Question

Honey is made from a mixture of fructose, glucose, and sucrose. How can you use Benedict's reagent to determine the number of moles of sucrose in a 1.0 g sample of honey? (Hint: If excess Cu²⁺ is present the difference in [Cu²⁺] before and after reaction is twice the concentration of reducing sugar present.)

Answer 1

Benedict's quantitative reagent is used to determine how much reducing sugar is present.

Sucrose (table sugar) contains two sugars (fructose and glucose) joined by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict's reagent.

Sucrose indirectly produces a positive result with Benedict's reagent if heated with dilute hydrochloric acid prior to the test, although after this treatment it is no longer sucrose. The acidic conditions and heat break the glycosidic bond in sucrose through hydrolysis. The products of sucrose decomposition are glucose and fructose, both of which can be detected by Benedict's reagent.

Thus the titration needs to be carried out twice. The first time on a cold sample of the honey, which will give you the amount of the reducing sugars (fructose and glucose) present. The second titration will be on a sample that has been heated with dilute hydrochloric acid prior to the test. This will give you the total amount of sugars present. The difference between the two results is the amount of sucrose present. The titrations will require calibration with a reducing sugar, such as glucose, and sucrose.