In a solution of a weak acid in water the rule is: [H3O+] = [A-] (These two appear in equal amounts from HA).
But note: This is not the case in a buffer solution where there is an excess of Ac-.
In a buffer we say: [H3O+] ≠ [A-].
A buffer solution contains not only the weak acid, but also a salt of that weak acid.
Adding a bit of a strong acid to a buffer solution (that means some extra hydronium or oxonium ion H3O+ ),
then immediately that extra added acid will be neutralised by the large amount of in the buffer present weak base (Ac-).
The following reaction takes place:
Ac-(from the buffer) + H3O+(from the added strong acid) HAc + H2O
NB: [Ac-] decreases a littlebit and [HAc] increases a bit.
Adding a bit of a strong base, for example OH-) to a buffer,
that extra base will immediately be neutralised by the large amount of weak acid HA in that buffer.
The following reaction will take place:
HAc(from the buffer) + OH-(from the added strong base) Ac- + H2O
NB: [Ac-] increases a littlebit and [HAc] will decrease a bit.
two possible definitions of the buffer:
An aqueous solution:
in which the pH hardly will change, even with adding acids or bases.
containing a mixture of a weak acid + its weak conjugated base, both in considerable concentrations.
The Buffer formula:
A 'buffer' of good quality will always have more or less equal amounts of (weak) acid en of the (weak conjugated) base.
When the pH of the blood (7.3) changes, even if it is just a littlebit, this can be mortal.
A human accepts nor survives a real pH-change in blood.
Yet the blood has te transport acids and bases like carbon dioxyde, lactic acid, amino acids, phosphates and more.
They may not change the pH of the blood.
How the body will manage that? What mechanism has the blood to prevent changes?
Of course blood contains the necessary buffers, containing:
the groups -NH2 (amino, BASE) and -COOH (carboxyl group, ZUUR)
groups present in proteins, blood proteins
the ions carbonate and bicarbonate
Buffer solutions always function in such a way that the buffer contains weak acids and bases.
If blood must transport an acid, then any basic ion/molecule like an amino group can be responsible for that.
When a base must be transported, an acid ion/molecule will be responsible.
Other locations in the human body with this kind of regulations:
in urine: the pH can vary between 5 and 8, or, urine must be close to neutral.
in the digestion system: different enzymes take care for a good digestion, and each of them will have an own preference for pH.