Iodometry and iodimetry are redox titrations used to determine the concentration of oxidizing agents/reducing agents by indirectly measuring the amount of iodine liberated/consumed during a chemical reaction.
In iodometry (determining an oxidizing agent), the oxidizing agent reacts with excess iodide ions to liberate iodine. The amount of iodine released is proportional to the amount of oxidizing agent present, via the equation of the reaction.
The iodide ions are oxidized according to the reaction:
\[\ce{2I^- -> I2 + 2e^-}\]
In iodimetry (determining a reducing agent), a known amount of excess iodine is added. Part of the iodine is reduced to iodide ions, while te excess iodine can be titrated. After some simple stoechiometrical calculations, the concetration of the reducer can be found.
Iodine is reduced according to the reaction:
\[\ce{I2 +2e^- -> 2I- + 2e^-}\]
The liberated/excess iodine is then titrated with sodium thiosulfate solution:
\[\ce{I2 + 2S2O3^{2-} -> 2I^- + S4O6^{2-}}\]
Starch is used as an indicator due to its ability to form a deep blue complex with iodine. Near the endpoint, the blue color disappears, indicating that all iodine has reacted. Aqueous solutions which have a relatively high concentration of iodine are brown/orange/yellow. Usually, the starch is added only after the yellow color is faint, to precisely find the endpoint.
The solution initially turns brown due to iodine. Upon addition of starch, a deep blue color appears which disappears sharply at the endpoint.
The concentration of the oxidizing/reducing agent is determined indirectly from the amount of iodine titrated.
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