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Testes para Açúcares Redutores: Benedict, Fehling, Barfoed, Nylander — Resultados Positivos

All detect reducing sugars (free anomeric carbon) via copper(II) reduction; differ in sensitivity, specificity, and conditions.

Benedict's Test

• Reagent: CuSO₄, sodium citrate, Na₂CO₃ (alkaline).
• Positive: Blue → green → yellow → orange → brick-red precipitate (Cu₂O).
• Semi-quantitative by color intensity.
• Detects all reducing sugars (glucose, fructose, lactose, maltose).

Fehling's Test

• Two solutions: A (CuSO₄), B (Rochelle salt + NaOH). Mixed fresh.
• Positive: Red precipitate (Cu₂O) — more specific for aldehydes (aldoses).
• Less sensitive; ketoses (fructose) react slowly.

Barfoed's Test

• Reagent: Copper acetate in acetic acid (pH ~4.5).
• Positive: Red Cu₂O precipitate in 2–3 min — monosaccharides only (disaccharides hydrolyze too slowly).
• Distinguishes monosaccharides from disaccharides.

Nylander's Test

• Reagent: Bismuth subnitrate, Rochelle salt, NaOH.
• Positive: Black/gray precipitate (Bi⁰) — very sensitive for reducing sugars.
• Also detects some non-sugar reductones.

Clinical Use

Benedict's: urine screening (historical). Modern: enzymatic glucose oxidase strips. Fehling's/Barfoed's/Nylander's: teaching labs for carbohydrate chemistry.