Agriculture Reference
In-Depth Information
3.2 pH BUFFER CAPACITY
The extent to which the pH of a solution is buffered against additions or removals
of protons is measured by the solution's pH buffer capacity. This is defined as
the amount of strong acid or base required to produce unit change in pH. The
buffering depends on the transfer of protons between donors and acceptors, i.e.
Bronsted acids and bases, which form conjugate acid-base pairs. The pH buffer
capacity of a solution is calculated from the buffer capacities of the individual
acid-base pairs present.
Consider a generic acid-base pair HX-X representing the various acid-base
pairs in a solution. The pH buffer capacity of the HX-X pair is defined as
d[X ]
dpH
b HX =
( 3 . 5 )
[X ]
The total concentration of the pair is [HX]
+
=
[X total ] and from the acidity
[H + ][A] /K , hence
constant, K, [HX]
=
[X total ]
[H + ] /K +
[X ]
=
( 3 . 6 )
1
Substituting in Equation (3.5) for [X ] from Equation (3.6)
b HX = [X total ] d1 / [H + ] /K + 1
dpH
( 3 . 7 )
Hence
b HX = [X total ] d[H + ]
dpH
d1 / [H + ] /K
1
+
K [H + ]
K + [H + ] 2
= 2 . 303[X total ]
d[H + ]
or
[HX][X ]
[HX]
b HX = 2 . 303
( 3 . 8 )
[X ]
+
The total buffer capacity of the solution is equal to the sum of the buffer
capacities of the individual acid-base pairs present, each given by an equation
like Equation (3.8). In an aqueous solution of acid HA, three acid-base pairs are
present: HA-A , H 3 O + -H 2 OandH 2 O-OH . Because [H 3 O + ]and[OH ]are
both negligible compared with [H 2 O], in Equation (3.8) [X ] = ( [HX] + [X ] )
for H 3 O + -H 2 O and [HX] = ( [HX] + [X ] ) for H 2 O-OH . Hence
b H 3 O + = 2 . 303[H 3 O + ]
(3.9)
b OH = 2 . 303[OH ]
(3.10)
Therefore
b solution = 2 . 303 [H 3 O + ] + [OH ] +
[HA][A ]
[HA] + [A ]
( 3 . 11 )
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