Definition of Henderson Hasselbalch Equation
A buffer is a solution that maintains its pH and contains a mixture that is formed by weak acid and salt of its conjugate base or by a weak base and salt of its conjugate acid. The resistance showed by the solution upon the addition of a small amount of an acid or base is due to the equilibrium that is formed between the weak acid/base and salt of conjugate base/acid.
The pH of the buffer can be determined by the Henderson Hasselbalch Equation. The formula to calculate the pH is
pH = pKa + log([A-]/[HA]).