Buffer Solutions

Science > Chemistry > Physical Chemistry > Ionic Equilibria >Buffer Solutions

In this article, we shall study the concept of buffer solution, its characteristics, its types, and preparations.

Buffer Solution:

A solution, which resists the change in its pH value, even on the addition of a small amount of strong acid or base is called a buffer solution or buffer.

Example: Mixture of acetic acid (CH₃COOH) and Sodium acetate CH₃COONa in water.

Characteristics of Buffer:

• It has a definite pH value.
• Its pH value doesn’t change on keeping for a long time
• Its pH value doesn’t change on dilution.
• Its pH value doesn’t change even with the addition of a small amount of a strong acid or a base.

Buffer Action:

The property of the solution to resist the changes in its pH value on the addition of small amounts of strong acid or base is known as buffer action.

Types of Buffer:

Acidic Buffer:

A mixture of a weak acid and its salt of a strong base in water is called an acidic buffer.  The pH value of acidic buffer is less than 7.

Preparation: Acidic buffer is prepared by mixing weak acid and its salt with a strong base in a water medium.

Examples: CH₃COOH + CH₃COONa (the mixture of acetic acid and sodium acetate in water) and HCOOH + HCOONa (the mixture of formic acid and sodium formate in water)

Basic Buffer:

A mixture of a weak base and its salt of strong acid in a water medium is called a basic buffer. Its pH value is greater than 7.

Preparation: It is prepared by mixing a weak base and its salt of strong acid in a water medium.

Examples: NH₄OH + NH₄CI (the mixture of ammonium hydroxide and ammonium chloride in water) and NH₄OH + NH4NO₃ (the mixture of ammonium hydroxide and ammonium nitrate in water in water)

A Single Salt Solution:

When a single salt of a weak acid and a weak base is dissolved in water a buffer solution is obtained. Its pH value depends on the relative strength of the weak acid and weak base.

Preparation: It is prepared by mixing single salt of a weak acid and a weak base in water.

Examples: CH₃COONH₄ (ammonium acetate)

Notes:

• Strong Acid Buffers: Strong acids such as nitric acid, hydrochloric acid or sulphuric acid can act as a buffer with low pH. As these acids are almost completely ionized, the concentration of hydrogen ions is high. The addition of a small amount of acid or base to these acids will have a negligible effect on the pH of the solution.
• Strong Base Buffers: A strong base such as sodium hydroxide, potassium hydroxide can act as a buffer with high pH. As these bases are almost completely ionized, the concentration of hydroxyl ions is high. The addition of a small amount of acid or base to these bases will have a negligible effect on the pH of the solution.

Mechanisms of Buffer Action:

Mechanism of Buffer Action of Acidic Buffer:

The property of the solution to resist the changes in its pH value on the addition of small amounts of strong acid or base is known as buffer action.

Consider an acidic buffer, a mixture of acetic acid (CH₃COOH) and sodium acetate (CH₃COONa). In an aqueous medium, CH₃COOH and CH₃COONa dissociates as,

CH₃COOH_(aq)  ⇌ CH₃COO^–_(aq)  + H⁺_(aq)  (Slight ionisation)

CH₃COONa_(aq) → CH₃COO^– _aq) + Na⁺_(aq)     (Complete ionisation)

If a strong acid like HCI is added to the buffer solution, the additional H⁺ ions combine with the acetate ions in the solution to produce undissociated CH3COOH.

H⁺_(aq) +   CH₃COO^–_(aq) →   CH₃COOH_(aq)

Since additional H⁺ ions of acid are consumed (neutralized), the pH of the solution remains unchanged. This resistance to change in pH on the addition of a strong base is called as reserve basicity and is due to CH₃COO^– ions.

If strong base like NaOH is added to the buffer solution, addition­al OH^– ions combine with CH₃COOH as

NaOH_(aq) +  CH₃COOH_(aq) → CH₃COONa_(aq) + H₂O

OH^–_(aq) +  CH₃COOH_(aq) → CH₃COO^–_(aq) + H₂O

Since additional OH^– ions of the base are consumed or neutralized, the pH of the solution remains unchanged.  This resistance to change in pH on the addition of base is called reserve acidity and is due to CH₃COOH.

Mechanism of Buffer Action of Basic Buffer:

The property of the solution to resist the changes in its pH value on the addition of small amounts of strong acid or base is known as buffer action.

