|Preparation of A Buffer|
|To prepare a buffer first choose an acid with a pKa
within a ±1 pH unit of the desired value. Then manipulate the ratios
to get the desired pH. A solution buffered at pH 5.00 is needed in a chemistry
experiment. Can we use acetic acid and sodium acetate to make it? If so, what
ratio of acetate ion to acetic acid is needed?
|Ka of acetic acid = 1.8 x 10-5 pKa
|the pKa is within a range of 5 ± 1
ie. between the values of 4 - 6.
therefore, acetic acid is okay to use.
|next use pH = pKa + log [anion]
5.00 = 4.74 + log [anion]
0.26 = log [anion]
|Therefore the ratio of anion to acid is 1.8 to 1. Therefore,
use a ratio of 1.8 moles of acetate ion to 1.0 mole of acetic acid. The solution
will then be buffered at a pH of 5.00
|We could use a ratio of 0.18 moles to 0.100 moles if we wanted a smaller buffering capacity.|