Fehling’s Test: Theory

Fehling's test is used to determine if a reducing sugar is present in a sample using a reagent called the Fehling's reagent. The reagent is an alkaline solution, containing a copper tartrate complex with Cu²⁺ ions. When the reagent reacts with the aldehyde group of a reducing sugar, the Cu²⁺ ions are reduced to Cu⁺ ions, forming a red precipitate of cuprous oxide. The chemical reaction of the key functional groups can be viewed in Figure 1.

Figure 1 - Simplified reaction of a reducing sugar with Fehling’s reagent.

Fehling’s reagent is made by mixing equal amounts of two solutions - Fehling's A and Fehling's B. Fehling's A is a copper sulfate solution. Fehling's B is an alkaline solution of sodium potassium tartrate (Rochelle’s salt). Fehling’s reagent deteriorates over time, because the copper tartrate complex is unstable, therefore the reagent must be freshly prepared before use.