Brownish Discoloration of Skin in Nitric Acid Poisoning

The Mechanism: Xanthoproteic Reaction

1. Nitration of aromatic amino acids - Nitric acid (HNO3) acts as a nitrating agent and reacts with aromatic amino acid residues in skin proteins, specifically:
   • Tyrosine
   • Tryptophan
   • Phenylalanine
   This introduces nitro (-NO2) groups onto the benzene rings of these amino acids, forming yellow nitro-derivatives (xanthoproteic acid / picric acid-like compounds).
2. Color shift from yellow to brown/orange - The initial color is yellow (xanthoproteic acid). When these yellow nitro-compounds encounter the alkaline environment of tissue fluids (tissue pH is slightly alkaline), they are converted into their sodium/potassium salts, which appear orange-brown to brownish-yellow in color.

This is why the discoloration is described as yellow on the skin surface but brownish in deeper tissues and mucous membranes - the more alkaline the environment, the more the color shifts toward brown/orange.

Key Clinical Features (from textbooks)

• Initially the skin/mucous membrane appears white (protein denaturation/coagulation), then turns yellow, and eventually brownish-yellow
• Staining affects: lips, tongue, oral mucosa, skin, teeth, clothing, and vomitus
• Vomitus is described as "yellowish-brown containing altered blood"
• At postmortem: tissues may appear brown or brown-black due to acid hematin (reaction of nitric acid with hemoglobin) in addition to the xanthoproteic staining
• Urine turns brown

Summary Table

Forensic Significance

• The Essentials of Forensic Medicine and Toxicology, 36th ed. (2026) - "In concentrated form it combines organic matter and produces a yellow discoloration of tissue due to the production of picric acid (xanthoproteic reaction)."
• PC Dikshit Textbook of Forensic Medicine and Toxicology - "Initially it is white then yellow due to formation of xanthoproteic acid."