Sometimes knowing your ingredients means knowing the many names under which they masquerade. Potassium metabisulfite and sodium metabisulfite are often lumped together under the ambiguous name "sulfites." Potassium metabisulfite is also known as known as potassium pyrosulfite, pyrosulfurous acid, dipotassium salt and disulfurous acid. Sodium metabisulfite is referred to as sodium pyrosulfite and disodium salt. These names don't begin to encompass the other numerical identifiers used for them. Fortunately, this confusion can be quickly sorted through by looking at the chemistry and uses of these ingredients.
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Sodium metabisulfite and potassium metabisulfite are chemically very similar. A molecule of sodium metabisulfite is made from two sodium, two sulfur and five oxygen atoms. If you swap out the two sodium atoms for potassium atoms, you have potassium metabisulfite. The properties of the chemicals are so alike because both potassium and sodium are Group I alkali metals on the periodic chart.
Sodium metabisulfate has a molecular weight of 190.1 g/mole. A maximum of 650 g of this chemical can be dissolved in 1 liter of water at 20 degrees Celsius. Potassium metabisulfate has a molecular weight of 222.32 g/mole and is less soluble in water. Only 450 g can be dissolved in 1 liter of water.
Both potassium metabisulfite and sodium metabisulfite are used in disinfectants, antioxidants, preservatives and photography. They are the principal ingredients in Campden tablets, which are used by home brewers and winemakers to stop the growth of yeasts.
Sodium metabisulfite is used in stump removal and as an inactive ingredient in drug preparations. Potassium metabisulfite is used in dying and printing fabrics.
The FDA has granted GRAS status to both potassium metabisulfite and sodium metabisulfite. GRAS status means that the ingredient "is not subject to premarket review and approval by FDA because it is generally recognized, by qualified experts, to be safe under the intended conditions of use." The Cosmetic Ingredient Review Expert Panel recognizes that sodium metabisulfite and potassium metabisulfite are safe as used in cosmetic formulations. Approval by these organizations does not preclude the possibility that either chemical can cause allergic reactions.