碘钟反应

过氧化氢型碘钟

向硫酸酸化的过氧化氢溶液中加入碘酸钾、硫代硫酸钠和淀粉的混合溶液。此时在体系中存在两个主要反应，离子方程式为：

H2O2(aq) + 3I−(aq) + 2H+ → I3− + 2H2O

I3−(aq) + 2S2O32−(aq) → 3I−(aq)+S4O62−(aq)

碘酸盐型碘钟

向用硫酸酸化的碘酸盐中加入亚硫酸氢钠（或先以较安定的焦亚硫酸钠溶于水生成亚硫酸氢钠）以及少量淀粉溶液，此时体系中出现如下反应：

IO3− (aq) + 3HSO3− (aq) → I− (aq) + 3HSO4−(aq)

然后过量的碘酸根离子与碘离子发生归中反应：

IO3− (aq) + 5I− (aq) + 6H+ (aq) → 3I2 + 3H2O (l)

接着亚硫酸氢钠将生成的碘还原：

I2 (aq) + HSO3− (aq) + H2O (l) → 2I− (aq) + HSO4−(aq) + 2H+ (aq)

过硫酸盐型碘钟

通过过硫酸钾、过硫酸钠或过硫酸铵将碘离子氧化成碘单质。加入硫代硫酸钠可以将碘单质还原回碘离子。离子方程式如下：

2I−(aq) + S2O82−(aq) → I2 (aq) + 2SO42−(aq)

I2 (aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)

氯酸盐型碘钟

• 将卢戈氏碘液、氯酸钠和高氯酸混合，离子方程式如下：
• I3− → I− + I2
• ClO3− + I− + 2H+ → HIO +HClO2
• ClO3− + HIO + H+ → HIO2 + HClO2
• ClO3− + HIO2 → IO3− + HClO2

The iodine clock reaction, sometimes called the Landolt reaction, is a classical chemical clock experiment that is performed as a mechanism to demonstrate chemical kinetics in action. It was first observed 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 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 commences 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:

H2O2 + 2 I− + 2 H+ → I2 + 2 H2O

In the second, fast reaction, iodine is reconverted to two iodide ions by the thiosulfate:

2 S2O32– + I2 → S4O62– + 2 I−

After some time the solution changes color to a very dark blue, almost black.

H2O2 and I− are both first order. H+ is more complicated.

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 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 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 I− + 2 H+ + H2O2 → I2 + 2 H2O

Iodate variation

An alternative protocol uses a solution of iodate ion (such as 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:

IO−3 + 3 HSO−3 → I− + 3 HSO−4

This first step is the rate determining step. Next, the iodate in excess will oxidize the iodide generated above to form iodine:

• IO−3 + 5 I− + 6 H+ → 3 I2 + 3 H2O
• However, the iodine is reduced immediately back to iodide by the bisulfite:
• I2 + HSO−3 + H2O → 2 I− + HSO−4 + 2 H+

When the bisulfite is fully consumed, the iodine will survive (meaning that it is no longer reduced by the bisulfite) to form the dark blue complex with starch.

Persulfate variation

This clock reaction uses sodium, 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 color.

• Iodine is generated:
• 2 I− + S2O2−8 → I2 + 2 SO2−4
• And is then removed:
• I2 + 2 S2O2−3 → 2 I− + S4O2−6

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 and perchloric acid that takes place through the following reactions.

Triiodide is present in equilibrium with iodide anion and molecular iodine:

I−3 ⇌ I2 + I−

Chlorate ion oxidizes iodide ion to hypoiodous acid and chlorous acid in the slow and rate-determining step:

ClO−3 + I− + 2 H+ → HOI + HClO2

Chlorate consumption is accelerated by reaction of hypoiodous acid to iodous acid and more chlorous acid:

ClO−3 + HOI + H+ → HIO2 + HClO2

More autocatalysis when newly generated iodous acid also converts chlorate in the fastest reaction step:

ClO−3 + HIO2 → IO−3 + HClO2

In this clock, the induction period 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.