Buffers Notes
Buffers
Definition
Solutions containing a weak conjugate acid-base pair.
Resist changes in pH upon addition of small amounts of strong acid or base.
Important in lab work and biological systems (e.g., blood).
Mechanism
Based on Le Chatelier's principle.
If you add hydroxide, acetic acid will react with water to replace the hydronium ions used up.
If you add hydronium ions, it will react with the acetate ions to use up the excess hydronium added.
Buffer Capacity
Limited capacity to maintain constant pH.
Depends on significant amounts of weak acid-base pair.
Compromised when concentrations of buffer pair members are lowered by added acid or base.
Increasing the concentration of the weak acid or base and its conjugate pair can increase the buffer capacity, while maintaining it's ratio.
Changing the ratio changes the pH maintained by the buffer.
Selection Criteria
Good buffers have about equal concentrations of both components.
Weak acids/salts are better for pH < 7; weak bases/salts for pH > 7.
A buffer loses effectiveness when one component is < 10% of the other.
Henderson-Hasselbalch Equation
Useful for dealing with buffers.
Important to note that the 'x is small' assumption must be valid to use this equation.