In the colorimetric chemical method, the S reagent produces what color product?

• A. Green Monosulfonic Acid
• B. Red Disulfonic Acid
• C. Orange Monosulfonic Acid
• D. Purple Disulfonic Acid

Answer: (B)

Colorimetric tests work by forming a colored product (a chromophore) when the reagent reacts with the analyte. The S reagent drives sulfonation of an aromatic system to create a disulfonic acid. This di-substituted, highly conjugated structure shifts the absorption to longer wavelengths, so it appears red. The two sulfonic groups enhance the chromophore’s intensity and red hue compared with monosulfonic products, which have less conjugation and absorb at shorter wavelengths (giving green or orange). A purple color would require a different electronic arrangement not typical for this reaction. So the red disulfonic acid is the color most consistent with the S reagent’s colorimetric product.