Fehling’s solution is used in classrooms as an example of a pH-dependent redox reaction. Johannes Waller and Philipp Kessler realised that glucose and fructose react at different speeds and that this phenomenon cannot be explained by their molecular structures. Their laboratory tests have demonstrated that Fehling’s solution follows more complex laws of chemistry than those presented in school books. During the reaction, the substances pass through various intermediate stages and form oxidation products, which, in turn, have reduction effects. The colouration follows a different mechanism for each sugar and depends on the formation of certain intermediate products. This explains why the fructose-based Fehling reaction generates a red colour more quickly than the glucose-based reaction.


Johannes Nicolas Waller

Philipp Nikolas Kessler