Benedict’s solution is a deep-blue complex mixture of sodium carbonate, sodium citrate and copper (II) sulphate pentahydrate. It is often used in place of Fehling’s solution to detect the presence of reducing sugars. The presence of other reducing substances also gives a positive reaction. A positive test with Benedict’s reagent is shown by a colour change from clear blue to a brick-red precipitate.
The substance to be tested is heated up to 95 °C (for example, in a water bath) with Benedict’s solution; formation of a brick-red precipitate indicates presence of the aldehyde group in relatively high concentrations. Since simple sugars (e.g., glucose) give a positive test, the solution is used to test for the presence of glucose in urine, a sign of diabetes.
The colour of the obtained precipitate gives an idea about the quantity of sugar present in the solution, hence the test is semi-quantitative. A greenish precipitate indicates about 0.5 g% concentration; yellow precipitate indicates 1 g% concentration; orange indicates 1.5 g% and red indicates 2 g% or higher concentration. If the solution remains clear or is a little blue, it means reducing sugar in not present in the solution.