Chemical piranha solution or piranha etch is a mixture of a strong acid or base with peroxide, mainly used to remove organic residue from glass and other surfaces. It’s a useful solution, but hazardous to make, use, and dispose of, so if you need to prepare this chemical, read over the precautions and disposal advice before you get started. Here’s what you need to know:
How to Make Piranha Solution
There are multiple recipes for piranha solution. The 3:1 and 5:1 ratios are probably the most common:
- 3:1 concentrated sulfuric acid (H2SO4) to 30% hydrogen peroxide (aqueous H2O2) solution
- 4:1 concentrated sulfuric acid to 30% hydrogen peroxide solution
- 5:1 concentrated sulfuric acid to 30% hydrogen peroxide solution
- 7:1 concentrated sulfuric acid to 30% hydrogen peroxide solution (less common)
- base piranha: 3:1 ammonium hydroxide (NH4OH) to hydrogen peroxide
- Prepare the solution in a fume hood and be certain you are wearing gloves, a lab coat, and safety goggles. Put the visor down on the hood to minimize the risk of damage or harm.
- Use a Pyrex or equivalent borosilicate glass container. Do not use a plastic container, as it will react with the solution and eventually fail. Label the container before preparing the solution.
- Make certain the container used for mixing is clean. If there is excessive organic matter, it may cause a vigorous reaction, possibly leading to a spill, breakage, or an explosion.
- Slowly add the peroxide to the acid. Do not add acid to peroxide! The reaction will be exothermic, may boil, and may splash out of the container. The risk of boiling or of sufficient flammable gas being released that could lead to an explosion increases as the amount of peroxide increases.
Another method used to prepare piranha solution is to pour sulfuric acid over a surface, followed by peroxide solution. After time is allowed for the reaction, the solution is rinsed away with water.
- Make piranha solution fresh before each use because the solution decomposes.
- Activity of the solution is increased by heating it, but don’t apply heat until after the reaction to make the solution has completed. It’s advisable to let the solution cool slightly after reaction before heating it.
- Do not leave hot piranha solution unattended on the lab bench.
- Do not store piranha solution in a sealed container. For that matter, do not store chemical piranha for later use, period.
- In case of skin or surface contact, rinse affected area immediately with large volumes of water. Continue rinsing at least 15 minutes. Seek appropriate emergency aid.
- In case of inhaling, remove affected person to fresh air and seek emergency medical aid. Be aware symptoms of exposure may be delayed.
- In case of suspected ingestion, seek immediate medical attention.
How to Use Piranha Solution
- To Clean Sintered Glass – Piranha solution is used to clean sintered glass or fritted glass because it doesn’t damage pores in the glass (which is why you don’t use a strong base instead). Soak the glassware overnight in piranha solution before rinsing it with water.
- To Clean Glassware – Piranha solution can remove contamination on glassware that is untouched by other chemicals. It’s important there is not excessive organic contamination. Soak the glassware overnight, then thoroughly rinse it.
- Apply as a surface treatment to glass to make it hydrophilic. Piranha solution increases the number of silanol groups on the glass surface by hydroxylating the silicon dioxide.
- Apply to remove residue from surfaces. Make sure you are removing residue and not a significant layer of material!
Disposal of Piranha Solution
- To dispose of piranha solution, allow the solution to cool completely, to allow for it to release oxygen gas. Make sure the gas has dissipated before proceeding.
- Neutralize piranha solution by diluting it with large amounts of water. Do not neutralize it by adding a base, as the rapid decomposition releases heat and pure oxygen gas. The exception is when the volume of piranha solution is small (~ 100 ml). Then, dilute the piranha by adding it to water until it is less than 10% of the volume. Add sodium hydroxide or sodium carbonate solution until the pH is 4 or higher. Expect heat, bubbling, and possibly foaming when the base is added to the acid solution.
- Usually, it’s okay to wash diluted piranha solution down the drain. However, some places prefer it be treated as toxic waste. Disposal also depends on the purpose of the solution, since some reactions may leave toxic residue in the container. Do not dispose of piranha solution with organic solvents, as a violent reaction and explosion will occur.