Nitric acid in Dr Stone S1E1

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In the heart of the pilot episode, Senku drags Taiju into a cave and reveals, with his trademark grin, what is for him a scientific miracle: the water dripping from the ceiling is nitric acid. “It’s miracle water, thanks to bat guano.”

This substance becomes the chemical keystone of the entire first season. Combined with ethanol (from wine distilled into brandy), it forms nital, the corrosive agent Senku applies as soon as the following episode to break through petrification.

The subject in depth

The molecule: a strong acid and a powerful oxidizer

Nitric acid has the formula HNO₃. Its structure is planar: a central nitrogen atom bonded to three oxygens, two by equivalent resonance and one carrying a labile acidic proton. Pure, it is a colorless, dense liquid (1.51 g/cm³) that fumes in air, releasing brown vapors of nitrogen dioxide (NO₂), hence its historic name, “fuming strong water” ^[Wikipedia] .

Two properties make it an exceptional substance in inorganic chemistry. First, it is a strong acid (pKa ≈ −1.4): in aqueous solution it dissociates almost completely into H⁺ and NO₃⁻. Second, and more importantly, it is a powerful oxidizer : it attacks most metals (copper, iron, zinc, lead), forming metal nitrates and releasing nitrous vapors. Only gold, platinum, and a few stainless alloys resist it, which is why it was historically used to verify gold’s authenticity. Combined with hydrochloric acid (3 to 1), it becomes aqua regia, capable of dissolving gold.

How bats make HNO₃

The mechanism shown in Dr Stone is rigorously accurate, just compressed. Here is the full chain:

1. Guano (droppings, urine, and runoff water) is rich in nitrogen compounds, mainly urea and uric acid.
2. On contact with moisture, these compounds hydrolyze into ammonia (NH₃), which dissolves in the cave water as ammonium (NH₄⁺).
3. A first bacterium, Nitrosomonas, oxidizes ammonium into nitrite (NO₂⁻). This is the first step of nitrification ^[Wikipedia] .
4. A second bacterium, Nitrobacter (or sometimes Nitrospira), oxidizes nitrite into nitrate (NO₃⁻).
5. In an environment acidified by dissolved atmospheric CO₂, the nitrate pairs with a proton to form dilute HNO₃, which slowly drips from the ceiling and pools on the floor.

This is exactly the process that naturally produces nitrates in farmland soil. The difference: in a closed cave with a dense bat population, nitrogen inputs are enormous and concentrated, and HNO₃ builds up over centuries or even millennia.

The modern industrial process: Ostwald, 1908

Outside caves, humanity today produces around 60 million tonnes of nitric acid a year via the Ostwald process (Wilhelm Ostwald, Nobel Prize 1909). The process chains three steps:

1. Catalytic oxidation of ammonia over platinum-rhodium grids at 850 °C: 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O.
2. Oxidation of the monoxide to dioxide: 2 NO + O₂ → 2 NO₂.
3. Absorption in water: 3 NO₂ + H₂O → 2 HNO₃ + NO (the NO is recycled).

Overall yield reaches 95%. Most of this output goes into ammonium nitrate fertilizers, then explosives (TNT, nitroglycerin), polymers (nylon via adipic acid), and metal treatment. Without the Ostwald process, there would be no modern intensive agriculture, it is one of the major technical levers behind the 20th century’s population explosion.

The fact-checker’s eye

Going further

• Discipline: Chemistry: all chemistry entries on the site
• Coming soon: Guano and nitrification: the biological detail behind the natural process
• Coming soon: Nital: the ethanol/HNO₃ mix used in metallography
• Coming soon: Fact-check, can nitric acid reverse petrification?

Glossary

Oxidation ↗

Chemical reaction in which a species loses electrons (often by gaining oxygen). Nitric acid is a powerful agent of this kind.

Nitrification ↗

Biological conversion of ammonium into nitrate, in two steps catalyzed by the bacteria Nitrosomonas and then Nitrobacter. A key step in the nitrogen cycle.

Ostwald process ↗

Industrial process developed by Wilhelm Ostwald in 1908 to produce nitric acid from ammonia, over a platinum catalyst at 850 °C.

Xanthoproteic reaction ↗

Reaction of nitric acid with skin proteins, which permanently turns them yellow.

Sources

1. Nitric Acid, French Wikipedia ^[Wikipedia]
2. Nitrogen Cycle, French Wikipedia ^[Wikipedia]
3. INRS, Nitric acid, toxicological fiche no. 9 ^[Web]
4. Robert P. Multhauf, The Origins of Chemistry, Oldbourne, 1966 ^[Livre]