Abstract
The reaction of Fehling’s and Benedicts’s test solutions with solutions of reducing sugars, e.g. glucose, is frequently misinterpreted with respect to the products formed. The tests are by no means suitable to detect aldehyde groups in organic molecules. The course of the reaction is known since long to be rather complex and the corresponding carboxylate, often assumed to be the major reaction product, does not occur at all. Both tests, as well as Tollens’ test using alkaline silver(I) solution, have some drawbacks for chemistry lessons, especially in students’ lab courses. The “iodine test”, well established to quantitatively determine glucose, could be a suitable alternative in many cases. It allows a quick detection, even of small amounts or reducing sugars at room temperature by decolourisation of a weakly alkaline iodine–starch–solution. The test is based on several coupled equilibria, and hypoiodous acid is the oxidizing agent. The complexity of the molecular description of the reaction can easily be reduced. Hence, the iodine test is suited to set different levels of difficulty, in lab work as well as in exams. The “iodine test” can even be used in Primary School, since the necessary solutions do not represent a danger.
Highlights
Sugars, i.e. carbohydrates of relatively low molecular weight, are nourishing contents of quite a lot of foods
I.e. carbohydrates of relatively low molecular weight, are nourishing contents of quite a lot of foods. They typically have a sweet taste which is highly regarded by many consumers
Given the same temperature and concentration of sugar, Fehling’s test proceeds quicker than Benedict’s, which is due to the higher concentration of hydroxide ions in the former
Summary
I.e. carbohydrates of relatively low molecular weight, are nourishing contents of quite a lot of foods. They typically have a sweet taste which is highly regarded by many consumers. In Primary School, pupils are usually only advised to reduce the intake of food containing added sugar or even avoid it at all. Fehling’s or Benedict’s test solutions as well as Tollens’ reagent are widespread and commonly used to test for reducing sugars. Despite their frequent use, they have a few drawbacks that will be outlined first
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