CHEM 311L Quantitative Analysis Laboratory Revision 2.3
A Volumetric Analysis (Redox Titration) of Hypochlorite in Bleach
In this laboratory exercise, we will determine the concentration of the active ingredient in Household Bleach; Hypochlorite Ion (ClO-). We will do this using a titrimetric analysis in which the hypochlorite is treated with Iodide (I-), resulting in the production of elemental Iodine (I2). This is then titrated with Thiosulfate (S2O32-) to a Starch Indicator Endpoint; a blue to colorless color change. The Thiosufate itself is standardized against primary grade Iodate (IO3-). All of these chemical processes are redox reactions and so this analysis is call a Redox Titration.
Generally, Redox Titrations involve Standard Solutions of Oxidants or Reductants; the titrant. The analyte, prepared to be in a single oxidation state, is treated with the Standard Solution such that a change in oxidation state is effected by the Oxidant or Reductant. This change of oxidation state accompanies a change in the Indicator, signaling the endpoint of the titration has been reached.
As an example, aqueous Iron can be analyzed using a redox titration. The Iron can exist in two oxidation states in Water; Fe2+ and Fe3+. An Analysis for aqueous Iron (Fe) might involve pre- treatment of the sample with a pre-reductant to ensure all the Iron is in the Fe2+ oxidation state.
Fe2+(aq) & Fe3+(aq) Fe2+(aq)
The resulting solution of Ferrous Ions (Fe2+) can then be titrated with a Standard Permanganate (MnO4-) solution (oxidant) causing the Ferrous Ions to be Oxidized to Ferric Ions (Fe3+):
The endpoint of this redox titration is indicated by a color change from clear to pink; due to the slight excess of purple Permanganate that exists when you add one additional drop of titrant beyond the equivalence point. And, at the equivalence point:
P a g e | 2
In an Iodometric Redox Titration, the analyte is first treated with the Iodide Ion (I-), a weak reducing agent, to produce Elemental Iodine, I2.
Analyte (aq) + 2 I- Reduced Analyte (aq) + I2(aq) (Eq. 1)
Note the Iodide is oxidized from an Oxidation State of -1 to an Oxidation State of 0.
The elemental Iodine then combines with any excess Iodide present to produce I3-, a species that forms a Blue colored complex with Starch:
I2(aq) + I-(aq) I3-(aq) (Eq. 2)
This is then titrated with Standardized Thiosulfate (S2O32-) to reduce the Iodine back to an Oxidation State of -1 :
I3-(aq) + 2 S2O32-(aq) 3 I-(aq) + S4O62-(aq) (Eq. 3) (Blue) (Clear)
What change in Oxidation State does the Sulfur undergo when Thiosulfate is oxidized to Tetrathionate (S4O62-)?
The endpoint of the titration is detected by using a Starch indicator. Starch, a complex carbohydrate, forms a blue-colored complex with I3- ion. So, at the point where the I3- is consumed by the titration, the titration solution turns from blue to colorless. The starch indicator is added to the solution near the end of the titration, at the point where dilute iodine...