Piranha solution

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). 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). 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 ammonia water 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 the 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 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 (often bubbling) solution cleans organic compounds off substrates and oxidizes or hydroxylates most metal surfaces. 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. The solution should not be stored, as it generates gas and therefore cannot be kept in a closed container. As the 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. Piranha solution is used frequently in the microelectronics industry, e.g. to clean photoresist residue from silicon wafers. Hobbyist electronics enthusiasts use the solution to etch home made circuit boards. A mask is applied to a blank copper board, and the piranha solution rapidly removes the exposed copper not covered by the mask. 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 (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 the solution to percolate backward through the sintered glass. Although cleaning sintered glassware will leave it as clean as possible without damaging the glassware it is not recommend due to the risk of explosion.