Which chemical may be used in the reduction of hexavalent chromium to trivalent chromium?

• A. Sodium Thiosulfate
• B. Sodium Bisulfite
• C. Ferrous sulfate
• D. Lime

Answer: (B)

Reducing Cr(VI) to Cr(III) requires a suitable reducing agent that donates electrons to the chromium species. Sodium bisulfite is a common, effective reducer in aqueous systems: it donates electrons and becomes sulfate, turning chromium into Cr(III), which then readily forms insoluble Cr(III) hydroxide for removal. Lime can raise the pH and help precipitate Cr(III), but it does not reduce Cr(VI) itself. Ferrous sulfate can also reduce Cr(VI), but it introduces iron and can complicate the treatment process or lead to reoxidation. Sodium thiosulfate has reducing power as well, but sodium bisulfite is the preferred, reliable choice for this reduction step.