Redox Titrations

Redox Titrations

Redox Titrations • Redox titrations are carried out in much the same way as acid-base titrations, using a burette and pipette to measure volumes accurately, and a standard solution of one reactant. • An indicator is usually used to signal the equivalence point, although some redox changes are accompanied by a color change and may not need an external indicator. • From the volume of the solution added from the burette to reach equivalence (known as the titre), the concentration of the other reactant can be determined. • The calculations are based on the redox equation, which is typically developed from the half-equations

Solution • Balance the reaction using the half-equation method. • Calculate the amount (in moles) of the reactants used to reach the equivalence point. Start with the reactant that the concentration and volume are known. In this case we start with KMnO4. • Use the mole to mole ratio to calculate the moles of the other reactant, in this case is the Fe2+.

From the moles, calculate the grams of the reactant, in this case the grams of Fe. • Using the grams of iron and the grams of sample, calculate the % Fe in the sample. • What would be observed during the titration and how can the equivalence point be detected?

Redox Titrations As the I2 is reduced to I- during the reaction, the blue color disappears, marking the equivalence point.

Solution • Balance the equation for the titration using the half-equation method. • Calculate the moles of Na2S2O3, as we know both the concentration and the volume used. • Calculate the moles of I2 using the mole to mole ratio. • Use the moles of I2 and the mole to mole ratio in the first titration to find the moles of OCl-. • Find the concentration of the OCl- using the equation M=mol/dm3

Redox Titrations • The dissolved oxygen content of water is one of the most important indicators of its quality, as oxygen is essential to the survival of aquatic life. • As the level of pollution in water increases, the dissolved oxygen content generally decreases, as the oxygen is used by bacteria in decomposition reactions. • The biological oxygen demand (BOD) is therefore used as a means of measuring the degree of pollution. • BOD is defined as the amount of oxygen used to decompose the organic matter in a sample of water over a specified time period, usually five days at a specified temperature. BOD is measured in ppm (mg/L) • A high BOD indicates a greater quantity of degradable organic matter in the water, which means, a lower level of dissolved oxygen.

Solution • Calculate the moles Na2S2O3 since we know the concentration and the volume used. • Use the mole to mole ratio in step 3 to calculate the moles of I2. • Use the mole to mole ratio in step 2 to calculate the moles of MnO2. • Use the mole to mole ratio in step 1 to calculate the moles of O2. • Finally, change the moles of O2 to grams of O2, and divide by the volume of water.

Redox Titrations