Piranha solution, also known as piranha etch, also known as Jesse's solution, is
a mixture of sulfuric acid and hydrogen peroxide, 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, making them
highly hydrophilic. Preparation and use
Many different mixture ratios are commonly used, and all are called
piranha. A typical mixture is 3 parts of concentrated sulfuric acid and 1 part of
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 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 in reverse. Mixing the solution is
extremely exothermic. If the solution is made rapidly, it will instantly boil,
releasing large amounts of corrosive fumes. Even when made with care, the
resultant heat can bring solution temperatures above 100 °C. It must be
allowed to cool reasonably before it is used. The sudden increase in temperature
can also lead to violent boiling of the extremely acidic solution. Solutions
made using hydrogen peroxide at concentrations greater than 50% may
cause an explosion. Once the mixture has stabilized, it can be further heated to
sustain its reactivity. The hot solution cleans organic compounds off substrates
and oxidize or hydroxylate most metal surfaces, it is strong enough to
dissolve elemental carbon. Cleaning usually requires about 10 to 40 minutes,
after which 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, as it generates
gas and therefore cannot be kept in a closed container. As piranha solution
reacts violently with many items commonly disposed of as chemical waste
if the solution has not neutralised, it must be left in clearly marked
containers. Applications
Piranha solution is used frequently in the microelectronics industry, e.g. to
clean photoresist residue from silicon wafers.
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 does not
contaminate glassware with heavy-metal ions.
Piranha solution is particularly useful when cleaning sintered 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. Piranha solution is used to make glass
hydrophilic by hydroxylating the surface, thus increasing the number of
silanol groups on the surface. 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 itself,
that makes concentrated sulfuric acid, and so piranha solution, very dangerous
to handle. H2SO4 + H2O2 → H2SO5 + 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", 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–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, 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. See also
Peroxymonosulfuric acid or Caro's acid Peroxydisulfuric acid
RCA clean References
External links How to make Piranha Solution on YouTube
Piranha solution dissolves chicken heart on YouTube