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The iodine clock reaction is a classical chemical clock demonstration experiment to display
chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in wh ...
in action; it was discovered by Hans Heinrich Landolt in 1886. The iodine clock reaction exists in several variations, which each involve iodine species ( iodide ion, free iodine, or
iodate An iodate is the polyatomic anion with the formula . It is the most common form of iodine in nature, as it comprises the major iodine-containing ores. Iodate salts are often colorless. They are the salts of iodic acid. Structure Iodate is pyram ...
ion) and redox reagents in the presence of starch. Two colourless solutions are mixed and at first there is no visible reaction. After a short time delay, the liquid suddenly turns to a shade of dark blue due to the formation of a triiodide–starch complex. In some variations, the solution will repeatedly cycle from colorless to blue and back to colorless, until the reagents are depleted.


Hydrogen peroxide variation

This method starts with a solution of hydrogen peroxide and sulfuric acid. 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: : + 2 + 2 → + 2 In the second, fast reaction, iodine is reconverted to two iodide ions by the thiosulfate: :2 + → + 2 After some time the solution always changes color to a very dark blue, almost black. When the solutions are mixed, the second reaction causes the iodine to be consumed much faster than it is generated, and only a small amount of iodine is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the iodine – starch complex appears. Anything that accelerates the first reaction will shorten the time until the solution changes color. Decreasing the pH (increasing concentration), or increasing the concentration of iodide or hydrogen peroxide will shorten the time. Adding more thiosulfate will have the opposite effect; it will take longer for the blue colour to appear. Aside from using sodium thiosulfate as a substrate, cysteine can also be used. iodide from potassium iodide is converted to iodine in the first reaction: 2 + 2 H+ + H2O2 → I2 + 2 H2O The iodine produced in the first reaction is reduced back to iodide by the reducing agent, cysteine. At the same time, cysteine is oxidized into cystine. 2 C3H7NO2S + I2 → C6H12N2O4S2 + 2 + 2 H+ Similar to thiosulfate case, when cysteine is exhausted, the blue color appears.


Iodate variation

An alternative protocol uses a solution of
iodate An iodate is the polyatomic anion with the formula . It is the most common form of iodine in nature, as it comprises the major iodine-containing ores. Iodate salts are often colorless. They are the salts of iodic acid. Structure Iodate is pyram ...
ion (for instance potassium iodate) to which an acidified solution (again with sulfuric acid) of sodium bisulfite is added. In this protocol, iodide ion is generated by the following slow reaction between the iodate and bisulfite: : + 3 → + 3 This first step is the rate determining step. Next, the iodate in excess will oxidize the iodide generated above to form iodine: : + 5 + 6 → 3 + 3 However, the iodine is reduced immediately back to iodide by the bisulfite: : + + → 2 + + 2 When the bisulfite is fully consumed, the iodine will survive (i.e., no reduction by the bisulfite) to form the dark blue complex with starch.


Persulfate variation

This clock reaction uses sodium, potassium or
ammonium persulfate Ammonium persulfate (APS) is the inorganic compound with the formula (NH4)2S2O8. It is a colourless (white) salt that is highly soluble in water, much more so than the related potassium salt. It is a strong oxidizing agent that is used as a catalys ...
to oxidize iodide ions to iodine. Sodium thiosulfate is used to reduce iodine back to iodide before the iodine can complex with the starch to form the characteristic blue-black color. Iodine is generated: :2 + → + 2 And is then removed: : + 2 → 2 + Once all the thiosulfate is consumed the iodine may form a complex with the starch. Potassium persulfate is less soluble (cfr. Salters website) while ammonium persulfate has a higher solubility and is used instead in the reaction described in examples from Oxford University.


Chlorate variation

An experimental iodine clock sequence has also been established for a system consisting of iodine potassium-iodide,
sodium chlorate Sodium chlorate is an inorganic compound with the chemical formula Na ClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leaves sodium chloride. Sev ...
and
perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous so ...
that takes place through the following reactions. Triiodide is present in equilibrium with iodide anion and molecular iodine: : + Chlorate ion oxidizes iodide ion to
hypoiodous acid Hypoiodous acid is the inorganic compound with the chemical formula HIO. It forms when an aqueous solution of iodine is treated with mercuric or silver salts. It rapidly decomposes by disproportionation: : 5 HIO → HIO3 + 2 I2 ...
and
chlorous acid Chlorous acid is an inorganic compound with the formula HClO2. It is a weak acid. Chlorine has oxidation state +3 in this acid. The pure substance is unstable, disproportionating to hypochlorous acid (Cl oxidation state +1) and chloric acid (Cl ...
in the slow and
rate-determining step In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of the ...
: : + + 2 → + Chlorate consumption is accelerated by reaction of hypoiodous acid to
iodous acid The iodite ion, or iodine dioxide anion, is the halite with the chemical formula . Within the ion the Iodine exists in the oxidation state of +3. Iodite anion Iodites (including iodous acid) are highly unstable and have been observed but never ...
and more chlorous acid: : + + → + More
autocatalysis A single chemical reaction is said to be autocatalytic if one of the reaction products is also a catalyst for the same or a coupled reaction.Steinfeld J.I., Francisco J.S. and Hase W.L. ''Chemical Kinetics and Dynamics'' (2nd ed., Prentice-Hall 199 ...
when newly generated iodous acid also converts chlorate in the fastest reaction step: : + → + In this clock the
induction period An induction period in chemical kinetics is an initial slow stage of a chemical reaction; after the induction period, the reaction accelerates. Ignoring induction periods can lead to runaway reactions. In some catalytic reactions, a pre-catalyst ...
is the time it takes for the autocatalytic process to start after which the concentration of free iodine falls rapidly as observed by UV–visible spectroscopy.


See also

* Clock reaction * Old Nassau reaction *
Chemical oscillator A chemical oscillator is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes. They are a class of reactions that serve as an example of non-equilibrium thermodynamics wi ...
* Briggs–Rauscher reaction


References


External links


Hydrogen peroxide variation

Sodium bisulfite variation with a high-speed camera
{{DEFAULTSORT:Iodine Clock Reaction Chemical kinetics Chemistry classroom experiments Clock reactions Clock reaction Thiosulfates Articles containing video clips