Sulfuric acid and hydrogen peroxide have various connections, including the production of hydrogen peroxide, laboratory experiments, and the creation of peroxymonosulfuric acid. This acid, also known as Caro’s acid, is a powerful oxidizing agent used in cleaning glassware, etching in the semiconductor industry, and separating metals in mining. The reaction of hydrogen peroxide and sulfur dioxide can also contribute to acid rain.
Sulfuric acid and hydrogen peroxide are among the most used chemicals, both industrially and in the laboratory. They are connected in several ways. Two methods of producing hydrogen peroxide involve the use of sulfuric acid, although these have largely been superseded. Several well-known laboratory experiments and demonstrations that are part of many school curricula call for these two compounds. Additionally, mixing sulfuric acid and hydrogen peroxide produces a highly corrosive solution with a variety of uses in the semiconductor, paper and mining industries.
Hydrogen peroxide was originally made by acidifying barium peroxide with hydrochloric acid. The barium chloride, also formed by this reaction, was eliminated by adding sulfuric acid; react to produce an insoluble precipitate of barium sulfate. A later method involved the hydrolysis of peroxydisulfuric acid, produced by the electrolysis of sulfuric acid. Today, however, almost all hydrogen peroxide is produced with the anthraquinone process, a cheaper process that does not involve the use of sulfuric acid.
The reaction of sulfuric acid and hydrogen peroxide produces an aqueous solution of peroxymonosulfuric acid (H2SO5): H2SO4 + H2O2 → H2SO5 + H2O. This is also known as a “piranha solution” due to its corrosiveness: it quickly destroys most organic materials. Another name is Caro’s acid, after the German chemist Heinrich Caro, who first produced the acid. Pure peroxymonosulfuric acid, a crystalline solid at room temperature, is prepared by a different method, but the acid is usually used as an aqueous solution. Piranha solution is usually prepared from concentrated sulfuric acid and 30% hydrogen peroxide; proportions can vary depending on use, but a 3:1 ratio of sulfuric acid to hydrogen peroxide is a common formulation.
This acid has several uses, but must be prepared and handled very carefully. It is a powerful oxidizing agent and is particularly useful for removing organic residues. For this reason it is sometimes used for cleaning glassware and other laboratory equipment. Caro’s acid is also used extensively in the semiconductor industry as an etchant and to ensure that silicon wafers and other delicate electronic components are free from organic contaminants. Other uses are in the mining industry – for separating metals and ores and decomposing toxic cyanide compounds from wastewater – and in the paper industry – for delignification and pulp bleaching.
Sulfuric acid can be produced by the reaction of hydrogen peroxide and sulfur dioxide: H2O2 + SO2 → H2SO4. This method is not used commercially; however, the reaction can occur in the atmosphere – where both sulfuric acid and hydrogen peroxide are found in small quantities – contributing to acid rain. Hydrogen peroxide can form naturally through photochemical reactions. Sulfur dioxide is produced by the combustion of sulfur-containing fossil fuels and naturally by volcanic activity. Although hydrogen peroxide is not required for the formation of acid rain from sulfur dioxide, the peroxide reaction is much faster.
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