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 = 4.74
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]

          100.26 = [anion]

                    = 1.8

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.
          Go to the Acid-Base Buffers Worksheet
          Go to the Acid-Base More Buffers Worksheet