Buffers Flashcards
Buffers Def’n:
A weak acid plus its conjugate
base that cause a solution to resist changes
in pH when an acid or base are added
Effectiveness of a buffer is determined by
– 1) the pH of the solution, buffers work best
within 1 pH unit above and 1 pH unit below
their pKa
(ie. within a 2 pH unit range)
– 2) the concentration of the buffer; the more
present, the greater the buffering capacity
A buffer is effective over a range of…
about
2 pH units, centered around the pKa of the
weak acid
– That is, pH = pKa ± 1
Buffer capacity is related to the
concentrations of
the weak acid and its
conjugate base.
– the greater the concentration of the weak acid
and its conugate base, the greater the buffer
capacity
Buffers are composed of…
a weak conjugate
acid-base pair.
- Resist large changes in pH when strong
acids or bases are added to them. - Ex: HC2H3O2 and NaC2H3O2
- Ex: NH3 and NH4Cl
How Do Buffers Work?
- Buffer = HX and X^(-)
- If strong acid is added to a buffer, the base
component (X-) neutralizes it.
X- (aq) + H3O+
(aq) → HX (aq) + H2O (l)
- This reaction goes to completion.
-If strong base is added to a buffer, the acid
component (HX) neutralizes it.
HX (aq) + OH- (aq) → X- (aq) + H2O (l)
- This reaction goes to completion.
Adding a strong base to
a weak acid solution will…
progressively convert
more and more HA to A-
- Notice that the pH
doesn’t change
significantly near the pKa: the solution is said
to be buffered.
The shape of a titration curve is…
sigmoidal.
- Most of the base is
consumed in a range of
about one pH unit on
either side of the pKa. - In this range, the change
in pH with moles of base
added is minimal; thus,
this is the best buffering
range.
pKa can be related to
pH
Buffer capacity:
the ability of a solution to resist
changes in pH
Most effective buffering occurs where:
solution pH = buffer pKa
- At this point: [weak acid] = [conjugate base]
Effective buffering range
is usually at pH values
equal to the pKa ± 1 pH unit
One ionizable group
can be titrated with
one equivalent of
strong base
The greatest
buffering capacity of
an ionizable group is
at its pKa
Each
ionizable
group has…
a
unique pKa
Buffered
solutions
resist…
changes in pH
Most biochemical processes depend heavily…
on the pH of
the solution. Thus various compartments of the body and its
cells are maintained at optimum pH values for the relevant
processes that occur there
Most important buffer systems
in humans:
- Bicarbonate/CO2 pKa=6.1
- Phosphate pKa=6.8
- Proteins pKa=Variety
Blood Plasma pH is 7.4
