Lab 6 Flashcards
what is a buffer
solutions capable of
maintaining a constant pH
buffer will be composed of:
mixture of both a weak acid (HA) and
its conjugate base (A-)
[Buffers can also be prepared from weak bases and their conjugate
acids]
function of buffer
The function of this buffer is to maintain the solution at a constant pH when small amounts of
either acid or base are added to it; if a small amount of acid is added then it is neutralised through
reaction with the conjugate base (A-) and the equilibrium (6.1) is displaced slightly to the left. If some
base is added it removes some of the H+ from the right hand side, shifting the equilibrium slightly to
the right.
pH of a buffer is determined by
The pH of a buffer system is determined by the acid dissociation constant, Ka, and the concentrations
of [HA] and [A-]. Rearranging the expression for the Ka for the weak acid, we can write an expression
for [H+]
in solving for pH;
the equation is put into logarithmic form. Recall that pH = -log [𝐻+], therefore if the
above equation is rearranged and the negative log of both sides is taken, an alternative form of the
equation is generated, known as the Henderson-Hasselbalch equation
Henderson Hasselbalch
pH= pK_a+log([A^-]/[HA])
if there is excess of the weak acid and the strong acid in the buffer solution, what happens?
then the changes in [HA] and [A-] on addition of small
quantities of acid or base are small and the pH remains essentially unchanged
buffers in everyday life
Blood plasma is a natural example in humans, where a bicarbonate buffer maintains
the pH of blood between 7.2 and 7.6.
Another is the phosphate buffer system which operates in the internal fluid of all cells. In mammalian cells, the phosphate buffer is effective in maintaining the pH in the range 6.9 to 7.4. Biochemists will typically undertake reactions in buffer solutions to mimic
‘physiological pH’.
when are buffers more effective
Buffers are most effective when the relative concentrations of acid and base are approximately
equal (ratio of [A-]/[HA] close to 1/1). If [HA] = [A-] then [A-]/[HA] = 1 and, because log(1) is equal to
zero, the Henderson-Hasselbalch equation reduces to:
pH=pKa
therefore…a buffer is most effective when (general rule)
A buffer is therefore most effective when the pH of the solution is equal to the pKa of the
ionizing group
- As the difference in this ratio increases, the ability of a buffer to resist changes in pH
decreases. As a guideline, the relative concentrations of acid and base should fall in the range 10 >
[HA]/[A-] > 0.1. Substituting these ratios into the Henderson-Hasselbalch equation one finds that the
effective pH range of a buffering system is in the range pKa ± 1
Buffer capacity
Buffer capacity (β) is the resistance of a buffer to pH change upon the addition of a strong acid
or base and can quantitatively be defined as the amount of acid or base necessary to change the pH of
one litre of a buffer by one unit
B=(mol of OH- or H+ added)/(ph)(volume of buffer in L)
buffer capacity depends on
buffer capacity is dependent on the total
concentration of the buffer system and on the acid to base ratio
EX.
if one buffer solution
comprises 100 mL of 0.10 M HA and 0.10 M A- and a second buffer contains 100 mL of 0.01 M HA and
0.01 M A- then both solutions should have the same buffer pH. However, the first solution has more
moles of HA and A- so would have a larger buffer capacity.
how can the buffer capacity be derived experimentally
The buffer capacity can be experimentally
derived in a manner such as titration or by calculating the pH change which results from the addition of
a strong acid or base to a buffer solution
what will be used to measure solutions when preparing buffers in this experiment
graduated pipet
the name of the weak acid and conjugate base used in this experiment
strong acid: sodium acetate
conjugate base: sodium hydroxide
