Wöhlk reaction

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The Wöhlk reaction , also Wöhlk test , also Wöhlk-Malfatti reaction , is a semi-quantitative detection reaction for lactose and maltose , which leads to a characteristic salmon-red color by adding ammonia solution while heating. It was named after its discoverer Alfred Wöhlk , who described it in 1904. In 1905 it was improved by the Innsbruck urology professor Hans Malfatti by adding a few drops of potassium hydroxide and until the 1960s it was used in medical and hospital laboratories to differentiate gestational diabetes from lactosuria (appearance of lactose in the urine, e.g. in one Milk congestion ) is used. In the English-speaking area, a chemically very similar variant was used for the same purpose, in which the ammonia solution is replaced by an alkaline methylammonium chloride solution. This variant is called " Fearon's test on methylamine" after its inventor William Robert Fearon and is described in an Indian textbook in 2000.

Although the Wöhlk reaction in the modern urological laboratory is obsolete due to more modern and better detection methods ( HPLC , GC , rapid laboratory test), it has been experiencing a renewed upswing in chemistry classes since 2016, because there it can be used as semi-quantitative evidence of the very different lactose content of dairy products . Even at the end of a standard school experiment, the breakdown of starch by salivary amylase , it is used to detect the disaccharide maltose, which is the main product, alongside glucose , isomaltose and other incidental residues of endohydrolytic breakdown.

There are now well-founded assumptions about the mechanism of the Wöhlk reaction and the structure of the red dye, which point towards a pyridinium salt with intramolecular charge separation. The betaine-like dye is only stable in the strongly alkaline range and cannot be shaken out with known solvents . Ketoses such as B. Lactulose and fructose react faster than the analogous aldoses , because the latter still have to carry out a Lobry-de-Bruyn-van-Ekenstein rearrangement in order to enter the reaction path to the red dye. After all, we now know that the salmon-red dye of the Wöhlk reaction has an absorption maximum at 527 nm and is also formed with other sugars that have a similar structure to lactose and maltose (e.g. cellobiose , maltotriose ), but also with glucose, if it carries a protective group on the OH group in position 4 .

Individual evidence

  1. ↑ Ruppersberg, Klaus, Hain, Julia: How can the lactose content of dairy products be made visible in school experiments? Rediscovering the Wöhlk reaction for chemistry class . In: Chemistry in concrete terms - ChemKon . tape 23 , no. 2 , 2016, urn : nbn: de: 0111-pedocs-145962 .
  2. ↑ Hans Malfatti: About the detection of milk sugar in the urine . In: Centralblatt for the diseases of the urinary and sexual organs . tape 16 , 1905, pp. 68–71 ( Textarchiv - Internet Archive ).
  3. ↑ Klaus Ruppersberg: On the trail of milk sugar - a European detective story . In: Practice of Natural Sciences - Chemistry in School: PdN . tape 65 , no. 8 . Aulis-Verlag, 2016, p. 30–33 , urn : nbn: de: 0111-pedocs-150938 .
  4. ^ WR Fearon: The detection of lactose and maltose by means of methylamine . In: Analyst . tape 67 , no. 793 , January 1, 1942, p. 130-132 , doi : 10.1039 / AN9426700130 .
  5. ↑ Kolhatkar, Arundhati A .: Medical laboratory science: theory and practice . Tata McGraw-Hill, New Delhi 2000, ISBN 0-07-463223-X , pp. 1339 .
  6. ↑ Klaus Ruppersberg, Julia Hain: The rediscovery of the Wöhlk sample . In: Chemistry in Our Time . tape 51 , no. 2 , April 2017, p. 106–111 , doi : 10.1002 / ciuz.201600744 .
  7. ↑ Klaus Ruppersberg: Starch digestion by saliva - what does that actually result? A simple maltose detection at the end of the enzymatic hydrolysis of amylose and the surprising presence of glucose in a ratio of 1:15 . In: MNU Journal . tape 69 , no. 5 , 2016, urn : nbn: de: 0111-pedocs-150973 .
  8. ↑ Klaus Ruppersberg, Stefanie Herzog, Manfred W. Kussler, Ilka Parchmann: How to visualize the different lactose content of dairy products by Fearon's test and Woehlk test in classroom experiments and a new approach to the mechanisms and formulas of the mysterious red dyes . In: Chemistry Teacher International . De Gruyter, October 17, 2019, ISSN  2569-3263 , doi : 10.1515 / cti-2019-0008 .
  9. ↑ Klaus Ruppersberg, Julia Hain, Petra Mischnick: On the trail of the red color: A historical proof of lactose rediscovered . In: CHEMKON . tape 24 , no. 4 , 2017, p. 302–308 , doi : 10.1002 / ckon.201790012 .
  10. ↑ Manfred W. Kussler, Klaus Ruppersberg: The dye from the Wöhlk sample . In: News from chemistry . tape 67 , no. 2 , February 2019, ISSN  1439-9598 , p. 63-65 , doi : 10.1002 / nadc.20194083855 .