🏾 Contents :
🏾 Benedict's test ?
The Benedict's test is a simple chemical test that can be used to check for the presence of reducing sugars. Reducing sugars are sugars that have free aldose or ketose groups capable of donating electrons to other molecules by oxidizing them. This includes all monosaccharides (eg glucose, fructose, galactose) and many disaccharides including lactose and maltose.
Benedict's reagent can also be used to detect glucose in urine. If the urine contains sugar, it is most often glucose, which indicates diabetes or another disorder of blood sugar regulation.
🏾 Principle
The principle is similar to Fehling's test, reducing sugars can reduce copper ion (Cu+2) to cuprous ion (Cu+) which is the basis of Benedict's test (and Fehling's test).
Copper sulfate in Benedict's solution reacts with reducing sugars and cupric ions to form cuprous ions, these are precipitated as red copper oxide, which is insoluble in water.
Sodium carbonate provides the alkaline conditions necessary for the redox reaction. The original color of Benedict's reagent is blue. It turns green, yellow, orange or red, depending on the concentration of reducing sugar present.
🏾 Composition
Benedict's reagent is a deep-blue aqueous solution.
- 173g sodium citrate per liter
- 17.3g copper sulfate per liter
- 100g anhydrous sodium carbonate per liter
The sodium carbonate and sodium citrate are mixed first, and then the copper sulfate is added slowly with constant stirring.
🏾 Benedict's test procedure and results
Procédure :
- 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.
Results :
The blue color of the reagent changes to green in the presence of small amounts of glucose.
A reddish precipitate forms over time
Higher glucose concentrations result in a yellow to red precipitate during heating.
| Color | Approximate glucose mg/dl | Indication |
|---|---|---|
| Blue Solution | No | |
| Green Solution | <500 mg/dl | Trace |
| Ppt green | 500-1000 mg/dl | + |
| Ppt yellow | 1000-1500 mg/ dl | ++ |
| Orange ppt | 1500-2000 mg/dl | +++ |
| Red to brick red ppt | > 2 000mg/dl | ++++ |
Benedict's test positive on the left and negative on the right.
Reference
- byjus - Benedict’s Test
- chemistrylearner - Benedict’s Test
- labster - Le réactif de Benedict
- nku - Benedict's Reagent: A Test for Reducing Sugars
- himedia - Benedict's Qualitative Reagent
- laboratoryinfo - Benedict’s Test : Principle, Reagent Preparation, Procedure and Interpretation
- biognost - BENEDICT'S REAGENT
- Reem.S.Najm - test for reducing sugars Lect.2. .