The Common Ion Effect: The extent of ionization of a weak

Unit 3b
Titration Curve: A graph of pH (of an acid or base) as a function of the volume of
base (or acid) added to neutralize the substance
Equivalence Point: Point in a titration where the added solute reacts completely
with the solute present in the solution.
Ka=Acid-Dissociation Constant
Kb=Base-Dissociation Constant
Kb=1.0x10-14 / Ka
Weak Acid-Strong Base vs. Strong Acid-Strong Base Titrations:
Solution involving the weak acid=higher initial pH than solution of a strong acid
of equal concentration
pH change at “rapid-rise” portion of curve near equivalence point is smaller for
weak acid than it is for strong acid
pH at equivalence point is over 7.00 for weak acid-strong base titration
pH Indicators:
Thymol Blue 1.2-2.8
Methyl yellow 2.9-4.0
Methyl orange 3.1-4.4
Methyl red 4.4-6.2
Phenol red 6.4-8.0
Phenolphthalein 8.0-10.0
Alizarin yellow 10.0-12.0
pH Meters and Potentiometric Titration:
pH Meter: Electronic instrument used to measure the acidity or basicity of a
liquid (consists of a glass electrode & electronic meter)
Potentiometric Titration: A volumetric method in in which the potential between
two electrodes is measured as a function of the added reagent volume.
Types of potentiometric titrations include acid-base, redox, precipitation, and
Back Titration:
Involves determining concentration of a solution by reacting it with certain # of
moles of excess reagent
Excess reagent is later titrated with another reagent
The Common Ion Effect: The extent of ionization of a weak electrolyte is
decreased by adding a strong electrolyte to the solution that has an ion in common with
the weak electrolyte.
Buffered Solution: A solutions containing a weak acid and its conjugate base or a
weak base and its conjugate acid.
A buffer is used to control the pH of a solution
Because of the common ion effect the mixture of a weak acid and its conjugate
base reduces the ionization of both
A buffer also reduces changes in pH due to the addition of a strong acid or
strong base.
Strong acid added to a buffer reacts with the conjugate (weak) base to form
more of the conjugate (weak) acid.
Strong base added to a buffer reacts with the conjugate (weak) acid to form
more of the conjugate (weak) base.
Henderson-Hasselbalch Equation:
Ka 
[H  ] [: A - ]
[HA ]
log K a  log (
[H  ] [: A - ]
[HA ]
log K a  log[H  ]  log
[A - ]
By Definition
- log [K a ]  pK a
- log[H  ]  pH
Therefore :
pK a  pH  log
[A - ]
Buffer Capacity:
The amount of acid or base the buffer can neutralize before the pH begins to
experience a noticeable change
Depends on the amount on the amount of acid and base from which the buffer is
The greater the amounts of the conjugate acid-base pair, the more resistant the
ratio of their concentrations (and therefore pH) is to change