Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an inorganic compound of chemical formula Na2S2O5. The substance is sometimes referred to as disodium metabisulfite. It is used as a disinfectant, antioxidant, and preservative agent.[2] When dissolved in water it forms sodium bisulfite. Preparation See also: Wellman–Lord process Sodium metabisulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide.[3] When conducted in warm water, Na2SO3 initially precipitates as a yellow solid. With more SO2, the solid dissolves to give the disulfite, which crystallises upon cooling.[4] SO2 + 2 NaOH → Na2SO3 + H2O SO2 + Na2SO3 → Na2S2O5 which yields a residue of colourless solid Na2S2O5. Chemical structure The anion metabisulfite consists of an SO2 group linked to an SO3 group, with the negative charge more localised on the SO3 end. The S–S bond length is 2.22 Å, and the "thionate" and "thionite" S–O distances are 1.46 and 1.50 Å, respectively.[5] Reactivity Main article: Bisulfite § Reactions Upon dissolution in water, bisulfite is generated: Na2S2O5 + H2O → 2 Na+ + 2 HSO3− Uses Sodium and potassium metabisulfite have many major and niche uses. It is widely used for preserving food and beverages. Sodium metabisulfite is added as an excipient to medications which contain adrenaline (epinephrine), in order to prevent the oxidation of adrenaline.[6] For example, it is added to combination drug formulations which contain a local anaesthetic and adrenaline, [6] and to the formulation in epinephrine autoinjectors, such as the EpiPen.[7] This lengthens the shelf life of the formulation, [6] although the sodium metabisulfite reacts with adrenaline, causing it to degrade and form epinephrine sulfonate.[8] In combination with sodium hydrosulfite it is used as a rust-stain remover[9] It is used in photography.[10] Concentrated sodium metabisulfite can be used to remove tree stumps. Some brands contain 98% sodium metabisulfite, and cause degradation of lignin in the stumps, facilitating removal.[11] It is also used as an excipient in some tablets, such as paracetamol. A very important health related aspect of this substance is that it can be added to a blood smear in a test for sickle cell anaemia (and other similar forms of haemoglobin mutation). The substance causes defunct cells to sickle (through a complex polymerisation) hence confirming disease. It is used as a bleaching agent in the production of coconut cream. It (or liquid SO2) is commonly used as an antimicrobial and antioxidant in winemaking; bottled wine indicates its use with the label "Contains Sulfites" in the US.