Fearon's test

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Fearon's test , more precisely: "Fearon's test on methylamine" is a semi-quantitative detection method for lactose and maltose that was first described by William Robert Fearon and is very similar to the Wöhlk reaction .

Since methylamine is less outgassed and used in about ten times lower concentration than the ammonia in the Wöhlk reaction, Fearon's test is the better choice for risk assessment and substitute testing . There are several variants for carrying out the detection reaction, u. a. with heating over an open flame; However, since alkaline solutions tend to have dangerous delays in boiling , it is better to carry out the reaction in a 70 ° C water bath. Instead of adding the reagents by dropping them as described in the original literature, it is advisable to use a ready-made solution in which methylamine is formed by adding methylammonium chloride to a 0.5 molar sodium hydroxide solution . This solution is called Fearon's reagent.

Preparation of 250 mL Fearon's reagent (can be stored in an amber bottle for approx. 12 months): Fearon's reagent is prepared by dissolving 1.25 g methylammonium chloride and 5 g NaOH in 250 ml deionized water.

Carrying out the detection reaction: Disposable nitrile gloves and protective goggles are worn during the experiment. 300 ml of water are heated to 70 ° C. in a 1000 ml beaker and kept at this temperature. 2 ml of the different 2.5% sugar solutions or milk products are pipetted into numbered test tubes using disposable pipettes. Depending on the viscosity of the dairy products, the tips of the disposable pipettes must be widened (cut off) a little.

Then 2 ml of Fearon's reagent are added with a pipette and shaken carefully and thoroughly until everything has mixed well.

The test tubes are placed in the hot water bath and left there for at least 5 minutes until the colors of the samples have developed well. The varying red coloration indicates whether the sample contains a lot or little lactose. Lactose-free samples turn yellow if they contained monosaccharides such as glucose, galactose or fructose. Non-reducing sugars such as B. sucrose give no color.

There are now well-founded assumptions about the mechanism of Fearon's test and the structure of the cherry-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 cherry-red dye from Fearon's test has an absorption maximum at 541 nm and is also formed with other sugars that are built up analogously to lactose and maltose (e.g. cellobiose , maltotriose ), but also with glucose, if they carries a protective group on the OH group in position 4 .

Individual evidence

  1. ↑ Klaus Ruppersberg: On the trail of milk sugar? A European detective story. In: Practice of Natural Sciences - Chemistry in School: PdN. Volume 65, No. 8, 2016, pp. 30-33. urn : nbn: de: 0111-pedocs-150938
  2. ↑ Klaus Ruppersberg, Horst Klemeyer: Lactose rapid test: How can you detect milk sugar in 60 seconds? In: CHEMKON . Wiley-Verlag, January 23, 2020, ISSN  0944-5846 , doi : 10.1002 / ckon.201900064 .
  3. ^ 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 .
  4. ↑ 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 .
  5. ↑ 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 .