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Buffer

What is Buffer in Chemistry?

A solution whose pH is not altered to any great extent by the addition of small quantities of either an acid or base is called buffer solution.

Buffer is also be defined as the solution of reserve acidity or alkalinity which resists change of pH upon the addition of small amount of acid or alkali.

Many chemical reaction are carried out a constant pH. In nature, there are many systems that use buffering for pH regulation. For example, the bicarbonate buffering system is used to regulate the pH of blood, and bicarbonate also acts as a buffer in the ocean.

Table of Contents

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Characteristics of buffer solution

(i) It has a definite pH.

(ii) Its pH does not change on standing for long periods of time.

(iii) Its pH does not change on dilution.

(iv) Its pH is slightly changed by the addition of small quantity of an acid or base.

Types of buffer solutions

(a) Acidic Buffer:

It is formed by the mixture of weak acid and its salt with a strong base.

Examples: (i) CH3COOH + CH3COONa, (ii) HCN + NaCN, (iii) Boric acid + Borax ect.

(b) Basic Buffer:

It is formed by the mixture of weak base and its salt with strong acid.

Examples: (i) NH4OH + NH4Cl, (ii) NH4OH + NH4NO3, (iii) Glycine + Glycine hydrochloride

(c) Simple Buffer:

It is formed by a mixture of an acid salt and normal salt of a polybasic acid,

example Na2HPO4 + Na3PO4

Or a salt of weak acid and a weak base. Example : CH3COONH4

Buffer Actions

(a) Acidic Buffer:

It is the mixture of CH3COOH and CH3COONa in aqueous solution.

CH3COOH ⇋ CH3COO + H+ (incomplete dissociation)

CH3COONa → CH3COO + Na+ (complete dissociation)

H2O ⇋ H+ + OH (incomplete dissociation)

Action of acid: when a drop of stong acid (HCl) is added in the above buffer solution H+ ions combine with CH3COO- ions to form feebly ionised CH3COOH. Whose ionisation is further suppressed due to common ion effect. So pH of the solution unaltered.

Action of base: when a drop of strong base (NaOH) is added to the above buffer solution it react with free acid to form undissociated water molecules. So pH of the solution unaltered.

CH3COOH + OH ⇋ CH3COO + H2O

(b) Basic Buffer:

It is the mixture of NH4OH and NH4Cl in aqueous solution.

NH4OH ⇋ NH4+ + OH (incomplete dissociation)

NH4Cl → NH4+ + Cl (complete dissociation)

H2O ⇋ H+ + OH (incomplete dissociation)

Action of acid: when a drop of HCl is added, the added H+ ions combine with NH4OH to form undissociated water molecules. So the pH of buffer is unaffected.

NH4OH + OH–   ⇋  NH4+ + H2O

Action of base: when a drop of NaOH is added, the added OH ions combine with NH4+ ions to form feebly ionised NH4OH. It is further suppressed due to common ion effect. So the pH of buffer is unaffected.

Hendersion’s Equation (pH of buffer)

(a) Acidic Buffer:

It is a mixture of CH3COOH and CH3COONa

CH3COOH ⇋ CH3COO + H+

CH3COONa → CH3COO +  Na+

By the law of chemical equilibrium,  Ka = {[CH3COO] [H+]} / [CH3COOH]

∴ [H+] = {Ka [CH3COOH]} / [CH3COO]

Taking negative log both sides, we obtain that

– log[H+] = – log Ka – log {[CH3COOH]/[CH3COO]}

pH = pKa + log {[CH3COO]/[CH3COOH]}

pH = pKa + log {[salt] / [acid]}

This equation is known as Hendersion’s Equation

Where, Ka = dissociation constant

[CH3COO] = initial concentration of salt

[CH3COOH] = initial concentration of acid

(b) Basic Buffer:

It is a mixture of NH4OH and NH4Cl

NH4OH ⇋ NH4+ + OH

NH4Cl → NH4+ + Cl

By the law of chemical equilibrium,   Kb = {[NH4+] [OH]} / [NH4OH]

∴ [OH] = {Kb [NH4OH]} / [NH4+]

Taking negative log both sides, we obtain that

– log [OH] = – log Kb – log {[NH4OH] / [NH4+]}

pOH = pKb + log { [NH4+] / [NH4OH]}

pOH = pKb + log {[salt] / [base]}

This equation is known as Hendersion’s Equation

Where, Kb = dissociation constant

[NH4+] = initial concentration of salt

[NH4OH] = initial concentration of base

pH + pOH = 14

Buffer capacity

Buffer capacity is defined as the number of moles of acid or base added in one litre of solution as to change the pH by unity.

Buffer capacity (Φ) = No. of moles of acid or base added to 1 litre solution / change in pH

Φ = ∂b /∂(pH)

Where ∂b – No. of moles of acid or base added to 1 litre

∂(pH) – change in pH

Applications of Buffer in chemistry

(i) Buffers are used in industrial processes such as manufacture of paper, dyes, inks, paints, drugs, etc.

(ii) Buffers are also employed in agriculture, dairy products and preservation of various types of foods and fruits.

(iii) It is used to determine the pH with the help of indicators.

(iv) Blood is the natural buffer, it maintenance of pH is essential to sustain life because, enzyme catalysis is pH sensitive process. The normal pH of blood plasma is 7.4.

(v) For the removal of phosphate ion in the qualitative inorganic analysis after second group using CH3COOH + CH3COONa buffer.

Frequently Asked Questions – FAQs

What is buffer and its types?

The solution which opposes the change in their pH value on addition of small amount of strong acid or strong base is known as buffer solution. These are mainly acidic buffer and basic buffer.

What are the properties of buffers?

The properties of buffet are (i) pH of buffer solution are reserved. (ii) Its pH does not change on standing for long periods of time. (iii) Its pH does not change on dilution.

What is pH stand for?

The abbreviation pH stands for potential hydrogen. It is a scale used to specify the acidity or basicity of an aqueous solution. pH is the negative of the base 10 logarithm of the activity of the H+ ion. Mathematically pH = – log [H+]

What is the pH of blood?

Blood has a normal pH range of 7.35 to 7.45. This means that blood is naturally slightly alkaline or basic.

Is milk an alkaline or acid?

The pH of milk is 6.7 to 6.9, making it slightly below neutral and therefore acid-forming. The exception is raw milk, which may be more alkalizing than pasteurized milk.

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