BUFFER

• A solution that resists changes in pH

Made from the combination of

• weak acid & its salt or

• weak base & its salt

example

• acetic acid & sodium acetate

• ammonia & ammonium chloride

Buffer Solution

• works best when the acid to salt ratio is 1:1

• works best when the base to salt ratio is 1:1

• buffering capacity of a solution works best when the pH is near the pKa of the acid component

Ka

• ionization constant for a weak acid

• pKa = -log Ka

  • ex.

    Weak acid → acetic acid → Ka = 1 x 10^-5

    pKa = -logKa

    = -log of 1 x 10^-5

    = 4.74

    For the effective buffer: pH of the buffer should be closer to 4.74

pKa

pKa = -log Ka

pKb

pKb = -log Kb

Kb

• ionization constant for a weak base

example: Acetic acid ionizes according to the ff. chemical equation:

Computation of Ionization Consant

Formula to compute for the pH of Buffer Solution

• Henderson-Hasselbach Eq.

  

  

Buffering Capacity

• capacity of the buffer to resist the change in the pH of a solution when an acid or alkali is added is called buffering capacity

• estimated by calculating the amount of acid or alkali required to change the pH of one litre of the buffer by one unit.

• Depends upon the ff. factors:

  • The concentration of the acid and base component of the buffer. As the concentration of acid and base components of the buffer increase, the capacity of the buffer also increases.

  • The pH of the Buffer can act best at pH = pKa, and its buffering range is about one pH unit above or below the pKa value