Benedict's Test: Detection of Reducing Sugars

By Chemca Editorial Team • Last Updated: January 2026 • 8 min read

Benedict's Test is a mild oxidizing test used to detect the presence of Reducing Sugars (such as glucose, fructose, maltose) and Aliphatic Aldehydes. It is chemically similar to Fehling's test but is more stable and convenient because it exists as a single solution.

1. Preparation of Benedict's Reagent

The reagent is a single solution containing:

Copper(II) Sulfate ($CuSO_4 \cdot 5H_2O$): Provides $Cu^{2+}$ ions (Blue).
Sodium Carbonate ($Na_2CO_3$): Provides the alkaline medium (weaker base than NaOH used in Fehling's).
Sodium Citrate: Acts as a complexing agent to keep $Cu^{2+}$ soluble in the alkaline medium (forming Copper Citrate complex).

2. General Reaction & Observation

Upon heating with a reducing sugar or aliphatic aldehyde, the blue Copper(II) ions are reduced to Copper(I) ions, which precipitate as insoluble red Cuprous Oxide.

$$ R-CHO + 2Cu^{2+} + 5OH^- \xrightarrow{\Delta} \underbrace{R-COO^-}_{\text{Carboxylate}} + \underbrace{Cu_2O \downarrow}_{\text{Brick Red Ppt}} + 3H_2O $$

Color Changes: The final color depends on the concentration of the reducing sugar:

Blue (None) Green (Trace) Yellow (Low) Orange (Mod) Brick Red (High)

3. Benedict's vs. Fehling's Test

While the chemistry is nearly identical (Oxidation of Aldehyde / Reduction of Copper), there are key differences:

Feature	Benedict's Reagent	Fehling's Reagent
Chelating Agent	Sodium Citrate	Sodium Potassium Tartrate (Rochelle Salt)
Base	Sodium Carbonate ($Na_2CO_3$) - Weaker	Sodium Hydroxide ($NaOH$) - Stronger
Stability	Stable (Single Solution)	Unstable (Mixed freshly from Sol A + Sol B)

4. Scope and Limitations

Positive Test (+ve)

Aliphatic Aldehydes (e.g., Acetaldehyde).
Reducing Sugars: Monosaccharides (Glucose, Fructose, Galactose) and some Disaccharides (Maltose, Lactose).
$\alpha$-Hydroxy Ketones: Fructose gives a positive test because it tautomerizes to an aldehyde in the alkaline medium.

Negative Test (-ve)

Aromatic Aldehydes: Benzaldehyde does not react (Reagent is too mild).
Simple Ketones: Acetone does not react.
Non-reducing Sugars: Sucrose (Table sugar) gives a negative test because the glycosidic bond locks the anomeric carbons.

5. Biological Significance

Benedict's test is classically used in medicine to detect Glucose in Urine (Glucosuria), which is an indicator of Diabetes Mellitus.

Benedict's Quiz

Test your concepts on Sugar identification. 10 MCQs with explanations.

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