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A bottle of iodine solution used on apples to determine the correct harvest time. The chart shows the level of residual starch. The cut surface of an apple stained with iodine, indicating a starch level of 4–5. The iodine–starch test is a chemical reaction that is used to test for the presence of starch or for iodine. The combination of ...
To this a solution containing potassium iodide, sodium thiosulfate, and starch is added. There are two reactions occurring simultaneously in the solution. In the first, slow reaction, iodine is produced: H 2 O 2 + 2 I − + 2 H + → I 2 + 2 H 2 O. In the second, fast reaction, iodine is reconverted to two iodide ions by the thiosulfate:
The first reaction of iodine with SO 2 and water is as follows: . SO 2 +I 2 +2H 2 O→H 2 SO 4 +2HI . As the reaction proceeds, all available SO 2 will be consumed and the starch indicator added to the solution will bind with the unconsumed iodine, turning the solution black.
For simplicity, the equations will usually be written in terms of aqueous molecular iodine rather than the triiodide ion, as the iodide ion did not participate in the reaction in terms of mole ratio analysis. The disappearance of the deep blue color is, due to the decomposition of the iodine-starch clathrate, marks the end point.
A redox titration [1] is a type of titration based on a redox reaction between the analyte and titrant. It may involve the use of a redox indicator and/or a potentiometer. A common example of a redox titration is the treatment of a solution of iodine with a reducing agent to produce iodide using a starch indicator to help detect the endpoint.
Starch is optionally added as an indicator to show the abrupt increase in iodide ion concentration as a sudden change from amber (free iodine) to dark blue (the "iodine-starch complex", which requires both iodine and iodide.) [7] Recently it has been shown, however, that the starch is not only an indicator for iodine in the reaction. [8]
The iodide and iodate anions can be used for quantitative volumetric analysis, for example in iodometry. Iodine and starch form a blue complex, and this reaction is often used to test for either starch or iodine and as an indicator in iodometry. The iodine test for starch is still used to detect counterfeit banknotes printed on starch ...
From the above stoichiometric equations, we can find that: 1 mole of O 2 → 2 moles of MnO(OH) 2 → 2 mole of I 2 → 4 mole of S 2 O 2− 3. Therefore, after determining the number of moles of iodine produced, we can work out the number of moles of oxygen molecules present in the original water sample.