The Iodine Test for Reducing Sugars - A Safe, Quick and Easy Alternative to Copper(II) and Silver(I) Based Reagents

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.