In the context of bilirubin chemistry, which solvent is used to dissolve unconjugated bilirubin before reacting with the Ehrlich diazo reagent?

• A. Acetone
• B. Ether
• C. Distilled water
• D. Caffeine

Answer: (D)

The key idea is that unconjugated bilirubin is not water-soluble, so a solubilizing agent is needed to bring it into solution before it can react with the Ehrlich diazo reagent. In common bilirubin assays, caffeine is used to form a bilirubin–caffeine complex that dissolves unconjugated bilirubin, allowing the diazo reaction to proceed and form the colored azobilirubin measured spectrophotometrically. Conjugated bilirubin, being more water-soluble, can react with the diazo reagent without such a solubilizer. Other solvents like acetone or ether can dissolve bilirubin but may interfere with the reaction or raise safety concerns, while distilled water won’t solubilize the unconjugated form effectively.