Nitric Acid vs Sulfuric Acid: Strong Acid Comparison
Nitric acid (HNO3) and sulfuric acid (H2SO4) are two of the most important industrial strong acids. Both are highly corrosive, but they differ fundamentally in their chemical behavior: nitric acid is a powerful oxidizing agent, while sulfuric acid is a powerful dehydrating agent. This comparison explains their distinct hazards, applications, and safety requirements.
Property Comparison
| Property | Nitric Acid | Sulfuric Acid |
|---|---|---|
| Acid Strength (pKa) | -1.3 (strong acid) | -3.0 (very strong acid; second dissociation pKa = 1.99) |
| Oxidizing Power | Strong — nitrate ion is a powerful oxidizer | Weak — concentrated H2SO4 acts as an oxidizer only when hot |
| Dehydrating Ability | Mild — can dehydrate some organic compounds | Extremely strong — chars organic matter on contact |
| Boiling Point | 83 °C (decomposes) | 337 °C (decomposes before boiling) |
| Density (conc.) | 1.51 g/mL (68% conc.) | 1.84 g/mL (98% conc.) |
| Vapor Hazard | High — emits toxic NOx fumes (brown gas) | Low — negligible vapor pressure at room temperature |
| Common Concentration | 68% (azeotrope), 90% (fuming) | 98% (concentrated), 93% (technical) |
| Reaction with Water | Exothermic (moderate) | Extremely exothermic — always add acid to water |
| Primary Industrial Use | Fertilizer (ammonium nitrate), explosives, metal etching | Fertilizer (phosphate), petroleum refining, battery acid |
| GHS Classification | GHS03 (oxidizer), GHS05, GHS06 | GHS05, GHS08 |
Safety Comparison
Uses Comparison
Verdict
Frequently Asked Questions
Why does nitric acid turn brown upon exposure to air?
Concentrated nitric acid decomposes when exposed to light or heat, producing nitrogen dioxide (NO2), a brown toxic gas. This is why nitric acid is stored in amber bottles. The brown color of old nitric acid indicates significant decomposition and reduced concentration.
Can I mix nitric acid and sulfuric acid?
Yes — the mixture is called 'nitrating acid' and is used for nitration reactions (producing nitrobenzene, TNT, etc.). The sulfuric acid acts as a catalyst and dehydrating agent, promoting the formation of the nitronium ion (NO2+). However, the mixing is extremely exothermic and must be done carefully with cooling.
Which acid is better for dissolving gold?
Nitric acid alone cannot dissolve gold. Gold is dissolved by aqua regia — a 3:1 mixture of hydrochloric acid and nitric acid. The nitric acid oxidizes the gold, and the chloride ions complex it as AuCl4-. Sulfuric acid does not dissolve gold under normal conditions.
Why does sulfuric acid not fume like nitric acid?
Sulfuric acid has a very low vapor pressure (0.001 kPa at 25°C) due to its high boiling point (337°C) and strong hydrogen bonding. Nitric acid has a much higher vapor pressure (6.4 kPa) and decomposes readily to NO2, creating visible brown fumes.