Fehling solution A

Fehling’s Solution A is a deep blue aqueous solution containing copper(II) sulfate pentahydrate, which serves as the copper ion source for detecting reducing sugars.

🏭⚗️ Composition
Fehling’s Solution A is composed of copper(II) sulfate pentahydrate dissolved in water, typically enhanced with a small amount of sulfuric acid for stability. Labs generally prepare it by dissolving copper sulfate crystals in distilled water to form a dark blue solution. This reagent is stored separately and only mixed with Fehling’s Solution B when needed to perform the test.

🔬 Properties
As a clear, intensely blue liquid, this solution supplies Cu²⁺ ions essential for the redox reaction in Fehling’s test. In diagnostic-grade formulations, such as those produced under clinical reagent specifications, the density is approximately 1.02–1.03 g/mL, and the pH is mildly acidic (typically around 3.5–4.5). It is fully miscible with water and stable when stored properly.

🧪 Applications
Although not typically used in histology, Fehling’s Solution A is crucial when combined with Solution B to create Fehling’s reagent a classic test for the presence of reducing sugars and aldehydes. In this test, the Cu²⁺ ions are reduced to copper(I) oxide (Cu₂O), precipitating as a brick-red solid when heated with a reducing compound. This reaction forms the foundation of qualitative carbohydrate testing, including glucose detection for diabetes screening in urine and evaluating sugar content in food.

⚠️ Safety
Fehling’s Solution A should be handled with care. It contains copper salts and is mildly acidic, so it may irritate skin and eyes. Appropriate protective equipment such as gloves and safety goggles and good laboratory ventilation are strongly recommended.