His solution made him famous: Fehling's test

Really simple?

Fehling's measurements and atomic weight calculations had led him to assume that one sugar molecule reacted with 10 molecules of copper sulphate. But it soon became clear that this ratio between copper sulphate and sugar was not universally valid. Yet this realisation did nothing to alter the sugar test method itself, which was not based on a calculation, but on the calibration table devised by Fehling. As long as Fehling's instructions for the test solution were adhered to, it was possible to work accurately with the figures he had given.

To this day it is still not possible to establish a fixed numerical relationship between the components of the reaction. In technical terms, Fehling's reaction is said not to be stochiometric, meaning that it is not possible to provide a detailed equation for reaction determining precisely which particles react with which and which product is formed at each stage of the process.

In practice, Fehling's solution is now primarily used as a qualitative test for sugar or aldehydes.

The colour change as a qualitative test for sugar is easy to recognise and is simple to use even for amateur chemists. Yet a lot of expertise is needed to be able to arrive at a precise quantitative result using Fehling's method. Here the key is to add just enough sugar solution to Fehling's solution so that the change of colour just begins to start - no more and no less.