Na2S2O3

Na2S2O3, commonly known as sodium thiosulfate, is an inorganic salt containing the thiosulfate (S2O3^2−) anion. It is the sodium salt of thiosulfuric acid and is also called sodium hyposulfite. In solution, the thiosulfate ion acts as a mild reducing agent and participates in several redox reactions.

Production and occurrence: Sodium thiosulfate is typically prepared by the direct reaction of sodium sulfite (Na2SO3) with elemental sulfur: Na2SO3 + S → Na2S2O3. It can also be formed by dissolving sulfur dioxide in aqueous sodium sulfite solution. The solid is highly soluble in water and is stable under ordinary storage conditions when kept dry.

Properties: Sodium thiosulfate is a white crystalline solid that is highly soluble in water. It decomposes on heating to form sodium sulfite and elemental sulfur. In aqueous solutions it can act as a dechlorinating agent, reducing chlorine to chloride.

Reactions: As a reducing agent, thiosulfate reduces oxidizing agents such as iodine to iodide, a property exploited in iodometric titrations. In strongly acidic solutions, thiosulfate is oxidized and fragments to sulfur and sulfur dioxide, for example Na2S2O3 + 2 HCl → 2 NaCl + S + SO2 + H2O.

Applications: The most well-known use is as a fixer in photography, where it dissolves unexposed silver halide from film and paper. Medical use includes the treatment of cyanide poisoning when administered with nitrite; thiosulfate provides a sulfur donor that enables conversion of cyanide to the less toxic thiocyanate. In water treatment and aquariums, it is used to dechlorinate residual chlorine. It also serves as a reducing agent in various iodometric determinations in analytical chemistry.

Safety: Sodium thiosulfate is relatively low in toxicity but can irritate skin and eyes. It should be handled with appropriate precautions, and solutions should be kept away from strong oxidizers and concentrated acids. Keep in a dry, well-ventilated area.