The Benedict’s test is a simple chemical that can be used to assess the presence of reducing sugars. Reducing sugars have free aldose or ketose groups, which capable of donating electrons to other molecules by oxidizing them.
Reducing sugars are mostly monosaccharides (eg glucose, fructose, galactose) and some of them are disaccharides which are lactose and maltose.
Principle of Benedict test
Reducing sugar under alkaline condition form enediols. Benedict’s solution contains milder alkali Na2CO3. Enediols are powerful reducing agents.
They can reduce cupric ions (Cu +2) to cuprous ions (Cu+) which is the basis for Benedict’s reaction. The cuprous hydroxide during the process of heating is converted to red cuprous oxide.
Benedict test procedure
- Add 5ml of Benedict’s qualitative reagent to a test tube.
- Add 8 drops (0.5 ml) of urine. Boil over high heat for 3 min.
- The contents of the tube become cloudy due to the precipitate which can vary from green to brick red depending on the amount of sugar present in the urine.
- If no sugar is present, the solution will remain clear or show slight turbidity.
How to prepare Benedict’s reagent
- Measure 17.3 grams of copper sulfate (CuSO4), 173 grams of sodium citrate (Na3C6H5O7), and 100 grams of anhydrous sodium carbonate (Na2CO3) (or 270 grams of sodium carbonate decahydrate (Na2CO3.10H2O).
- Put all the measured chemicals in a volumetric flask of 1000 mL.
- Pour distilled water up to 1000 mL marking.
- Dissolve all the components properly by shaking gently.
Benedict reagent components
- Benedict’s reagent
- Water bath
- Dry test tubes
- Positive control (5% glucose)
- Negative control (distilled water)
Benedict test Result with interpretation
- Positive Benedict test : color change from blue to brick red ppt (glucose)
- Negative Benedict’s test: no change in color( sucrose)
Benedict positive means?
When tube color change from blue to Black. It is the sign of reducing sugar in the sample.