A nitrating mixture combines concentrated nitric acid with concentrated sulfuric acid: the nitric acid supplies the nitrating group and the sulfuric acid takes up the water the reaction produces, which would otherwise dilute the acid and stall it. Mixing the two releases heat, the mixture is corrosive and oxidising, and its composition is one of the settings that decides whether cellulose ends up as plastic or as explosive, so the hazard is explained below and no proportions are given.
Part of Plastics and Polymers
A nitrating mixture, or mixed acid, is a combination of nitric and sulfuric acids used to introduce nitrate groups into organic compounds such as cellulose.
Why The Mixture Uses Two Acids
Nitric acid alone can nitrate cellulose, but each nitration step makes a molecule of water. The water dilutes the acid and weakens it as the reaction goes on. Sulfuric acid takes up that water, and it also helps nitric acid form the reactive species that does the nitrating, the nitronium ion (general knowledge). The result is a stronger, more consistent nitrating agent than nitric acid alone. Nitric Acid and Sulfuric Acid cover the two acids on their own.
Because water in the mixture controls how far nitration goes, the composition of the acids is one of the parameters that separates celluloid grade from explosive grade. That is the reason for the limit below.
The Mixture Is Hazardous Before Any Cellulose Is Added
- It generates heat on mixing. Concentrated sulfuric acid heats strongly when diluted or combined, and the mixture can spit or boil.
- It is a strong oxidiser. The nitric acid article warns that the concentrated acid can ignite combustibles. Wood, cloth, straw, paper and grease are all fuel.
- It gives off nitrogen dioxide. The same article notes these fumes are toxic at very low concentrations and can cause delayed fluid in the lungs, so a victim may feel fine for hours and then deteriorate rapidly. Any significant exposure needs rest and medical care.
- It burns tissue and eyes. Chemical burns keep damaging until the acid is removed: see Chemical Burns. Protective Gear describes layered barriers, and the first rule there is eye protection.
- It must not be sealed. A mixed acid can release gas as it stands, and a sealed container can burst. Container choice for acids is in Container Selection.
Where This Article Stops
This page gives no proportions, no strengths, no mixing order, no temperatures and no quantities. That is a deliberate limit, explained in Cellulose Nitration. The same acid mixture, differently proportioned and handled, is how nitrocellulose and other explosives are made, and a written recipe cannot check who is reading it or how prepared they are.
A reader can come away with the essentials: why two acids, what each does, what the dangers are, and that the proportions are a control variable and not a detail. A reader who wants more should work with someone who has handled mixed acids and has the instruments to measure their strength.
Mixed Acid Plus Cotton Is Fuel Soaked In Oxidiser
Mixed acid is an oxidiser strong enough to ignite organic material, so a spill on a wooden bench, a rag or sawdust is a fire, not a stain. Cotton cellulose, the starting material, is itself organic fuel, and putting it into the oxidiser is the reaction. That combination is why the process is run cold, with active cooling, by those who do it, which the next article takes up.
Common Mistakes
- Treating the acid mixture as a reagent to improvise — strength and water content matter.
- Mixing in a metal or plastic vessel — the vessel material must resist both acids.
- Wiping spills with rags — a fire.
- No water supply to hand — flooding the skin with water is the first response for an acid splash.
Corrosive, oxidising, toxic fume
Mixed acid burns skin and eyes, ignites combustibles, and gives off nitrogen dioxide that can cause delayed lung injury. Wear eye protection and acid-resistant gloves, work outdoors with the wind at your back, keep a large supply of water for flushing, and keep combustibles away. This page is not a procedure and gives no proportions.
What’s Next
- Learn next: Nitration Process — what must be controlled, and how it fails
- Combine with: Celluloid Safety — why a poorly made product is dangerous