Preparation of Buffer Solution

A buffer solution can be made by controlling the salt acid or salt base ratio if the dissociation constants of the acid (pKa) and the base (pKb) are known. Weak bases and their conjugate acids, or weak acids and their respective conjugate bases, are used to make these solutions. The Handerson-Hasselbalch equation and the preparation of acidic buffer and basic buffer.

Henderson-Hasselbalch Equation

Henderson-Hasselbalch equation is the equation that provides a relationship between the pH of acids and their pKa.

The Henderson-Hasselbalch equation is,

pH = pKa + log10 ([A^–] / [HA])

where 
[A^–] denotes the molar concentration of the Conjugate Base (of the Acid)
[HA] denotes the molar concentration of the Weak Acid

Importance of Henderson Equation

The significance of the Henderson Equation is,

• It helps to determine the pH of the buffer obtained from a mixture of salt and weak acid or base.
• To determine the pKa value.
• To maintain the buffer solution of the required pH.

Limitations of Henderson-Hasselbalch Equation

The Henderson-Hasselbalch Equation is not applicable in many cases are:

• This equation does not explain the water self-dissociation tendency. 
• The assumptions stated by this equation are not applicable when strong acids or bases are associated.

A buffer solution can easily be prepared using Henderson-Hasselbalch Equation

Preparation of Acid Buffer

In an acid buffer solution with a strong base (KOH), consider a weak acid (HA) and its salt (KA). The weak acid (HA) ionizes, and the equilibrium is as follows:

H₂O + HA ⇋ H⁺ + A^–

The acid dissociation constant is,

K_a = ([H⁺] [A^–])/[HA]

RHS and LHS take negative logs:

-log K_a = -log [H⁺] – log ([A^–]/[HA])

pK_a = pH – log ([salt]/[acid])

pH = pK_a + log ([salt]/[acid])

pH of acid buffer solution = pK_a + ([salt]/[acid])

Preparation of Base Buffer

Consider a basic buffer solution with strong acid, salt (BA), and a weak base (B).

As a result, the basic buffer solution will be,

pOH = pKb +  log ([salt]/[base])

pOH of a basic buffer solution = pK_b +  log ([salt]/[acid])

pH of a basic buffer solution = pK_a – log ([salt]/[acid])

Buffering Capacity

Buffering Capacity is the property of solutions which is represented by the symbol β. It is defined as the ratio of the moles of acid or base required to change the pH of the solution by 1 and the pH change and the volume of buffers.

Β = millimoles /(ΔpH)

Buffer Solution is a special aqueous solution that resists the change in its pH when some quantity of acid and Base is added. Many fluids, such as blood, have specific pH values of 7.14, and variations in these values indicate that the body is malfunctioning.  The change in pH of Buffer Solutions on adding a small quantity of acid or bases is very minimal and hence they are used to make solutions that resist the change in pH.

Let us learn about Buffer solution, its types, and others in this article.