Iodine clock reaction

The Landolt reaction is named after the Swiss chemist Hans Heinrich Landolt and describes the delayed formation of iodine from iodic acid and sulfurous acid.

With the addition of starch, the resulting iodine leads to a blue coloration of the solution ( iodine sample ). A relative became famous for this reaction, because under certain conditions, the blue color does not occur immediately, but suddenly after a longer time period after mixing the starting materials. Depending on temperature and concentration of the solutions can be predicted accurately to a few seconds, when the color change is to occur. For this reason, the Landolt reaction is also called Landoltsche reaction time or Ioduhr and often presented in show trials as an introduction to the chemistry and chemical kinetics.

Chemical basis

The following reaction equation (1) describes the overall process of the redox reaction occurring in the individual steps in the chemical formulas:

In fact, several partial reactions occur with different rates of reaction and lead to delayed Iodbildung and so blue. The starting materials used in general not directly acids, but their salts, Iodate and sulfites or hydrogen sulfites, in acid solution. However, the relevant partial reactions are always the same:

Iodate and iodide form sulfite ions in a redox reaction.

Iodide and iodate react in a comproportionation to iodine

But which is immediately reduced back to iodide, as long as the solution is still present sulfite.

Reaction (4) is so fast that no iodine in the solution is practically available. Only when the sulfite, the reactions (2) and (4) is consumed in the formation of a detectable amount of iodine which forms with an excess of iodide polyiodide.

Is stabilized in the linear helical channels of amylose in starch and appears blue ( iodine sample ).

Historical

The reaction is named after the Swiss chemist Hans Heinrich Landolt, who described extensive trials in 1886 for the first time.

He also identified an equation for calculating the time from mixing the raw materials until the color changes of the solution.

Is the temperature in degrees Celsius, the concentration of the sulfur dioxide and the concentration of the iodic acid, in each case in g · m - third This equation is empirical and only has validity in the temperature range 5-40 ° C, and.

Landolt also discussed other factors whose influence extends to the time until the color changes not directly from the reaction equations forth but is clearly present, including sulfuric acid, various other acids, sodium chloride ( " sodium chloride " ) and the wall of the reaction vessels.