Sulfites contain the SO₃²⁻ anion, while thiosulfates contain S₂O₃²⁻ — essentially a sulfite with one oxygen replaced by sulfur. This chemical distinction places them in separate subheadings (283210/283220 for sulfites, 283230 for thiosulfates), even though both are reducing agents with overlapping industrial uses.

Sodium sulfite used as a food additive or preservative falls under FDA jurisdiction. Importers should confirm that the product meets applicable food-grade purity specifications and that the supplier is registered with FDA. Sulfite allergen labeling requirements may also apply to downstream food manufacturers.