His solution made him famous: Fehling's test

Really Fehling's achievement?

The process documented by Fehling had been known as a method of testing for sugar for a long time. Fehling's achievement was to precisely quantify this reaction. In experiments using self-prepared solutions with a known amount of sugar he devised a kind of calibration table, which allowed the sugar content of a liquid to be precisely calculated. It was only thanks to Fehling's list of figures that the familiar yet purely qualitative test method was transformed into a versatile and precise measurement method, which was also very simple to administer.

However, in practice the method naturally was more complicated than it sounds. Especially the stability of Fehling's solution caused problems. As some of its components reacted with each other, the mixture could not be stored for any length of time. Both Fehling and other scientists worked on improving the method over the next ten years. Yet the fact that the process was described in a popular textbook and that it rapidly became established in practice linked his name for ever with using a copper sulphate solution to test for sugar - even if some people thought that this was unfair.