Sodium thiosulfate, na2s2o3, is used as a "fixer" in black and white photography. identify the reducing agent in the reaction of thiosulfate with iodine. 2s2o32 - (aq) + i2 (aq) → s4o62 - (aq) + 2i - (aq)

Answer is: reducin agent in this reaction is thiosulfate (S₂O₃²⁻).

Balanced chemical reaction: 2S₂O₃²⁻ (aq) + I₂ (aq) → S₄O₆²⁻ (aq) + 2I⁻ (aq).

Reducing agent is element or compound who loose electrons in chemical reaction. Sulfur in thiosulfate change oxidation number from + 2 to + 5 tetrathionate anion (two sulfur atoms in the ion have oxidation state 0 and two atoms have oxidation state + 5).