Consider a basic buffer, the mixture of Ammonium hydroxide (NH₄OH) and Ammonium chloride (NH₄Cl) In an aqueous medium NH₄OH and NH₄Cl dissociates as

NH₄OH_(aq)  ⇌ NH₄⁺_(aq) +  OH^–_(aq)          (Slight ionisation)

NH₄Cl_(aq) → NH₄⁺_(aq) +  Cl^–_(aq)           (Complete ionisation)

If a strong acid like HCI is added to the buffer solution, additional H⁺ ions of acid combine with NH₄OH, to produce ammonium ions and water.

HCl_(aq) +  NH₄OH_(aq)  → NH₄Cl_(aq) + H₂O

H⁺_(aq) +  NH₄OH_(aq)  → NH₄⁺_(aq) + H₂O

Since additional H⁺ ions of acid are consumed (neutralized), the pH of the solution remains unchanged.  This resistance to the change in pH upon the addition of strong acid is called reserve basicity and is due to NH₄OH molecules.

If a strong base like NaOH is added to the buffer solution, addi­tional OH^– ions of the base combine with NH₄⁺ ions to produce undissoci­ated NH₄OH molecules.

OH^– _(aq)   +    NH₄⁺ _(aq) →   NH₄OH_(aq)

Since additional OH^–  ions of the base are consumed (neutralized) pH of the solution remains unchanged.  This resistance to change in pH on addition base is called as reserve acidity and is due to NH₄⁺  ions in a solution.

Mechanism of Buffer Action of Single Salt Solution:

The property of the solution to resist the changes in its pH value on the addition of small amounts of strong acid or base is known as buffer action.

Consider a single salt buffer solution of ammonium acetate (CH₃COONH₄). In an aqueous medium CH₃COONH₄ dissociates as,

CH₃COONH_4(aq) ⇌ CH₃COO^–_(aq)+    NH₄⁺ _(aq)

If a strong acid like HCI is added to the buffer solution, additional H⁺ ions of acid combine with CH₃COO^–, to produce practically undissociated CH₃COOH

H⁺_(aq) +  CH₃COO^–_(aq)  →   CH₃COOH_(aq)

Since additional H⁺ ions of acid are consumed (neutralized), the pH of the solution remains unchanged.  This resistance to the change in pH upon the addition of strong acid is called reserve basicity and is due to CH₃COO^– ions.

If a strong base like NaOH is added to the buffer solution, additional OH^– ions of base combine with NH₄⁺ ions to produce practically undissociated NH₄OH molecules.

OH^– _(aq)  +  NH₄⁺_(aq)  →  NH₄OH_(aq)

Since additional OH – ions of the base are consumed (neutralized) pH of the solution remains unchanged.  This resistance to change in pH upon addition base is called as reserve acidity and is due to NH₄⁺ ions in a solution.

Reserve basicity:

The resistance of a buffer solution to change in pH upon addition of a strong acid is called ‘reserve basicity’

Reserve acidity:

The resistance of a buffer solution to change in pH upon addition of a strong base is called ‘reserve basicity’

pH of a Buffer Solution:

pH of a buffer solution is calculated by applying the Henderson-Hasselbalch equation. Let us consider an acidic buffer consisting of weak acid HA and its salt NaA

Consider dissociation of the acid

HA  + H₂O  ⇌ H₃O⁺+  A^–     (Slight ionisation)

_NaA   → Na⁺_(aq)  +  A^– _(aq)    (Complete ionisation)

Now the salt NaA is completely dissociated. Hence [A^–] = [NaA] = [Salt]. and as HA is almost non dissociated [HA] = [Acid]

Similarly for the basic buffer

Application of Buffer Solution:

• Buffers are used in the laboratory.  In inorganic qualitative and quantitative analysis.
• It is often necessary to adjust the pH of solutions by the calorimetric method.
• They are used in various industrial process viz electrodeposition of metals, tanning of leather, brewing of alcohols, manufacture of paper, etc.
• They are used in the pathological analysis.
• Buffers are also used as stabilizers and preservatives e.g. sodium citrate is used to stabilize penicillin preparations, while sodium benzoate is used as a buffer to preserve jams and jellies. Sulphate preparations are preserved by acetate or acetate buffers,
• Buffers are used to maintain the pH of the soil for a particular crop or horticulture.
• The chemical changes occurring in life processes take place at a definite pH. e.g. the pH of the blood of normal human beings is 7.4. The electrolyte present in the blood act as a buffer solution to maintain the desired value of the pH of the blood.

Science > Chemistry > Physical Chemistry > Ionic Equilibria >Buffer Solutions