Weak Acid Buffer Solutions
The degree of water dissociation, and therefore the hydrogen ion
concentration, is also influenced by charges derived from weak acids. These are
partially dissociated
compounds whose degree of dissociation is determined by the prevailing temperature
and pH. The predominant molecules in this group are albumin and phosphate. Stewart
used the term ATOT
to represent the total
concentration of weak ions that influenced acid-base balance.[10]
The acid, HA, only partly dissociates, represented by the following
equilibrium:
[HA] = KA
[H+
][A−
]
KA
is the weak acid (A-
) dissociation constant. If we assume
that HA and A-
play no further part in this reaction (because of the law
of mass conservation), the amount of A-
present in the solution must equal
the amount initially present:
[HA][A−
] = [ATOT
]
In this equation, [ATOT
] is the total weak acid concentration.
To calculate the effect of weak acid dissociation on [H+
],
we must take into account water dissociation and electrical neutrality:
[H+
× [OH−
] = Kw
(water dissociation)
[SID] + [H+
] − [A−
]
− [OH−
] = 0 (electrical neutrality)
The four previous simultaneous equations determine the [H+
] in this solution
containing strong ions and weak acids. SID and ATOT
are independent variables,
whose concentration was determined during the production of the system. Kw and KA
are constants. Consequently, the other variables [HA], [H+], [OH-
], and
[A-
] must adjust to satisfy the previous equations. They are dependent
variables.
