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 (H 2 O 2, BaO 2 ) where it is 1- Hydrogen is 1+ except in metal hydrides (NaH, CaH 2 ) 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