pH and buffers
pH is the negative logarithm to the base 10 of the H+ ion concentration.
Conc. Of H+ varies from 0.1M in stomach to 10-7 in cytosol of cell. It’s difficult to plot such a wide range of
data hence a logarithmic scale is used.
There’s an inverse relationship between H+ and pH.
pH can be measured:
● Using a pH meter
● Using pH paper
● Addition of pH indicators to solution
An acid: Proton donor pH<7
A base: Proton acceptor pH>7
The strength of an acid can be determined by its dissociation constant, Ka.
Strong acids and Bases
They fully dissociate into ions in aqueous solution.
Weak acids and bases
They partially dissociate in aqueous solutions.
Need to know a dissociation constant (Ka) to calculate H+/OH- conc.
A weak acid dissociates into conjugate base giving off a H+
A weak base dissociates into conjugate acid accepting a H+
The larger the Ka, the stronger the acid, because most of
the HA has dissociated into H+ and A–. Conversely, the smaller the Ka, the less acid has dissociated and,
therefore, the weaker the acid.
Similar to pH, the value of Ka can also be represented as pKa.
pKa = -log Ka.
, The larger the pKa, the weaker the acid.
pKa is a constant for each conjugate acid and its conjugate base pair.
Henderson-Hasselbalch Equation:
pH=pka when HA=A-
Buffers:
A buffer is a solution that resists a change in pH upon addition of a small amount of an acid or a base.
It is a mixture of a weak acid and its conjugate base.
Example:
Acetic acid/acetate pair
Maximum buffering capacity
occurs at a pH equal to the pKa,
but a conjugate acid–base pair can
still serve as an effective buffer
when the pH of a solution is within
approximately ±1 pH unit of the
pKa.
If the amounts of weak acid and conjugate base are equal, the pH is
equal to the pKa.
pH is the negative logarithm to the base 10 of the H+ ion concentration.
Conc. Of H+ varies from 0.1M in stomach to 10-7 in cytosol of cell. It’s difficult to plot such a wide range of
data hence a logarithmic scale is used.
There’s an inverse relationship between H+ and pH.
pH can be measured:
● Using a pH meter
● Using pH paper
● Addition of pH indicators to solution
An acid: Proton donor pH<7
A base: Proton acceptor pH>7
The strength of an acid can be determined by its dissociation constant, Ka.
Strong acids and Bases
They fully dissociate into ions in aqueous solution.
Weak acids and bases
They partially dissociate in aqueous solutions.
Need to know a dissociation constant (Ka) to calculate H+/OH- conc.
A weak acid dissociates into conjugate base giving off a H+
A weak base dissociates into conjugate acid accepting a H+
The larger the Ka, the stronger the acid, because most of
the HA has dissociated into H+ and A–. Conversely, the smaller the Ka, the less acid has dissociated and,
therefore, the weaker the acid.
Similar to pH, the value of Ka can also be represented as pKa.
pKa = -log Ka.
, The larger the pKa, the weaker the acid.
pKa is a constant for each conjugate acid and its conjugate base pair.
Henderson-Hasselbalch Equation:
pH=pka when HA=A-
Buffers:
A buffer is a solution that resists a change in pH upon addition of a small amount of an acid or a base.
It is a mixture of a weak acid and its conjugate base.
Example:
Acetic acid/acetate pair
Maximum buffering capacity
occurs at a pH equal to the pKa,
but a conjugate acid–base pair can
still serve as an effective buffer
when the pH of a solution is within
approximately ±1 pH unit of the
pKa.
If the amounts of weak acid and conjugate base are equal, the pH is
equal to the pKa.
