new carbonyl group on the neighbouring carbon whose bonds were disrupted: The carbon removed by the 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. With time, piranha solutions in which organic materials have been immersed typically return to complete clarity, with no visible traces of the original organic materials remaining. A last secondary contribution to the piranha solution cleaning is its high acidity, which dissolves deposits such as metal oxides, hydroxides, and carbonates. However, since it is safer and easier to remove such deposits using milder acids, the solution is more typically used in situations where high acidity facilitates cleaning instead of complicating it. For substrates with low tolerance for acidity, an alkaline solution consisting of ammonium hydroxide and hydrogen peroxide, known as base piranha, is preferred. Safety and disposal Piranha solution is dangerous to handle, being both strongly acidic and a strong oxidizer. Solution that is no longer being used should never be left unattended if hot. It should never be stored in a closed receptacle because of the risk of gas overpressure and explosive burst with spills (especially with fragile thin wall volumetric flask). Piranha solution should never be disposed of with organic solvents (e.g. in waste solvent carboys), as this will cause a violent reaction and a substantial explosion, and any aqueous waste container containing even a weak or depleted piranha solution should be labelled appropriately to prevent this. The 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 taking care to leave the receptacles open under a ventilated fume cupboard. This allows the spent solution to degrade prior to disposal and is especially important if a large portion of peroxide was used in the preparation. While some institutions believe that used piranha solution should be collected as hazardous waste, others consider that it can be neutralized and poured down the drain with copious amounts of water. Improper neutralization can cause a fast decomposition, which releases pure oxygen (increased risk of fire of flammable substances in a close space).