Experiment 4. Ionization of Acids and Bases, pH and Buffers
Goal: To experiment is designed gain experience in the use of pH
electrodes to accurately measure the pH of aqueous solutions and
to use the measured pHs to calculate acid dissociation constants
and investigate the properties of buffers.
Objectives: On completion of this
lab you should be able to:
- Draw and annotate a diagram of a pH electrode.
- Identify the components of a pH electrode and
explain their functions.
- Calibrate a pH electrode / meter with standard
buffers and use that electrode to measure the pH of aqueous solutions.
- Calculate weak acid and weak base dissociation
constants from pH measurements.
- Investigate and assess the buffering capacity of
Acids and bases are classified as being either strong or weak. A
strong acid (or base) in an aqueous solution dissociates
completely to give a proton (or hydroxide ion) and a neutral
anion (or cation). The protons become associated with the aqueous
HA + H2O → A- + H3O+
or BOH → B+ + OH-.
A weak acid (or base) however only partially dissociates to give
a proton (or hydroxide) and anion (or cation). The produced anion
(or cation) is a weak base (or weak acidic), called the conjugate
base (or conjugate acid) of the weak acid (or weak base):
HA + H2O ⇆ A- + H3O+
or BOH ⇆ B+ + OH-.
The extent to which weak acids (or weak bases) dissociate is
expressed by the acid (or base) dissociation constant given by:
KHA = [A-][H3O+]/[HA]
or KBOH = [B+][OH-]/[BOH]
Since A- (B+) is a weak (conjugate) base (weak (conjugate) acid)
then the equilibrium
A- + H2O ⇆ HA + OH- and KA- = [HA][OH-]/[A-]
or B+ + 2H2O ⇆ BOH + H3O+
and KB+ = [BOH][H3O+]/[B+].
Note that for a weak acid conjugate base pair KHA x KA- =
[H3O+][OH-] = Kw, the dissociation constant of water:
2H2O ⇆ H3O+ + OH- and Kw = [H3O+][OH-].
Similarly for a weak base and its conjugate acid KBOH x KB+ =
[OH-][H3O+] = Kw.
If the acid (or base) is a cation (or anion) then the
dissociation constant can equally be called a hydrolysis
constant. For example:
NH4+ + H2O ⇆ NH3 + H3O+ Ka = Kh =
or CH3COO- + H2O ⇆ CH3COOH + OH-. Kb = Kh =
The pH (-log[H3O+]) of an aqueous solution of an acid (or base)
therefore depends on whether it is a strong or weak acid (or
base) and, if it is a weak acid (or base), the magnitude of the
acid (or base) dissociation constant.
When an aqueous solution contains both a weak acid (or weak base)
and its conjugate base (or conjugate acid) it is called a buffer
- a solution whose pH does not change very much when acid (or
base) is added to it. The capacity of the buffer is a measure of
how much acid (or base) must be added to it to change the pH by
During this experiment you will be working with a weak and
strong acids and bases. You must wear eye protection at all
times. In the event that any reagent used in this investigation
comes in contact with your skin or eyes, wash the affected area
immediately with lots of water. Notify your instructor. Ammonia
is a toxic gas, you should not inhale it. Use ammonia in a fume
Your demonstrator will show you how to use the pH electrode and
meter. Users manuals for the electrode and meter accompany your
instrument. Be very careful with your electrode. The
pH sensing membrane (see electrode users manual) is made of very
thin glass and is very fragile; always use such an electrode with
an electrode guard in place.
Record all observations directly onto
Following the procedure in the electrode meter manual calibrate
your electrode with the supplied buffers and then measure and
record the pHs of the buffers.
Use your meter and calibrated electrode to do the following:
Part 1: pHs of dilute solutions of acids, bases and salts
- Pour 2.5 cm3 of each of the supplied 0.010 mol dm-3 solutions
of acetic acid, hydrochloric acid, sodium hydroxide and ammonia
into test tubes and record their pHs. Discard the solutions.
- Pour 2.5 cm3 of each of the supplied solutions of 0.050 mol
dm-3 sodium chloride, ammonium chloride and sodium acetate into
test tubes and record their pHs. Discard the solutions.
Part 2: Buffers
Prepare an acetic acid-acetate buffer by mixing 1 cm3 of 1 mol
dm-3 CH3CO2H with 1 cm3 of
1 mol dm-3 CH3CO2Na and diluting the
mixture to 100 cm3 in a 100 cm3 measuring cylinder. Measure the
pH of the solution and then divide it equally into two 100 cm3
To one half of the solution add dropwise with stirring enough 1
mol dm-3 HCl until the pH of the solution changes by 1 unit.
(Make a note of the total no. of drops added).
To the other half of the solution add dropwise with stirring
enough 1 mol dm-3 NaOH until the pH of the solution also changes
by 1 unit. (Make a note of the total no. of drops
Finally measure the pHs of the standard buffers to see if the
electrode calibration has "drifted" during your work.
Apparatus per student:
Access to a pH electrode and meter and associated manuals. Seven
test tubes of large enough diameter for the pH electrode to fit
into and a test-tube rack. Two 50 cm3 beakers, one 100 cm3
measuring cylinder, one dropper and teat, one stirring rod, one
Chemicals per student:
10 cm3 of each of the following:
0.010 mol dm-3 acetic acid, hydrochloric acid, sodium
hydroxide and ammonia
0.050 mol dm-3 sodium chloride, ammonium chloride and
1 mol dm-3 Sodium acetate and acetic acid
1 mol dm-3 NaOH and HCl.
Chemicals and apparatus per pH electrode and meter:
100 cm3 of standard pH buffers 4, 7 and 10.
Three 25 cm3 beakers.
One 250 cm3 beaker.
Copyright © 1997-2014 by The Department
of Chemistry UWI, Jamaica, all rights reserved.
Created and maintained by Prof. Robert J.
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created March 1997. Links checked and/or last
modified 27th September 2014.