Buffer solutions

Buffered and Isotonic Solutions
Buffers are compounds or mixtures of compounds that resist changes in pH upon the addition of small quantities of acid or base. The resistance to a change in pH is known as buffer action
A combination of a weak acid and its conjugate base (i.e., its salt) or a weak base and its conjugate acid acts as a buffer. If 1 mL of a 0.1 N HCl solution is added to 100 mL of pure water, the pH is reduced from 7 to 3. If the strong acid is added to a 0.01 M solution containing equal quantities of acetic acid and sodium acetate, the pH is changed only 0.09 pH units because the base Ac- ties up the hydrogen ions according to the reaction



If a strong base, sodium hydroxide, is added to the buffer mixture, acetic acid neutralizes the hydroxyl ions as follows:



The Buffer Equation

Common Ion Effect and the Buffer Equation for a Weak Acid and Its Salt

The pH of a buffer solution and the change in pH upon the addition of an acid or base can be calculated by use of the bufferequation. This expression is developed by considering the effect of a salt on the ionization of a weak acid when the salt and the acid have an ion in common.
the buffer equation or Henderson–Hasselbalch equation, for a weak acid and its salt:



The term pKa, the negative logarithm of Ka, is called the dissociation exponent.
The buffer equation is important in the preparation of buffered pharmaceutical solutions; it is satisfactory for calculations within the pH range of 4 to 10.

The Buffer Equation for a Weak Base and Its Salt

Buffer solutions are not ordinarily prepared from weak bases and their salts because of the volatility and instability of the bases and because of the dependence of their pH on pKw, which is often affected by temperature changes.
Pharmaceutical solutions—for example,