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