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Piranha solution, also known as piranha etch, is a mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2), used to clean organic residues off substrates. Because the mixture is a strong oxidizing agent, it will remove most organic matter, and it will also hydroxylate most surfaces (add OH groups), making them highly hydrophilic (water compatible).
Preparation and use
Many different mixture ratios are commonly used, and all are called piranha. A typical mixture is 3:1 concentrated sulfuric acid to 30% hydrogen peroxide solution; other protocols may use a 4:1 or even 7:1 mixture. A closely related mixture, sometimes called "base piranha", is a 3:1 mixture of ammonium hydroxide (NH4OH) with hydrogen peroxide.
Piranha solution must be prepared with great care. It is highly corrosive and an extremely powerful oxidizer. Surfaces must be reasonably clean, and completely free of organic solvents from previous wash steps, before coming into contact with piranha solution. Piranha solution cleans by dissolving organic contaminants, and a large amount of contaminant will cause violent bubbling and a release of gas that can cause an explosion.
Piranha solution should always be prepared by adding hydrogen peroxide to sulfuric acid very slowly, never vice versa. Mixing the solution is exothermic. The resultant heat can bring solution temperatures exceeding 100 °C. It must be allowed to cool reasonably before applying any heat. The sudden increase in temperature can also lead to violent boiling, or even splashing of the extremely acidic solution. Also, explosions may occur if the peroxide solution concentration is more than 50%.  The hot (often bubbling) solution will clean organic compounds off substrates, and oxidize or hydroxylate most metal surfaces. Cleaning usually requires about 10 to 40 minutes, after which time the substrates can be removed from the solution.
The solution may be mixed before application or directly applied to the material, applying the sulfuric acid first, followed by the peroxide. Due to the self-decomposition of hydrogen peroxide, piranha solution should be used freshly prepared. Piranha solution should not be stored.
Immersing the substrate (such as a wafer) into the solution should be done slowly to prevent thermal shock that may crack the substrate material.
In the laboratory, this solution is sometimes used to clean glassware, though it is discouraged in many institutions and it should not be used routinely due to its dangers. Unlike chromic acid solutions, piranha will not contaminate glassware with heavy metal ions.
Piranha solution is particularly useful when cleaning sintered (or 'fritted') glassware. The size of the pores of sintered glassware is critical for its function, so it should not be cleaned with strong bases, which gradually dissolve the sinter. Sintered glass also tends to capture material deep within the structure, making it difficult to remove. Where less aggressive cleaning methods fail, piranha solution can be used to return the sinter to a pristine white, free flowing form without excessive damage to the pore dimensions. This is usually accomplished by allowing piranha solution to percolate backward through the sintered glass.
Mechanism of action
The effectiveness of piranha solution in removing organic residues is due to two distinct processes that operate at noticeably different rates. The first and faster process is removal of hydrogen and oxygen as units of water by the concentrated sulfuric acid. This occurs because hydration of concentrated sulfuric acid is thermodynamically strongly favorable, with a ΔH of -880 kJ/mol. It is this rapid dehydrating property, rather than acidity per se, that makes concentrated sulfuric acid, and so piranha solution, very dangerous to handle.
- H2SO4 + H2O2 → H2SO5 (Caro's acid) + H2O
The dehydration process exhibits itself as the rapid carbonisation of common organic materials, especially carbohydrates, when immersed in piranha solution. Piranha solution was named in part for the vigour of this first process, since large quantities of organic residues immersed in piranha solution are dehydrated so violently that the process resembles a piranha feeding frenzy. The second and more definitive rationale for the name, however, is the ability of piranha solution to "eat anything," including in particular elemental carbon in the form of soot or char.
This second and far more interesting process can be understood as the sulfuric-acid boosted conversion of hydrogen peroxide from a relatively mild oxidizing agent into one sufficiently aggressive to dissolve elemental carbon, a material that is notoriously resistant to room temperature aqueous reactions. This transformation can be viewed as the energetically favourable dehydration of hydrogen peroxide to form hydronium ions, bisulfate ions, and, transiently, atomic oxygen:
- H2SO4 + H2O2 → H3O+ + HSO4− + O
It is this extremely reactive atomic oxygen species that allows piranha solution to dissolve elemental carbon. Carbon allotropes are difficult to attack chemically because of the highly stable and typically graphite-like hybridized bonds that surface carbon atoms tend to form with each other. The most likely route by which piranha solution disrupts these stable carbon-to-carbon surface bonds is for an atomic oxygen first to attach directly to a surface carbon to form a carbonyl group:
In the above process, the oxygen atom in effect "steals" an electron bonding pair from the central carbon, forming the carbonyl group and simultaneously disrupting the bonds of the target carbon atom with one or more of its neighbours. The result is a cascading effect in which a single atomic oxygen reaction initiates significant "unraveling" of the local bonding structure, which in turn allows a wide range of aqueous reactions to affect previously impervious carbon atoms. Further oxidation, for example, can convert the initial carbonyl group into carbon dioxide and create a new carbonyl group on the neighbouring carbon whose bonds were disrupted:
The carbon removed by piranha solution may be either original residues or char from the dehydration step. The oxidation process is slower than the dehydration process, taking place over a period of minutes. The oxidation of carbon exhibits itself as a gradual clearing of suspended soot and carbon char left by the initial dehydration process. In time, piranha solutions in which organic materials have been immersed typically will return to complete clarity, with no visible traces of the original organic materials remaining.
A final minor contribution to the piranha solution cleaning is its high acidity, which dissolves deposits such as metal oxides and carbonates. However, since it is safer and easier to remove such deposits using milder acids, piranha solution is more typically used in situations where high acidity complicates cleaning instead of assisting it. For substrates with low tolerance for acidity, the alkaline oxidising solution known as base piranha is preferred.
Safety and disposal
Piranha solution is very dangerous, being both strongly acidic and a strong oxidizer. Solution that is no longer being used should never be left unattended if hot. It should not be stored in a closed container. Piranha solution should not be disposed with organic solvents (e.g. in waste solvent carboys), as this will cause a violent reaction and a substantial explosion.
Piranha solution should be allowed to cool, and oxygen gas should be allowed to dissipate prior to disposal. When cleaning glassware, it is both prudent and practical to allow the piranha solution to react overnight. This allows the spent solution to degrade prior to disposal. While some institutions believe that used Piranha solution should be collected as hazardous waste, others believe that it can be neutralized and poured down the drain with copious amounts of water. Attempting neutralization by adding bases can instead cause fast decomposition, which releases pure oxygen.
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- Piranha Waste Fact Sheet, University of Illinois at Urbana-Champaign