Redox Titrations

Redox titrations involve reactions where electrons are transferred, and oxidation state of components changes. This is especially significant when dealing with metals and their properties.

Basic Concepts and Definitions

Redox reaction is short for reduction-oxidation reaction. It includes any chemical reaction involving transfer of electrons between two species.
Oxidation is the loss of electrons by a substance during a reaction, leading to an increase in its oxidation state.
Reduction is the gain of electrons by substance during a reaction, leading to a decrease in its oxidation state.
An oxidising agent (or oxidant) is a substance which causes oxidation by accepting electrons, and is itself reduced in the process.
A reducing agent (or reductant) is a substance which causes reduction by donating electrons, and is itself oxidised in the process.

Redox Titration Techniques

The solution of known concentration (titrant) is added to the solution of the substance being analysed (analyte) until the reaction between them is complete. This point is known as the equivalence point or end point.
The end point of a redox titration can be detected by indicators, which change colour when all the analyte has reacted, or by using a potentiometer (an instrument for measuring electric potential).
Common indicators include substances such as starch (used in iodometric titrations) and methylene blue.
The amount of titrant required to react completely with the analyte can be used to calculate the concentration of the analyte solution using the concept of moles.

Example Redox Titrations

A common redox titration is the reaction between potassium permanganate (KMnO4) and iron (II) sulphate (FeSO4). In this titration, the iron (II) sulphate is oxidised to iron (III) sulphate, and the potassium permanganate is reduced to manganese(II) ions.
Another example is the titration of sodium thiosulphate (Na2S2O3) with iodine. Here, iodine is reduced to iodide ions, and the sodium thiosulphate is oxidised to tetrathionate ions.

Calculations in Redox Titrations

The simple formula to use in redox titration calculations is: Concentration x Volume = Moles.
Reactant ratios can be determined from the balanced equation for the redox reaction.
The molarity (concentration) or volume of the unknown can be solved using stoichiometry once the moles have been calculated.
It is important to balance the redox equation before performing the calculation.

Safety in Redox Titrations

Certain redox indicators and reagents can be hazardous or irritant. Exercise appropriate safety measures, including wearing gloves and using a fume cupboard where necessary.