The gas behind the smell of a burnt match also guards wine
Strike a match and the sharp whiff that follows is sulfur dioxide. Volcanoes pour it out by the million tons, it fills the thin air of Jupiter's moon Io, and Roman winemakers burned sulfur candles in empty vessels to keep the vinegar smell away. Winemakers still depend on it today.
Earth's air holds only about 15 parts per billion of SO2, but other worlds carry far more. On Venus it ranks third among atmospheric gases at 150 ppm and combines with water to make clouds of sulfurous acid. Io's thin atmosphere is 90% sulfur dioxide, and frozen SO2 is thought to lie in the crusts of Europa, Ganymede and Callisto. The James Webb Space Telescope has detected it on the exoplanet WASP-39b, where starlight drives its formation.
Most of what industry makes goes into sulfuric acid. Molten sulfur, held at 140 to 150 °C, is sprayed as a fine mist and burned at up to 1,600 °C, and the heat is captured as steam for electricity. In 1979 the United States turned 23.6 million metric tons of SO2 into acid while using just 150,000 tons for everything else. Whitehaven in England made acid and cement together until the 1970s by roasting calcium sulfate with shale or marl.
Chemists recognise it as an unusual gas that is both acidic and reducing. Moist litmus paper turns pink in it and then bleaches white, and it changes an orange dichromate solution to green. Medieval alchemists knew it as the volatile spirit of sulfur.
Food makers label it E220 in Europe and use it to keep dried apricots and figs bright and free of rot. In wine it fights bacteria and oxidation, which would otherwise brown the wine and flatten its varietal character. Only the molecular form, favoured when pH is low, does that work, and above about 50 ppm of free SO2 drinkers can smell and taste it. Even wines made without added sulfites can contain up to 10 mg/L, and bottles above 10 ppm total must carry a sulfite warning in the US and EU.
Source: Sulfur dioxide
