Sodium Thiosulphate

The inorganic compound sodium thiosulfate (sodium thiosulphate) has a formula Na₂S₂O₃.5H₂O. In its solid-state, it is indeed a crystalline solid that readily loses water. Sodium thiosulfate, as well known as sodium hyposulfite, is really a water-soluble salt.

The thiosulfate ion does have a tetrahedral shape. Here, the thiosulfate ion’s distance between two sulphur atoms is comparable to the distance between two sigma bonded sulphur atoms. It thus means that the sulphur that isn’t bonded to any oxygens has a negative charge.

Physical Properties of Sodium Thiosulfate

• Sodium thiosulfate does have a molar mass of 158.11 grammes per mole in its anhydrous form. The molar mass of the more commonly available pentahydrate from, Na₂S₂O₃.5H₂O, is 248.18 g/mol.
• As a solid, it does have a white, crystalline appearance and is odourless.
• Sodium thiosulfate really does have a density of 1.667 grammes per cubic centimetre.
• The pentahydrate of such a salt has a melting point of 321.4 K and a boiling point of 373 K.
• At 20^oC, sodium thiosulfate dissolves in water at a rate of 70.1g/100 mL and at 100^oC, it dissolves at a rate of 231g/100 mL.
• Na₂S₂O₃ crystals have a monoclinic crystal structure.

Chemical Properties of Sodium Thiosulfate

• The sodium thiosulfate salt seems to have no charge. Even so, in water and some other polar solvents, it dissociates to give Na⁺and S₂O₃^2-.
• But besides being stable at room temperature, the sodium thiosulfate salt decomposes at high temperatures to produce sodium sulphate and sodium polysulfide.

4Na₂S₂O₃ ⟶ 3Na₂SO₄ + Na₂S₅

• Once exposed to dilute acids such as dilute hydrochloric acid, the sodium thiosulfate salt decomposes to produce sulphur and sulphur dioxide.

Na₂S₂O₃ + 2HCl ⟶ 2NaCl + SO₂ + H₂O + S

• S-alkyl thiosulfates are produced by the alkylation of Na₂S₂O₃. Bunte salts are the common name for these compounds.

Production of Sodium Thiosulfate

The sodium thiosulfate salt has been created in the laboratory by heating aqueous sodium sulfite solutions with sulphur.

The boiling of aqueous NaOH (sodium hydroxide) with sulphur can also result in the production of Na₂S₂O₃.

6NaOH + 4S ⟶ Na₂S₂O₃ + 2Na₂S + H₂O

Sodium thiosulfate has been manufactured in the industrial sector from the liquid waste generated during the production of sulphur dye.

Uses of Sodium Thiosulfate

• In the medical treatment of cyanide poisoning, Na₂S₂O₃ is indeed a critical chemical compound.
• Medically, sodium thiosulfate is used to treat dermatophytosis (ringworm) and tinea versicolor.
• Na₂S₂O₃ is being used to treat the side effects of chemotherapy and hemodialysis (blood purification).
• It is indeed a crucial compound in analytical chemistry because it stoichiometrically reacts with iodine to reduce it to the iodide ion while oxidising it to the S₄O₆^2- (tetrathionate) ion.
• Because of the ability of the thiosulfate ions to react with the silver halides that comprise photographic emulsions, sodium thiosulfate salts are used as photographic fixers.