Iodine sample

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Iodine samples on mash when brewing beer. Left two blue sample results that still contain starch. Yellowish on the right, i.e. without starch.

The iodine test ( iodine-starch reaction ) is a chemical method for the detection of starch using a solution containing iodine . In most cases, either an iodine-potassium iodide solution ( Lugol's solution ) or an iodine tincture (alcoholic solution of elemental iodine) is used, which is dripped onto a solid sample or mixed with a sample solution. The result is “iodine starch”, an inclusion compound that - depending on the concentration of the iodine solution used - has a deep blue, blue-violet to black color.

Conversely, the iodine sample can also be used to detect iodine by using a starch solution.

Applications of the test for strength

Bottle with iodine solution for the iodine strength test to determine when apples are harvested
Sliced ​​apple of starch degradation level 4–5

In fruit growing, an iodine sample is used to determine when to harvest. For this purpose, the cut surfaces of freshly picked apples are wetted with a solution of 3 g of iodine and 10 g of potassium iodide in 1 liter of water. The starch reduction pattern is then assessed using a template.

Chemically functioning bank note test pens are felt-tip pens with which some iodine solution is painted on a bank note . Banknote papers are based on cotton and / or polymer and do not contain any starch, the slightly yellowish coating tends to fade, while most office and copier papers contain starch, which quickly makes the coating appear blue.

Chemical background

Schematic representation of an I 3 - ion embedded in the amylose helix

The amylose present in the starch has a helical conformation with a channel-like cavity in the middle. Polyiodide chains (I 3 - , I 5 - , I 7 - , I 9 - ) can be embedded in these . More recent studies (from 2016), on the other hand, assume unlimited chains of neutral iodine. In these I made two - molecules built up chains that cater to an existing solution in the iodide ion accumulate, all seven are valence of iodine atom delocalized . This reduces the excitation energy of the valence electrons so that they absorb light in the longer-wave range and appear complementary blue-black. This color is used in the analytical method of iodometry .

When the deep blue to blue-violet solution is warmed up, discoloration takes place. To explain this, one must consider the prevailing equilibrium . The formation of the complex is exothermic, so the reverse reaction, i.e. the formation of polyiodide and amylose from the polyiodide-amylose complex, must be endothermic. According to the principle of Le Chatelier and Braun , an increase in temperature leads to a shift of the equilibrium to the left, since the reverse reaction is endothermic. As a result, the number of starting materials increases and discoloration goes hand in hand. Cooling down leads to the preferential formation of the complex, since according to the principle of Le Chatelier the forward reaction is favored.

By adding ethanol or carbon disulfide , the polyiodide ions are released from the helix.

Web links

Individual evidence

  1. ↑ Entry on starch iodine reaction. In: Römpp Online . Georg Thieme Verlag, accessed on June 14, 2014.
  2. ↑ Lucas' Instructions for Fruit Growing, 31st edition 1992, pp. 308–309.
  3. ↑ Banknote detection pens - for more security in the wallet kriminalberatung.de, with video from 2012, updated 2019, accessed June 20, 2019.
  4. ↑ effectiveo: Instructions for banknote test pin Money test pin function youtube.com, September 18, 2012, accessed June 20, 2019. - Video (0:44)
  5. ↑ X. Yu, C. Houtman, RH Atalla, Carbohydr. Res. 1996, 292, 129-141.
  6. ↑ Sheri Madhu, Hayden A. Evans, Vicky VT Doan-Nguyen, John G. Labram, Guang Wu, Michael L. Chabinyc, Ram Seshadri, Fred Wudl: Infinite Polyiodide Chains in the Pyrroloperylene-Iodine Complex: Insights into the Starch-Iodine and Perylene-Iodine Complexes. In: Angewandte Chemie. 128, 2016, p. 8164, doi : 10.1002 / anie.201601585 .
  7. ↑ Spektrum.de: Iodine starch sample: Mystery of school experiment solved after 200 years - Spectrum of Science , accessed on September 8, 2016.
  8. ↑ Dr. Sheri Madhu et al .: Infinite Polyiodide Chains in the Pyrroloperylene – Iodine Complex: Insights into the Starch – Iodine and Perylene – Iodine Complexes Ztschr. Angewandte Chemie, May 30, 2016, onlinelibrary.wiley.com, abstract available, accessed June 20 2019.