most metal surfaces. Cleaning usually requires about 10 to 40 minutes, after which the substrates can be removed from the solution and rinsed with deionized water. 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 always be used freshly prepared (extemporaneous preparation). The solution should not be stored, as it generates gas and therefore cannot be kept in a closed container because of the risk of overpressure and explosion. As the solution violently reacts with many oxidizable substances commonly disposed of as chemical waste, if the solution has not yet been completely self-decomposed, or safely neutralized, it must be left in an open container under a fume hood, and clearly marked. Applications Piranha solution is used frequently in the microelectronics industry, e.g. to clean photoresist or organic material residue from silicon wafers. It's also widely employed in wet etching of wafers in the semiconductor fabrication process. 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 ions. Piranha solution is particularly useful when cleaning sintered (or "fritted") glass filters. A good porosity and sufficient permeability of the sintered glass filter is critical for its proper function, so it should never be cleaned with strong bases (NaOH, , , ...) which dissolve the silica of the glass sinter and clog the filter. Sintered glass also tends to trap small solid particles deep inside its porous structure, making it difficult to remove them. 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 achieved by allowing the solution to percolate backward through the sintered glass. Although cleaning sintered glass with piranha solution will leave it as clean as possible without damaging the glass it is not recommend due to the risk of explosion. Piranha solution is also used to make glass more hydrophilic by hydroxylating its surface, thus increasing the number of silanol groups present on its surface. Mechanism The effectiveness of piranha solution in decomposing organic residues is due to two distinct processes operating at noticeably different rates.