sodium thiosulfate

chemical compound

Also known as: hypo, sodium hyposulfite

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Last updated

July 13, 2026

Quick Summary

Sodium thiosulfate, also known as hypo or fixer, is a crystalline salt with the chemical formula Na₂S₂O₃. It is the most important salt of thiosulfuric acid. This compound is hygroscopic, meaning it absorbs moisture from the air.12

Sodium thiosulfate is primarily used as a photographic fixing agent. It also functions as a reducing or bleaching agent. The term "thiosulfato-" refers to the presence of the S₂O₃ radical, which is characteristic of thiosulfates.13

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4 Britannica Sources

sodium thiosulfate
hyposulfite of soda
thiosulfato-
thiosulfonic acid

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sodium
Principal compounds
Sodium thiosulfate (sodium hyposulfite), Na₂S₂O₃, is used by photographers to fix developed negatives and prints; it acts by dissolving the part of the silver salts coated onto film which remain unchanged by exposure to light.
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Sodium thiosulfate (sodium hyposulfite), Na₂S₂O₃, is used by photographers to fix developed negatives and prints; it acts by dissolving the part of the silver salts coated onto film which remain unchanged by exposure to light.
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history of photography
Photogenic drawing
…with sodium hyposulfite (now called sodium thiosulfate) and waxing them before printing, which reduced the grain of the paper.
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…with sodium hyposulfite (now called sodium thiosulfate) and waxing them before printing, which reduced the grain of the paper.
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