepts hydrogen ion or rather protons, the Arrhenius theory of bases defines them as substances that release hydroxide ion in the aqueous solution. It has to be understood that definitions of all these theories (including the Lewis theory of acids and bases) may have some limitations and depending on their applicability, scientists use them to explain properties exhibited by acids and bases.

Weak acids and bases have poor ability to ionize in an aqueous solution.This means that when a base is put in water, it will take up a hydrogen ion (H⁺) from water, leaving an hydroxide ion (OH^-) behind.

When we use Bronsted - Lowry concept of bases, we define weak bases as chemical substances in which addition of protons or hydrogen ions remains incomplete. This makes the pH of weak bases very low as compared to that of strong bases. It must be known that strong bases have a high pH close to 14 on the pH scale while acids have pH value less than 7. The number 7 stands for neutral nature of a chemical compound.

The pH level of an acidic substance is given by pH = -log₁₀[H⁺]. However, since we're dealing with weak bases, it is good to give the pH for hydroxide ion. That is as follows.

POH = - log10[OH-]

While we can understand weak bases by knowing its exact definition, it is good to know what are strong bases, to make our understanding about bases more crystal clear.

Strong bases are chemical substances in which the metal ions and hydroxide ions are fully separated. Consider the examples of strong bases, sodium hydroxide (NaOH) and potassium hydroxide (KOH). When dissolved in water, NaOH separates into Na ions and hydroxide ions completely.

Similarly, weak bases are chemical substances like Ammonia which itself doesn't produce hydroxide ions but it reacts with water to do so. Simply stated, in weak bases, hydroxide ions are not completely ionized in the solution.

How to Determine Strength of Weak Bases

It is easier to measure pH level of acids and bases using pH level formula when hydrogen and hydroxide ion concentrations are known. However, there is a better way to determine strength of bases by a more accurate method. It is based on behavior of bases in water. In chemical equilibrium, the measure of strength of bases is given by measuring base ionization constant (K_b) or the base dissociation constant. The same factor for weak acids is known as acid ionization constant (K_a).

Base Ionization Constant (K_b): It is the equilibrium constant of a chemical reaction that involves the dissociation of hydroxide ions.

Consider, B^- + H₂O ↔ OH^- + BH

The base ionization constant (K_b) equals to products of concentration of the products divided by equilibrium concentration of the original base.

That is, K_b = [BH^-] [OH]^-/[B]

Due to numerous orders of magnitude of K_b, logarithmic value is preferred in calculations. The relationship between K_b and pK_b is just like the pH formula relation. That is,

pKb = - log10pKb

Higher the K_b value, stronger is the base strength. Generally, strong bases have K_b > 1, while weak bases have K_b < 1.

Another formula that can come handy in finding strength of weak bases is the product of K_b, K_a and K_w (ionic product of water). The relation between the three is given by,

Kb x Ka = Kw = 1.0 x 10-14

Ammonia (NH₃), Trimethyl ammonia [N(CH₃)₃], Pyridine (C₅H₅N), Ammonium hydroxide (NH₄OH) are some common weak bases examples. It is a great idea to make a note of any weak base that you get to know daily. Gradually, in your course of studying, you'll be delighted to have found many examples of weak bases.