Buffer - A mixture of a conjugate acid-base pair that can resist changes in pH when small amounts of strong acids or bases are added

Conjugate Acid-Base Pairs

The part of the acid remaining when an acid donates a H⁺ ion is called the conjugate base.  The acid formed when a base accepts a H⁺ ion is called the conjugate acid.  For the generic acid HA:

For the generic base A^-:

More examples of conjugate acid-base pairs:

Four ways to prepare a buffer solution

1. The most common preparation method for a buffer solution is combining a weak acid with its conjugate base.  The conjugate base comes from an aqueous salt which dissociates in water to give the base.
2. Though less common, the exact opposite of the first method can be done by combinding a weak base with its conjugate acid.
3. A third way to make a buffer solution is to start with a weak acid and add half as many moles of strong base.
4. Conversely, a buffer solution can also be prepared by starting with a weak base and adding half as many moles of a strong acid.

The main thing with any of the above preparation methods is that the starting solution is a WEAK acid or base or else the starting acid or base would already dissociate 100%.  Both components of a conjugate acid-base pair must remain in the solution to be able to neutralize any added acid or base.  

• The ACID neutralizes the OH^- ions formed if a strong base is added.  
• The BASE will neutralize the H₃O⁺ ions formed if a strong acid is added.

Example of a buffer solution

Suppose there is a buffer solution with acetic acid and its conjugate base, the acetate ion.

		When strong base is added
		When strong acid is added

Buffer Capacity - By definition, the buffer capacity of a solution is the number of moles of strong acid or strong base needed to change the pH of 1 Liter of buffer solution by 1 pH unit.

• A general estimate of the buffer capacity is 40% of the sum of the molarities of the conjugate acid and conjugate base:

How to chose which acid to use for a buffer solution

The obtain a specific pH for a buffer solution, an acid with a pK_a value close to the desired pH of the buffer solution should be picked.

pK_a is by definition the negative of the logarithm of the acid-dissociation equilibrium constant, K_a:

Chosing an acid with a pK_a near close to the pH wanted ensures that the amounts of each component of the conjugate acid-base pair will be very close to equal.  This makes the buffer more effective in neutralizing either the addition a strong acid or a strong base.

Henderson-Hasselbach Equation

• Derrivation of the first Henderson-Hasselbach Equation

Next:  "Gas-Phase Equilibrium"