Iodine clock reaction
Encyclopedia
The iodine clock reaction (also known as the Harcourt-Esson reaction or the Harcourt-Essen reaction) is a classical chemical clock
demonstration experiment to display chemical kinetics
in action; it was discovered by Hans Heinrich Landolt
in 1886. Two colorless 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. The iodine clock reaction exists in several variations. In some variations, the solution will repeatedly cycle from colorless to blue and back to colorless, until the reagents are depleted.
with sulfuric acid
. To this is added a solution containing potassium iodide
, sodium thiosulfate
, and starch
. There are two reactions occurring in the solution.
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 triiodide
ion to be consumed much faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium
. Once the thiosulfate ion has been exhausted, this reaction stops and the blue color caused by the triiodide – starch complex appears.
Anything that accelerates the first reaction will shorten the time until the solution changes color. Decreasing the pH (increasing H+ 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 color to appear.
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:
This is the rate determining step. The iodate in excess will oxidize the iodide generated above to form iodine:
However, the iodine is reduced immediately back to iodide by the bisulfite:
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.
, potassium or ammonium persulfate
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 colour.
Once all the thiosulfate is consumed the Iodine may form a complex with the starch. Potassium persulfate has a low solute potential, according to documents on the Salters website. Ammonium persulfate has a higher solubility and is used instead in reaction document examples from Oxford University.
and perchloric acid
that takes place through the following reactions.
In this clock the induction period
is the time it takes for autocatalytic process to start after which the concentration of free iodine
falls rapidly as observed by UV/VIS spectroscopy.
Chemical clock
A chemical clock is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes....
demonstration experiment to display chemical kinetics
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
in action; it was discovered by Hans Heinrich Landolt
Hans Heinrich Landolt
Hans Heinrich Landolt was the Swiss chemist who discovered iodine clock reaction. He is also one of the founders of Landolt-Börnstein database.-Biography:...
in 1886. Two colorless 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. The iodine clock reaction exists in several variations. 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 reaction starts from a solution of hydrogen peroxideHydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
with sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
. To this is added a solution containing potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
, sodium thiosulfate
Sodium thiosulfate
Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...
, and starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
. There are two reactions occurring in the solution.
- In the first, slow reaction, the triiodideTriiodideIn chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...
ion is produced . - H2O2(aq) + 3 I−(aq) + 2 H+ → I3− + 2 H2O.
- In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate.
- I3−(aq) + 2 S2O32−(aq) → 3 I−(aq) + S4O62−TetrathionateThe tetrathionate anion, S4O62−, is a sulfur oxoanion derived from the compound tetrathionic acid, H2S4O6. Two of the sulfur atoms present in the ion are in oxidation state 0 and two are in oxidation state +5. Alternatively, the compound can be viewed as the adduct resulting from the binding of...
(aq)
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 triiodide
Triiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...
ion to be consumed much faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium
Dynamic equilibrium
A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state...
. Once the thiosulfate ion has been exhausted, this reaction stops and the blue color caused by the triiodide – starch complex appears.
Anything that accelerates the first reaction will shorten the time until the solution changes color. Decreasing the pH (increasing H+ 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 color to appear.
Iodate variation
An alternative protocol uses a solution of iodateIodate
An iodate is a conjugate base of iodic acid. In the iodate anion, iodine is bonded to three oxygen atoms and the molecular formula is IO3−. The molecular geometry of iodate is trigonal pyramidal....
ion (for instance potassium iodate) to which an acidified solution (again with sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
) of sodium bisulfite
Sodium bisulfite
Sodium bisulfite is a chemical compound with the chemical formula NaHSO3. Sodium bisulfite is a food additive with E number E222. This salt of bisulfite can be prepared by bubbling sulfur dioxide in a solution of sodium carbonate in water...
is added.
In this protocol, iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
ion is generated by the following slow reaction between the iodate and bisulfite:
- IO3− (aq) + 3HSO3− (aq) → I− (aq) + 3HSO4−(aq)
This is the rate determining step. The iodate in excess will oxidize the iodide generated above to form iodine:
- IO3− (aq) + 5I− (aq) + 6H+ (aq) → 3I2 + 3H2O (l)
However, the iodine is reduced immediately back to iodide by the bisulfite:
- I2 (aq) + HSO3− (aq) + H2O (l) → 2I− (aq) + HSO4−(aq) + 2H+ (aq)
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 sodiumSodium persulfate
Sodium persulfate is a chemical compound. It is a strong oxidizer. It is a severe irritant of skin, eyes, and respiratory system. It is almost non-hygroscopic and has particularly good ability to be stored for long time. It is easy and safe to handle...
, potassium or ammonium persulfate
Ammonium persulfate
Ammonium persulfate 2S2O8 is a strong oxidizing agent. It is very soluble in water; the dissolution of the salt in water is endothermic. It is a radical initiator. It is used to etch copper on printed circuit boards as an alternative to ferric chloride solution...
to oxidize
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
ions to iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
. Sodium thiosulfate
Sodium thiosulfate
Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...
is used to reduce Iodine back to Iodide before the Iodine can complex with the starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
to form the characteristic blue-black colour.
- Iodine is generated:
- 2I−(aq) + S2O82−(aq) → I2 (aq) + 2SO42−(aq)
- And is then removed:
- I2 (aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)
Once all the thiosulfate is consumed the Iodine may form a complex with the starch. Potassium persulfate has a low solute potential, according to documents on the Salters website. Ammonium persulfate has a higher solubility and is used instead in reaction document examples from Oxford University.
Chlorate variation
An experimental iodine clock sequence has also been established for a system consisting of iodine potassium-iodide, sodium chlorateSodium chlorate
Sodium chlorate is a chemical compound with the chemical formula . When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride...
and perchloric acid
Perchloric acid
Perchloric acid is the inorganic compound with the formula HClO4. Usually encountered as an aqueous solution, this colourless compound is a strong acid comparable in strength to sulfuric and nitric acids. It is a powerful oxidizer, but its aqueous solutions up to appr. 70% are remarkably inert,...
that takes place through the following reactions.
- iodide anions present in equilibrium with triiodideTriiodideIn chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...
ion and iodineIodineIodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
: - I3− → I− + I2
- chlorate ion oxidizes iodide ion to hypoiodous acidHypoiodous acidHypoiodous acid is formed when mercury oxide is shaken with aqueous iodine. It rapidly decomposes by disproportionation:Reactive solutions of hypoiodites can be prepared by reacting I2 with alkali hydroxides....
and Chlorous acidChlorous acidChlorous 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 and chloric acid :Although the acid is difficult to obtain in pure substance, the conjugate base,...
in slow and rate-determining stepRate-determining stepThe rate-determining step is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. In...
: - ClO3− + I− + 2H+ → HOI + HClO2
- chlorate consumption is accelerated by reaction of hypoiodous acid to iodous acid and more chlorous acid.
- ClO3− + HOI + H+ → HIO2 + HClO2
- more autocatalysisAutocatalysisA single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product itself is the catalyst for that reaction....
when newly generated iodous acid also converts chlorate in the fastest reaction step. - ClO3− + HIO2 → IO3− + HClO2
In this clock the induction period
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....
is the time it takes for autocatalytic process to start after which the concentration of free iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
falls rapidly as observed by UV/VIS spectroscopy.