Piranha solution is an aqueous mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) that is used to remove organic residue from surfaces and make them more hydrophilic. It is also known as piranha etch because another common use is etching circuit boards. Piranha solution is wicked stuff, so read all the instructions for its preparation, safe usage, and disposal before making it. Even then, consider less dangerous alternatives, such as potassium hydroxide in ethanol or pre-made etch solutions containing peroxymonosulfuric acid. But, if piranha solution is what you need, here’s how to prepare it and use it safely.
How to Prepare Piranha Solution
There is more than one recipe for piranha solution. The basic recipe combines concentrated sulfuric acid and aqueous hydrogen peroxide. Use 30% hydrogen peroxide in water. A higher concentration, such as 50% peroxide, greatly increases the temperature of the reaction and risk of explosion.
The 3:1 and 5:1 ratios of concentrated sulfuric acid to hydrogen peroxide solution are most common, but other ratios may be used:
- 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
- Select a borosilicate glass container (e.g., Pyrex or Kimax). Do not prepare piranha solution in a plastic container, as the mixture will react with the plastic. Be aware, any mixture over 100 ml in volume should be prepared in an ice bath.
- Check to make sure the container is clean. Piranha solution reacts vigorously (possibly explosively) with organic matter!
- Label the container and include appropriate warnings.
- Put on acid-resistant gloves, safety goggles, and a lab apron or acid smock. Tie back long hair and wear long pants and closed-toe shoes.
- Lower the sash of the fume hood as much as possible.
- Slowly add the hydrogen peroxide to the acid. Do not add acid to peroxide. The reaction often reaches 100⁰C or higher, so boiling may occur. For some applications, acid is poured over a surface, followed by peroxide solution, and rinsed away with water.
- Research indicates chilling the peroxide solution before adding it to the sulfuric acid increases the potency of the piranha solution.
- Prepare the minimum volume needed for the application.
- Piranha solution rapidly decomposes, so make the solution fresh before use. Do not seal the container!
- Only use the solution within the fume hood. Do not leave piranha solution unattended.
- Do not attempt to store the solution.
- The activity of piranha solution is increased by heating it. However, it’s best to let fresh piranha solution cool and stabilize before heating it for use. If the solution is heated, it should be heated quickly (total time less than 1 hour), as heat accelerates decomposition of the acid and peroxide.
- Never add any organic chemical to piranha solution (e.g., alcohol, acetone, detergent). Even a small amount could cause an explosion.
- In case of a spill: Do not mop up the spill with paper towels or cloth rags, as they may ignite. Neutralize the area with sodium bicarbonate or calcium carbonate.
- In case of eye or skin contact: Flush with water at least 15 minutes. Seek medical attention for any eye injury or if skin is burned.
- In case of inhalation: Remove the victim to fresh air. Seek medical attention in case of cough or respiratory irritation. Be aware symptoms may be delayed.
- In case of ingestion: Seek medical attention immediately.
How Piranha Solution Works
Piranha solution works by three processes:
- Sulfuric acid and hydrogen peroxide react to form Caro’s acid, which is also known as peroxymonosulfuric acid or persulfuric acid (H2SO5):
H2SO4 + H2O2 → H2SO5 (Caro’s acid) + H2O
The dehydration rapidly carbonizes organics, particularly carbohydrates. Caro’s acid also attacks elemental carbon, including soot. In fact, the solution gets its name because it violently dehydrates organics in a manner resembling a piranha feeding frenzy.
- Piranha solution is highly acidic, so it can remove metal oxides and carbonates.
- Sulfuric acid boosts the oxidizing power of hydrogen peroxide.
H2SO4 + H2O2 → H3O+ + HSO4− + O
The atomic oxygen can dissolve elemental carbon, which normally resists attack because of the hybridized bonds formed between carbon atoms. Oxygen takes an electron bonding pair from carbon, forming a carbonyl group, and disrupting the bond between atoms. The reaction cascades and attacks newly exposed carbon surfaces.
Storage and Disposal
Piranha solution should be prepared fresh and not stored. Sulfuric acid should be stored in an acid cabinet, while hydrogen peroxide solutions decompose more slowly stored in a refrigerator.
Neutralize piranha solution immediately following use. There are two methods:
Neutralization With Ice and Strong Base
The ice method is preferable because it’s less likely to bubble and overflow the container.
- Use a container large enough to accommodate the piranha solution, ice, and base solution.
- Place ice into the container. Use five times more ice than solution (e.g., 500 g of ice for 100 ml piranha solution).
- Pour the piranha solution over the ice.
- Slowly pour 1M sodium hydroxide (NaOH) or potassium hydroxide (KOH) until the pH of the mixture is neutral. The acid-base reaction will generate heat and melt the ice.
Neutralization With Baking Soda and Water
Use the baking soda (sodium bicarbonate) method if ice is unavailable.
- Choose a neutralization container that is at least 10 times the volume of piranha solution. Allow space for foaming and bubbling. Select a container that can withstand the heat generated by the neutralization reaction.
- Pour baking soda into the bottom of the container and spread into an inch-thick layer.
- Add enough water to cover the baking soda.
- Slowly pour piranha solution onto the baking soda. The reaction produces carbon dioxide, which will bubble. If the mixture threatens to overflow the container, allow time for the bubbles to dissipate before adding more piranha solution. Add more sodium bicarbonate if it gets used up (no undissolved material is visible).
- Test the pH of the mixture to verify the acid is neutralized.
Neutralized piranha solution may be poured down the drain if the mixture does not contain regulated metals (arsenic, barium, cadmium, chromium, copper, lead, mercury, nickel, selenium, silver, zinc), radioactive materials, or other regulated waste. Facilities working with regulated materials may have special requirements for piranha waste, which may involve temporary storage without neutralization. In this situation, keep the piranha solution in an uncapped container within a chemical fume hood and follow other disposal protocols.
How to Use Piranha Solution
- To Clean Sintered Glass – Piranha solution is preferred over a strong base to clean sintered glass or fritted glass because it doesn’t damage pores in the glass. Soak the glassware overnight in piranha solution before rinsing it with water.
- To Clean Glassware – Piranha solution can remove contamination on glassware. It’s important there is not excessive organic contamination. Soak the glassware overnight, then thoroughly rinse it.
- To Make Glass Hydrophilic – 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.
- To Remove Surface Residue – Only use piranha solution to remove residue, not a substantial layer.
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- Kemsley, Jyllian (January 16, 2015). Piranha solution explosions. The Safetyzone by C&EN.
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- Urben, P. G. (Ed.) (1999). Bretherick’s Handbook of Reactive Chemical Hazards (6th ed.). Oxford:Butterworth-Heinemann.