Medium-strength acid (international name dihydrogensulphite) formed when sulphur dioxide is introduced into water. Especially in warm environments, it tends to change into the much more toxic and stronger sulphuric acid by oxidation. The salts and esters of sulphurous acid are called Sulphites or neutral or secondary sulphites, as well as hydrogen sulphites or formerly bisulphites or also primary or acid sulphites. When sulphur dioxide is added to mash, must or wine, most of it is converted into sulphurous acid, only a small part remains as sulphur dioxide. The sulphurous acid immediately begins to split into sulphites and hydrogen sulphites. Both enter into compounds with various substances in the wine. Only when these processes are complete is the so-called free sul phurous acid present. The amount depends on the temperature and the pH value, but is usually about 20%. This means that the major part of the sulphurous acid, about 80%, is present in the wine in bound form.