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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.
In an iodine clock reaction, colour changes after a time delay.. A chemical clock (or clock reaction) is a complex mixture of reacting chemical compounds in which the onset of an observable property (discoloration or coloration) occurs after a predictable induction time due to the presence of clock species at a detectable amount. [1]
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]
This is an accepted version of this page This is the latest accepted revision, reviewed on 9 January 2025. This article is about the chemical element. For other uses, see Iodine (disambiguation). Chemical element with atomic number 53 (I) Iodine, 53 I Iodine Pronunciation / ˈ aɪ ə d aɪ n, - d ɪ n, - d iː n / (EYE -ə-dyne, -din, -deen) Appearance lustrous metallic gray solid, black ...
The Old Nassau reaction or Halloween reaction is a chemical clock reaction in which a clear solution turns orange and then black. This reaction was discovered by two undergraduate students at Princeton University researching the inhibition of the iodine clock reaction (or Landolt reaction) by Hg 2+ , resulting in the formation of orange HgI 2 .