When people mention piranha they envision this scary looking fish that rips apart other lifeforms and that’s how chemists view this chemical. The name originated from the fact that this acid can rip apart organic materials just like its fish counterpart.
Piranha acid is not actually one compound, but a mix of two: sulfuric acid and hydrogen peroxide. Due to this, it has two nasty properties: corrosive and oxidizer. It also has some hidden properties which are not easily shown on any SDS. In fact, you shouldn’t be seeing piranha on any SDS as it needs to be made on the spot as transport is strictly forbidden.
There is a way that this solution must be prepared and it is 3 parts of 96% sulfuric acid to 1 part of 30% hydrogen peroxide while slowly heated.
Always slowly add hydrogen peroxide to sulfuric acid. If you’ve ever had to prepare solutions with any mineral acids then you know to always slowly add water to acid and not the other way around. Mixture of water and acid is very exothermic which can result in boiling of the solution. If you add acid to water there will be a very high amount of heat released that can cause instant boiling of the water which can spatter the solution out of your container. This spattering can cause dermal exposures to the worker.
A 30% solution of hydrogen peroxide is 70% water so we want to be mindful when we commingle this mixture.
So what’s happening when piranha is added to organic material? Due to the sulfuric acid, piranha has a very strong dehydrating effect which rips oxygen and hydrogen away forming water. When this happens you’ll see the material turn black because that’s carbon that’s left over. The hydrogen peroxide reaction with sulfuric acid produces oxygen radicals which break apart carbon-carbon and carbon-hydrogen bonds forming alkyl radicals. These alkyl radicals then react with more oxygen radicals that form the gas, carbon dioxide.
So when you’re watching these videos on YouTube, you’ll notice that people put all sorts of fun stuff in piranha. You’ll see that the material and solution start to bubble, turn black, then turn clear again. The reason why is because the carbon (black material) has now been oxidized to carbon dioxide and because carbon dioxide is a gas at room temperature it will off-gas and not be in solution. All that is left is some water with your piranha. This is why it seems like the piranha is “eating” the material.
Woah, now that is a fun dangerous chemical. So why do people even use it? Well the semi-conductor industry uses it to clean their silicon wafers of organic materials, cleaning glassware, and hydroxylation for surfaces.
Since this is a safety blog, lets go over some safety stuff I guess. We’ll go through the hierarchy of controls.
Elimination: Do we actually need to use piranha and take on more risk than we need to?
Substitution: Okay, let’s find some less hazardous options.
- Base bath (potassium hydroxide and ethanol/isopropanol)
- Nano-Strip
- NoChromix
- Ozonation
Engineering Controls:
- Must be used in a fume hood at all times.
- The sash can be used as a primary way to protect against spattering and splashes.
- The ventilation will protect against the hazardous vapors that are produced.
Administrative Controls:
- Work in pairs
- Create an SOP
- Train users on SOP and handling of piranha
PPE: Make sure to always wear the correct PPE
- Chemical Safety Goggles (ensuring full eye protection)
- Face shield (full face coverage to protect against spattering and splashes)
- Lab coat (Aramid material blend)
- Gloves (Viton or Neoprene material)
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