Volumetric Analysis
Volumetric Analysis
• Volumetric analysis involves the analysis of a
solution of unknown concentration with a
standard solution.
• A pipette is used to transfer a known quantity
of one of the solutions into a conical flask.
This sample is called an aliquot
• The other solution is added to the burette.
The amount of this solution that is needed to
react with the aliquot is called the titre.
Volumetric Analysis (cont.)
• The point where the reactants are in exact
mole ratio proportion is called the equivalence
point.
• The point where we detect the change
through an indicator or pH meter is called the
end point.
• Titrations are repeated until three titrations
are concordant, i.e. within 0.1ml of each other
Standard Solutions – Primary Standard
• A standard solution is a solution of precisely
known concentration. For a solution to be
used as a primary standard it must
– be readily obtainable in a pure form
– have a known formula
– be easy to store without deteriorating or reacting
with the atmosphere
– have a high molar mass to minimise the effect of
errors in weighing
– be inexpensive.
Standard Solutions – Secondary
standards
• If a primary standard solution can not be made a
secondary standard solution is created by
titrating it with a primary standard to find the
exact concentration.
• Substances such as strong acids and strong bases
are not suitable for primary standards as the
concentrations prepared are not accurate and
they can react with the atmosphere or containers
• Examples of recommended substances used to
make secondary standards can be found in the
table on the next slide
Flow Chart for a Titration
Back Titrations
• If an end point cannot be easily detected in a
titration or the substance is toxic, volatile, a
mixture of gases, in low concentration or
insoluble in water a back titration is needed.
• A back titration involves first reacting the sample
with an excess amount of a standard solution and
titrating the product or excess with another
suitable standard solution.
• Flow charts of back titrations can be found on the
following slides
Product Back tTtration
Excess Reactant Back Titration
Acid-Base Titrations
• When completing an acid-base titration it is
important to select an appropriate indicator to
determine the end point.
• The pH of the equivalence point varies
depending on the reactants that are used.
• The following slides show the pH curves of
different acid and base reactions.
Strong Acid – Strong Base
Strong Acid – Weak Base
Weak Acid – Strong Base
Weak Acid – Weak Base
(unsuitable for titration)
Choosing an appropriate indicator
• When selecting an indicator it is necessary to
find one that changes colour in the range that
has the sharp change in pH.
Redox Reactions
• Redox reactions occur when reactant loses
reactions (oxidation) and another reactant
gains electrons (reduction).
• These reactions occur spontaneously in
solution and usually result in a change in
colour, which can be used to find the end
point.
Oxidation Number Rules Revision
• All free elements have an oxidation number of 0
• Ionic compounds containing two ions have oxidation
numbers equal to their ions (related to their group number)
• Oxygen is 2- except in peroxides (H2O2, BaO2) where it is 1• Hydrogen is 1+ except in metal hydrides (NaH, CaH2) where
it is 1• The sum of oxidation numbers in a neutral compound is 0
• The sum of oxidation numbers in a polyatomic ion is equal
to the charge on the ion.
• The most electronegative element has the negative
oxidation number