Buffer Capacity
Buffer Capacity (B )
Buffer Capacity is the ratio of the increment of strong base or strong
acid to the change in pH.
B = ęB / ępH
= the small increment in gram equivalents/liter of strong base
(or acid) added to buffer solution to produce a pH change of ępH.
B = (2.3 x C x Ka [H+] ) / (Ka +
[H+])2
B = 2.3 C a (1 - a)
C = [Acid] + [Salt]
or
C = [Base] + [Salt]
Example:
Calculate the buffer capacity (B) for a mixture of 0.01 moles of
acetic acid and 0.03 moles of NaOAc in 100 mL of total solution.
pKa = 5.24
- pH = pKa + log (salt/acid) = 4.76 + log 0.03/0.01 =
5.24
- [H+] = 5.75 x 10-6
- C = (0.01 + 0.03) moles/100 mL = 0.4 M
- pKa = 4.76
- Ka = 1.74 x 10-5
B = (2.3 x C x Ka [H+] ) / (Ka +
[H+])2
B = ( 2.3 x 0.4 x 1.74 x 10-5 x 5.75 x 10-6
) / (1.74 x 10-5 + 5.75 x 10-6)2
B = ( 9.20 x 10-11 ) / ( 5.36 x 10-10 ) =
0.172
Using other equation:
B = 2.3 C a (1- a )
C = (0.01 + 0.03) moles/100 mL = 0.4 M
a = (0.03)/(0.03 + 0.01) = 0.75
B = 2.3 x 0.4 x 0.75 x (1 - 0.75)
B = 2.3 x 0.4 x 0.75 x 0.25 = 0.172
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