Sodium Thiosulfate

Preparation

• Sodium thiosulfate is prepared by reacting sulfur dioxide with a sodium sulfite solution, followed by the addition of elemental sulfur:

SO2 + 2Na2SO3 + S → 2Na2S2O3

Properties

• A colorless, odorless, and crystalline compound.

• Highly soluble in water.

• Also known as "hypo."

Uses

1. Antidote for Cyanide Poisoning: Used in conjunction with sodium nitrite.

2. Medical Treatment: Treats calciphylaxis in patients with end-stage renal disease.

3. Food Industry: Acts as a preservative.

4. Photography: Used as a fixer to remove unexposed silver halide from photographic films.

Storage of Sodium Thiosulfate

• Store in a cool, dry place, away from heat, moisture, and incompatible materials.

• Keep in a tightly closed container.

Assay

• Method: Iodometric titration.

Procedure:

1. Prepare a standard iodine (I₂) solution of known concentration.

2. Weigh and dissolve a sample of sodium thiosulfate in distilled water.

3. Add excess standardized iodine solution to the sample.

4. Sodium thiosulfate reacts with iodine to form sodium iodide (NaI) and sodium tetrathionate (Na₂S₄O₆).

5. Add a starch indicator, which turns blue in the presence of excess iodine.

6. Titrate the remaining unreacted iodine with standardized thiosulfate solution until the blue color disappears.

7. Calculate the concentration of sodium thiosulfate using the titrant volume and iodine solution concentration.