Lecture 7: Buffer Solutions (pH); Reducing and Enlarging (11.11.24)

Buffer solution

A solution that has the ability to maintain a desired pH at a relatively constant level, when a small quantity of an acid or a base is added to it

Types of buffer solutions

1) A weak acid with its conjugate base (a salt of the acid)

2) A weak base with its conjugate acid (a salt of the base)

The most common buffer systems used in pharmacy

1) Preparation of dosage forms that are placed directly into pH-sensitive body fluids

2) Manufacture of formulations in which the pH must be maintained relatively constant to ensure maximum product stability

3) Pharmaceutical tests and assays requiring a specific pH for analytic purposes

Preparation of dosage forms that are placed directly into pH-sensitive body fluids

Dosage forms could be injections, ophthalmic solutions, etc

A weak acid with its conjugate base (a salt of the acid)

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

A weak base with its conjugate acid (a salt of the base)

pH = pKa + log [base] / [salt]

Ka

Acid dissociation constant (at a given temperature)

Kb

Base dissociation constant (at a given temperature)

pKa or pKb

Measurement of the degree of dissociation of an acid or base

pKa

1) Strength of acid in solution

2) Lower pKa = stronger acid

pKa =

1) -log (Ka)

2) 14 - pKb

pKb

1) Strength of base in solution

2) Higher pKb = stronger base

pKb =

-log (Kb)

A weak acid with its conjugate base (a salt of the acid): When [salt] = [acid]

pH = pKa

A weak acid with its conjugate base (a salt of the acid): When [salt] > [acid]

pH > pKa

A weak acid with its conjugate base (a salt of the acid): When [salt] < [acid]

pH < pKa

A weak base with its conjugate acid (a salt of the base): When [salt] = [base]

pH = pKa

A weak base with its conjugate acid (a salt of the base): When [salt] > [base]

pH < pKa

A weak base with its conjugate acid (a salt of the base): When [salt] < [base]

pH > pKa

Calculations for the preparation of a buffer solution: Step 1

Given pH and pKa (or Ka), solve for [salt] / [acid] OR given pH and pKb (or Kb), solve for [base] / [salt]

Calculations for the preparation of a buffer solution: Step 2

Known [salt] / [acid] or [base] / [salt] and given total concentration, solve for [salt] and [acid] or [salt] and [base]

Calculations for the preparation of a buffer solution: Step 3

Given the volume of the buffer solution, solve for the number of moles of salt and acid or salt and base, respectively

Calculations for the preparation of a buffer solution: Step 4

Given the molecular weights, solve for the quantity of salt and acid or salt and base

Buffer capacity (β)

1) The amount of strong acid or base in Eq (typically moles), that need to be added to 1 L of the buffer to change the pH by 1 unit

2) Reflect the efficiency of a buffer solution; is a measure of its ability to resist changes in pH

β =

Acid or Base added (Eq/L) / ∆pH

With the addition of a given amount of acid or base

The smaller the pH change the greater the buffer capacity of the system

Unites States Pharmacopeia - National Formulary

Formulas based on 1000 mL or 1000 g

Pharmacists

May need to reduce or enlarge formulas for pharmaceutical preparations

Irrespective of quantity prepared

The correct proportion of one ingredient to the other in a given formula, must be maintained

Factor =

Required Quantity / Formula Quantity