Potassium Metabisulfite

Potassium metabisulfite (K₂S₂O₅; CAS 16731-55-8) is the potassium analog of sodium metabisulfite and serves essentially identical roles in photographic solutions: preservative, acid buffer, and Cr(VI) reducer for dichromate waste disposal.^[1] The choice between potassium and sodium metabisulfite is a practical one — potassium metabisulfite is slightly more soluble and stocked more commonly by wine- and beer-brewing suppliers (where it serves the same SO₂-releasing preservative function).

Photographic uses

Functionally interchangeable with sodium metabisulfite in most darkroom formulas. Key situations:

• Acid stop baths: 2–3% potassium metabisulfite provides odorless stop-bath action.^[2]
• Acid fixer preservation: holds pH around 4.5 and scavenges dissolved oxygen.
• Dichromate disposal: same reduction chemistry as sodium metabisulfite.
• Wine/brewing crossover: Some DIY darkroom formulas call for potassium metabisulfite specifically because the worker has it on hand from home fermentation hobbies — the grade is indistinguishable.

Practical notes

Supplied as a white-to-off-white crystalline powder. Shelf-stable in a sealed container for years. Slightly more soluble than sodium metabisulfite (about 45 g/100 mL vs 40 g/100 mL at 20 °C). Substitution math: 1 g potassium metabisulfite ≈ 0.85 g sodium metabisulfite for equivalent sulfite activity (MW ratio 190/222).

Related compounds

Sodium metabisulfite — the direct sodium analog, the primary photographic-grade metabisulfite. Sodium sulfite is the neutral-pH sulfite without the bisulfite-strength acidity.