Sodium Thiosulfate - Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>

What is Sodium Thiosulfate?

Sodium thiosulfate, which is also referred to as sodium sulphate, is a chemical compound which has the formula Na2S2O3. It is typically found in its pentahydrate form which is either white in colour, or colourless altogether. This pentahydrate of sodium thiosulfate is described by the following chemical formula: Na2S2O3.5H2O.

In its solid form, it is a crystalline solid which has a tendency to readily lose water. Sodium thiosulfate is readily soluble in water and is also referred to as sodium hyposulfite. The structure of the Na2S2O3 molecule is illustrated below.

Sodium Thiosulfate

It can be noted that the shape of the thiosulfate ion is tetrahedral in the solid-state of sodium thiosulfate. The distance between the two sulfur atoms in the thiosulfate ion is comparable to the distance between two sigma bonded sulfur atoms. This implies that the sulfur which is not bonded to any oxygens holds a negative charge.

Properties of Sodium Thiosulfate

The physical and chemical properties of Na2S2O3 are listed below.

Physical Properties

  • In its anhydrous form, sodium thiosulfate has a molar mass of 158.11 grams per mole. The more commonly available pentahydrate from, Na2S2O3.5H2O has a molar mass of 248.18 g/mol.
  • It has a white, crystalline appearance as a solid and is odourless.
  • The density of sodium thiosulfate corresponds to 1.667 grams per cubic centimetres.
  • The pentahydrate of this salt has a melting point of 321.4 K and a boiling point of 373 K.
  • The solubility of sodium thiosulfate in water is 70.1g/100 mL at 20oC and 231g/100mL at 100o
  • The crystal structure of Na2S2O3 crystals is monoclinic.

Chemical Properties

  • The sodium thiosulfate salt is neutral in charge. However, it dissociates in water and some other polar solvents to yield Na+ and S2O32-
  • Despite being stable at standard conditions, the sodium thiosulfate salt decomposes at high temperatures to yield sodium sulfate along with sodium polysulfide.
  • The chemical equation for the reaction described above is given by 4Na2S2O3 ⟶ 3Na2SO4 + Na2S5
  • When exposed to dilute acids such as dilute hydrochloric acid, the sodium thiosulfate salt undergoes a decomposition reaction to yield sulfur along with sulfur dioxide. Na2S2O3 + 2HCl ⟶ 2NaCl + SO2 + H2O + S
  • The alkylation of Na2S2O3 yields S-alkyl thiosulfates. These compounds are commonly referred to as Bunte salts.

Production of Sodium Thiosulfate

  • The laboratory preparation of the sodium thiosulfate salt involves the heating of aqueous sodium sulfite solutions along with sulfur.
  • The production of Na2S2O3 can also be accomplished by the boiling of aqueous NaOH (sodium hydroxide) with sulfur.
  • The reaction for the method described above is given by 6NaOH + 4S ⟶ Na2S2O3 + 2Na2S + H2O.
  • Industrially, sodium thiosulfate is prepared from the liquid waste generated from the manufacture of sulfur dye.

It can be noted that upon heating with Al3+ containing samples, sodium thiosulfate gives a white-coloured precipitate. This is due to the reaction between the aluminium cation and the thiosulfate anion which forms aluminium hydroxide along with sulfur and sulfur dioxide.

Uses of Sodium Thiosulfate

Some important uses of sodium thiosulfate are listed below.

  • Na2S2O3 is a very important chemical compound in the medical treatment of cyanide poisoning cases.
  • Sodium thiosulfate is also used medically to treat dermatophytosis (ringworm) and tinea versicolor.
  • The side effects of chemotherapy and hemodialysis (purification of blood) are treated with Na2S2O3.
  • It is a very important compound in analytical chemistry since it stoichiometrically reacts with iodine to reduce it to the iodide ion while it is oxidized to the S4O62- (tetrathionate) ion.
  • Sodium thiosulfate salts are used as photographic fixers due to the ability of the thiosulfate ions to react with silver halides, which make up photographic emulsions.
  • Ammonium thiosulfate and sodium thiosulfate make up lixiviants (liquid mediums used in hydrometallurgy) which are used in the extraction of gold from its ores.
  • In order to reduce the chlorine levels in water bodies, Na2S2O3 is used in the dechlorination process.

Thus, it is evident that sodium thiosulfate is a very important chemical compound in the lives of human beings. To learn more about this compound and its reaction with hydrochloric acid, register with BYJU’S and download the mobile application on your smartphone.

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