One of the most popular tests used for the estimation or detection of reducing sugars and non-reducing sugars is the Fehling’s test. The test developed by German chemist H.C. Von Fehling is also used to differentiate between ketone functional groups and water-soluble carbohydrates.
Fehling’s test consists of a solution that is usually prepared fresh in laboratories. Initially, the solution exists in the form of two separate solutions which are labelled as Fehling’s A and Fehling’s B. Fehling’s A is a solution containing copper(II) sulphate, which is blue in colour. Fehling’s B is a clear liquid consisting of potassium sodium tartrate (Rochelle salt) and a strong alkali, usually sodium hydroxide. During the test solutions A and B are prepared individually and stored.
The two solutions are later mixed in equal volumes to get the final Fehling solution which is deep blue in colour. The deep blue ingredient is the bis(tartrate) complex of Cu2+. The tartrate tetra-anions serve as a chelating agent in the solution.
The procedure can be conducted as follows;
Notably, the result is positive if there is a formation of reddish brown precipitate while the result is negative if there is no indication of such change.
Precaution: Fehling’s solution is mostly corrosive in nature. Therefore it is always good to wear protective gear like goggles and gloves.
In Fehling’s solution the reaction between copper(II) ions and aldehyde is represented as;
RCHO + 2 Cu2+ + 5 OH− → RCOO− + Cu2O + 3 H2O
When tartrate is added:
RCHO + 2 Cu(C4H4O6)22− + 5 OH− → RCOO− + Cu2O + 4 C4H4O62− + 3 H2O
Some of the common uses of Fehling’s test are; it is used to determine whether a carbonyl group is an aldehyde or a ketone. Aldehydes tend to get oxidized and give positive result. Ketones apart from alpha-hydroxy-ketones do not react.
Fehling’s test is also used as a general test for monosaccharides where a positive result is obtained for aldose monosaccharides and ketose monosaccharides. Apart from these, Fehling’s test is used in the medical field to determine the presence of glucose in urine. It helps to know whether the person is diabetic or not.