ACID-BASE TITRATION
Purpose of the experiment
To determine the concentration of an unknown solution of sodium hydroxide by titration, a volumetric method of
analysis, using the known concentration of a hydrochloric acid standard solution.
Background information
Titration is an analytical method used to determine the exact amount of a substance by reacting that substance with
a known amount of another substance. The completed reaction of a titration is usually indicated by a color change or
an electrical measurement. An acid/base neutralization reaction will yield salt and water. In an acid-base titration, the
neutralization reaction between the acid and base can be measured with either a color indicator or a pH meter.
Acid + Base ® Salt + Water [1]
The concentration of a basic solution can be determined by titrating it with a volume of a standard acid solution (of
known concentration) required to neutralize it. The purpose of the titration is the detection of the equivalence point,
the point at which chemically equivalent amounts of the reactants have been mixed. The amount of reactants that
have been mixed at the equivalence point depends on the stoichiometry of the reaction.
In the neutralization reaction of HCl and NaOH, the equivalence point occurs when one mole of HCl reacts with one
mole of NaOH. However, in the reaction of H3PO4 and NaOH, the equivalence point occurs when one mole of H3PO4
reacts with 3 moles of NaOH.
HCl (aq) + NaOH (aq) ® NaCl (aq) + H2O (aq) [2]
H3PO4 (aq) + NaOH (aq) ® Na3PO4 (aq) + 3 H2O (aq) [3]
To determine when neutralization occurs, an indicator such as phenolphthalein can be used. An indicator is a
substance, which undergoes a distinct color change at or near the equivalence point. The point at which the indicator
changes color and the titration is stopped is called the endpoint. Ideally, the endpoint should coincide with the
equivalence point. Phenolphthalein is colorless in acidic solution and reddish violet in basic solution.
Example. A 25.00 ml sample of 0.1030 M HCl is titrated with exactly 24.42 ml of dilute NaOH solution. What is the
concentration of the NaOH solution?