Sodium Metabisulfite: The Gold Precipitant and Food Preservative

Sodium metabisulfite (Na₂S₂O₅, also called SMBS) is an inorganic salt that dissolves in water to release sulfite and bisulfite ions. It is a moderate reducing agent with applications spanning gold refining, food preservation, water treatment, and winemaking. ## Gold Recovery In gold refining from Aqua Regia: The Only Acid That Dissolves Gold solutions, SMBS acts as a selective reducing precipitant. Adding it to a filtered chloroauric acid (HAuCl₄) solution causes gold to precipitate as a fine metallic powder: the Au³⁺ is reduced to Au⁰ while sulfite is oxidized to sulfate. The reaction is selective enough to deposit gold while leaving base metal chlorides in solution, making it the preferred finishing step in small-scale chlorine-based gold refining and Gold Recovery from Laptop Hard Drives via Aqua Regia: Process and Economics. ## Food Preservation As food additive E223, SMBS serves as an antioxidant and antimicrobial agent, preventing enzymatic browning in dried fruits, preserving seafood freshness, and inhibiting wild yeast and bacteria. It can cause asthmatic reactions in sulfite-sensitive individuals (~1% of the general population, higher among asthmatics). ## Winemaking In winemaking, SMBS releases sulfur dioxide (SO₂) when dissolved, inhibiting wild yeast and oxidative spoilage. It's added at multiple stages — crushing, fermentation, and bottling — and is the most common source of the "contains sulfites" label on wine bottles. ## Water Treatment Municipal water treatment plants use SMBS to dechlorinate treated water before discharge into waterways, reducing residual chlorine that would harm aquatic life.