Equilibrium :  Buffers

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

• 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.
• Though less common, the exact opposite of the first method can be done by combinding a weak base with its conjugate acid.
• A third way to make a buffer solution is to start with a weak acid and add half as many moles of strong base.
• 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 pKa value close to the desired pH of the buffer solution should be picked.

pKa is by definition the negative of the logarithm of the acid-dissociation equilibrium constant, Ka:

Chosing an acid with a pKa 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

Derivation of the first Henderson-Hasselbach Equation