Buffer Solution: Buffer Equation And Buffer Capacity

Buffer solution

A buffer solution is a solution that resist the changes in pH values. It consists of a mixture of weak acid and its salt or a weak base and its salt. This solution is able to neutralize the effect of small amount of acid or alkali added to solution. Buffer solutions are prepared by mixing a weak acid with its salt or a weak base with its salt. Buffer solutions are used in formulation analysis of drug products to maintain a stable pH in various stages of drug development and production. In this article we will discuss about types of buffer solutions, buffer equation, mechanism of buffer solution, buffer capacity and standard buffer solution.

Table of Contents

Types of buffer solutions

Acidic buffer solution: This buffer solution contains a mixture of weak acid and its salt. For example, weak acid: acetic acid, salt: sodium acetate.

Basic buffer solution: This buffer solution contains a mixture of weak base and its salt. For example, weak base: ammonia, salt: ammonium chloride.

Properties of buffer solution

It has a constant pH.
Its pH does not change on dilution.
Its pH does not change on standing for long time.
Its pH does not change by addition of small quantities of acid or bases.

Buffer equation

The buffer equation for weak acid and its conjugate base is given by the following Handerson-Hasselbalch equation,

pH = pKa + log ([A^–]/[HA])

where, pH is the pH of buffer solution

pKa is the dissociation constant of weak acid

[A^–] is the concentration of conjugate base

[HA] is the concentration of weak acid

The buffer equation for a weak base and its conjugate acid is given by the following Handerson-Hasselbach equation,

pOH = pKa + log ([BH⁺]/[B])

Where, pH is the pH of buffer solution

pKa is the dissociation constant of weak base

[BH⁺] is the concentration of conjugate acid

[B] is the concentration of weak base

pH + pOH = 14

Mechanism of buffer action

The mechanism of buffer action involves the interaction between weak acids and their conjugate bases or weak bases or their conjugate acids, to resist change in pH when small amount of acid or base added to solution. The buffer action depends on the acid base equilibrium and the ability of buffer system to nullify the effect of addition.

Buffer action of acidic buffer

Consider an acidic buffer containing acetic acid and sodium acetate. Acetic acid is a weak electrolyte and sodium acetate is a strong electrolyte. Dissociation of acetic acid is supressed due to common ion (CH₃COO^–). As a result, the solution contains less H⁺ ions and more Na⁺ and CH₃COO^– ions.

When a small quantity of an acid is added H+ ions from solution combine with CH₃COO^–and forms undissociated CH₃COOH.

H⁺ + CH₃COO^– = CH₃COOH

When small quantity of base is added the OH^– ions are neutralized by acetic acid.

OH- + CH₃COOH = H₂O + CH₃COO^–

The net result is pH does not change with addition of small quantity of acid and base.

Buffer action of basic buffer

Consider a basic buffer containing ammonium hydroxide and ammonium chloride. When a small quantity of base added OH^– combines with NH₄⁺ ions.

COH^– + NH₄⁺ = NH₄OH

When a small quantity of acid is added NH₄OH neutralizes it.

H⁺ + NH₄OH = NH₄⁺ + H₂O

The net result is pH does not change with addition of small quantity of acid and base.

Buffer capacity

Buffer capacity is the moles of strong acid or strong base required to change the pH of 1 lit of solution by 1 unit. The buffer solution having higher buffer capacity accommodate more acid or base without change in pH, hence considered as good buffer solution. The buffer capacity is the ratio of pH difference caused by addition of an acid or base by the moles or grams of an acid or base needed. It can be expressed mathematically as follows,

β = Δn / ΔpH

Where β is the buffer capacity

Δn is the number of moles of an acid or base added to buffer

ΔpH is the change in pH

Standard buffer solution

A standard buffer solution is a solution with known and precise pH value that is used for reference purpose in pH measurements and carry out pharmacopoeial test. Standard buffer solutions for various pH range between 1.2 to 10 can be prepared by appropriate combination of 0.2 N HCl or 0.2 N NaOH with solutions like potassium hydrogen phthalate, potassium dihydrogen phosphate, boric acid-potassium chloride as described in pharmacopoeia. For example,

Preparation of potassium hydrogen phthalate 0.2 M: Dissolve 40.846 gm of potassium hydrogen phthalate in water and dilute with water to 1000 ml.

Preparation of potassium dihydrogen phosphate 0.2 M: Dissolve 27.218 gm of potassium dihydrogen phosphate in water and dilute with water to 1000 ml.

Preparation of boric acid-potassium chloride 0.2 M: Dissolve 12.366 gm of boric acid and 14.911 gm of potassium chloride in water and dilute with water to 1000 ml.

Conclusion

Buffer solution is a crucial aspect in the study of pharmaceutical formulations, analytical techniques and quality control processes. Buffers play an important role in ensuring the efficacy, safety and stability of medication. The applications of buffers range from stabilizing drug pH and enhancing solubility to ensuring accurate measurements in pharmaceutical analysis.

Frequently asked questions

What is a buffer solution?

How buffer solutions are prepared?

Buffer solutions are prepared by mixing a weak acid with its salt or a weak base with its salt.

What are the types of buffer solutions?

There are two types of buffer solutions i.e. acidic and basic buffer solutions. Acidic buffer solutions have pH below 7 and basic buffer solution have pH above 7.

What is the main function of buffers?

The main function of buffer is to resist change in pH of solution due to addition of small quantity of acid or bases.

Buffer Solution: Buffer Equation and Buffer Capacity