Sulfuric acid (H2SO4) is hands-down the most important chemical in the chemical industry. Over 170 million tons of sulfuric acid are produced annually, and about 30% of this production comes from the U.S., ranking sulfuric acid number one in terms of national production of one chemical. Sulfuric acid is the least voltatile of the common strong acids and therefore it can be used to derive almost any mineral acid from its salt. Concentrated sulfuric has a strong affinity for water and hence is an important drying agent. It is also used as an acid in its own right to produce sulfate salts. It is these three properties of sulfuric acid that make it such a valuable important chemical both in industry and in the home lab.
All methods of producing sulfuric acid involve first manufacturing its acid anhydride, sulfur trioxide. In industry, sufluric acid is produced via the contact process. Sulfur dioxide is first produced by burning sulfur or roasting sulfide ores. The sulfur dioxide then reacts with oxygen over a vanadium pentoxide catalyst at temperatures around 400-500C. The trioxide thus formed is dissolved into concentrated sulfuric acid to form oleum, which can be represented by the formula H2S2O7. Oleum is then reacted with water to form twice the amount of original sulfuric acid. Sulfur trioxide is not dissolved directly into water because this is an highly exothermic reaction which causes excessive splattering.
Vanadium pentoxide is difficult for the amateur experiment to procur, but luckily several other routes to obtaining sulfuric acid exist. Sulfur, which can be obtained in a relatively pure form as a dust to prevent mold from growing on plants, can be burned to produce sulfur dioxide which can then be bubbled in hydrogen peroxide.
SO2 + H2O2 --> H2SO4
I have yet to try to this reaction because it is highly uneconomical. The highest concentration of hydrogen peroxide I can obtain is 3%, which would produce a 3% sulfuric acid solution. This solution would then have to be boiled down to achieve the 98.3% azeotropical sulfuric acid. Hydrogen peroxide is quite expensive when you consider that you are really buying 97% water.
The alchemists prepared sulfuric acid by thermally decomposing metal sulfates such as ferrous and cupric sulfate into sulfur trioxide and their respective oxides. This method works nicely, but high temperatures are required.
Before the contact process for manufacturing sulfuric acid was invented, the lead chamber process proliferated. This involved burning a mixture of sulfur and potassium nitrate in a closed lead chamber and allowing the gasses formed to dissolve in water. In this process, nitrogen monoxide acts as a catalyst to convert sulfur dioxide into sulfur trioxide. The process can be represented by the serious of chemical equations below.
3S + 2KNO3 --> K2S + 2SO2 + 2NO
S + O2 --> SO2
NO + O2 --> NO2
NO2 + SO2 --> SO3 + NO
SO3 + H2O --> H2SO4
I have experimented with this method before to produce small amounts of sulfuric acid. I did this purely out of curiousity, however, because I now get concentrated sulfuric acid from drain cleaner. Although it is colored black, it has worked for all my intents and purposes so far.
I used an emptied 2-liter grape juice bottle and fastened a piece of copper wire on its lid. To the other end of the copper I attached a brass plumbing end-fitting. I mixed 0.5g of 92% sulfur and 0.5g of potassium nitrate stump remover and added it inside the fitting. Inside the reaction chamber I added 4mL of distilled water. I then lit the mixture which burned vigorously and smelt strongly (almost chokingly) of sulfur. I put this in the chamber and screwed on the lid. The flame went out rather quickly and then I unscrewed the lid again and tried to light the mixture once more, but I failed to do so. I rescrewed the lid and let the chamber set. The vapors became less cloudy after a few hours. Hopefully, this was due to the formation of sulfur trioxide and not due to gas escaping the reaction vessel. I ended up with an orange liquid that had a pH of 0.45. I boiled this down to dryness and near the end of the boiling I got the characteristic thick white smoke of SO3 so I guess I was successful. I am curious as to how much nitric acid was in the mixture because I would think that some nitrogen dioxides from the reactions would dissolve to form nitric acid. If, however, you let the chamber sit for a very long time, the mixture would achieve equilibrium and at this point all nitrogen oxides would be converted to sulfur trioxide.