Landolt reaction

The “Ioduhr”: Salt solutions of iodide and iodate react with a time delay to iodine , which turns the starch solution blue

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

When starch is added , the resulting iodine turns the solution blue ( iodine sample ). This reaction became relatively well known because under certain conditions the blue coloration does not appear immediately, but only suddenly after a long period of time after the starting materials have been mixed. Depending on the temperature and concentration of the solutions, it can be predicted to within a few seconds when the color change should occur. For this reason the Landolt reaction is also called Landolt's time reaction or Ioduhr and is often presented in demonstration experiments to introduce chemistry and chemical kinetics .

Chemical basics

The following reaction equation (1) describes the entire process of the redox reaction, which takes place in individual steps, in chemical formulas :

${\displaystyle \mathrm {(1)\ \ 2\,HIO_{3}+5\,H_{2}SO_{3}\longrightarrow 5\,H_{2}SO_{4}+I_{2}+H_{2}O} }$

In fact, several partial reactions take place at different reaction rates and lead to delayed iodine formation and thus blue coloration. In general, the acids are not used directly as starting materials, but their salts, iodates or sulfites or hydrogen sulfites , in acidic solution. The relevant partial reactions are always the same:

Iodate and sulfite form iodide ions in a redox reaction .

${\displaystyle \mathrm {(2)\ \ IO_{3}^{-}+3\,SO_{3}^{2-}\longrightarrow I^{-}+3\,SO_{4}^{2-}} }$

Iodide and iodate react in a comproportionation to iodine

${\displaystyle \mathrm {(3)\ \ IO_{3}^{-}+5\,I^{-}+6\,H^{+}\longrightarrow 3\,I_{2}+3\,H_{2}O} }$

which, however, is immediately reduced to iodide again as long as sulfite is still present in the solution.

${\displaystyle \mathrm {(4)\ \ SO_{3}^{2-}+I_{2}+H_{2}O\longrightarrow 2\,I^{-}+SO_{4}^{2-}+2\,H^{+}} }$

Reaction (4) is so fast that there is practically no iodine in the solution. Only when the sulfite has been used up as a result of reactions (2) and (4) does a detectable amount of iodine form, which forms polyiodide with excess iodide.

${\displaystyle \mathrm {(5)\ \ 2\,I_{2}+I^{-}\longrightarrow I_{5}^{-}} }$

${\displaystyle \mathrm {I_{5}^{-}} }$is stabilized in starch in the linear helical channels of the amylose and appears blue ( iodine sample ).

Historical

The reaction is named after the Swiss chemist Hans Heinrich Landolt, who first described detailed experiments in 1886.

He also found an equation to calculate the time from mixing the raw materials to the color of the solution.

${\displaystyle t(T,c_{\mathrm {H_{2}SO_{3}} },c_{\mathrm {HIO_{3}} })={\frac {906,05-23,01\,{\frac {T}{\mathrm {^{\circ }C} }}+0,1888\,\left({\frac {T}{\mathrm {^{\circ }C} }}\right)^{2}}{\left({\frac {c_{\mathrm {H_{2}SO_{3}} }}{\mathrm {g\cdot m^{-3}} }}\right)^{0,904}\cdot \left({\frac {c_{\mathrm {HIO_{3}} }}{\mathrm {g\cdot m^{-3}} }}\right)^{1,642}}}\ \mathrm {s} }$

This contains the temperature in degrees Celsius , the concentration of sulphurous acid and the concentration of iodic acid, each in mol · m ⁻³ . This equation is empirical and is only valid in the temperature range 5–40 ° C, and . ${\displaystyle T}$ ${\displaystyle c_{\mathrm {H_{2}SO_{3}} }}$ ${\displaystyle c_{\mathrm {HIO_{3}} }}$ ${\displaystyle t<60\,\mathrm {s} }$ ${\displaystyle C_{\mathrm {S} }\leq 3\,C_{\mathrm {J} }}$

Landolt also discussed other factors whose influence on the time until the color change is not immediately apparent from the reaction equations, but is clearly present, including sulfuric acid, various other acids, sodium chloride (“sodium chloride”) and the wall of the reaction vessels.

literature

• FR Kreißl, O. Krätz: Fire and flame, sound and smoke. Wiley-VCH, 1999.

Individual evidence

1. ^ HH Landolt : About the duration of the reaction between iodic acid and sulphurous acid. In: Ber. German Chem. Ges. 19, 1886, pp. 1317-1365, doi : 10.1002 / cber.188601901293 .
2. ↑ ^{a b} H. Landolt : About the duration of the reaction between iodic acid and sulphurous acid. In: Ber. German Chem. Ges. 20, 1887, pp. 745-760, doi : 10.1002 / cber.188702001173 .

Web links

• Description of the experiment ( memento from October 18, 2009 in the Internet Archive )