Na₂S₂O₃.5₂O: It is manufactured by saturating a solution of sodium carbonate with SO₂ which gives a solution of sodium sulfite,

Na₂CO₃ + SO₂ + H₂O — Na₂SO₃ + CO₂ + H₂O

                                                                                                               373K
The resulting solution is boiled with powdered sulfur as, Na₂SO₃ + S ———> Na₂S₂O₃

The solution is then cooled to get crystals of sodium thiosulfate.

Physical properties : (i) Sodium thiosulfate is a colorless crystalline solid. In the hydrated form, it is called hypo. (ii) It melts at 320 K and loses its water molecules of crystallization on heating to 490K.

Chemical properties

(i) Action with halogens : It reacts with halogens as,

(a) Chlorine water oxidizes sodium thiosulphate to sodium sulphate and sulphur is precipitated,

Na ₂S₂O₃ + Cl₂ + H₂O —> 2HCl + Na₂SO₄ +S

This property enables it to act as an antichlor in bleaching i.e. it destroys the unreacted chlorine in the process of bleaching.

(b) Bromine water also oxidizes sodium thiosulfate to sodium sulfate and sulfur,

Na ₂S₂O₃ + Br₂ + H₂O —> Na₂SO₄ + 2HBr + S

(c) With iodine it forms a soluble compound called sodium tetrathionate,

Therefore, hypo is commonly used to remove iodine stains from the clothes.

(ii) Action of heat : Upon heating, sodium thiosulfate decomposes to form sodium sulfate and sodium pentasulfide,

(iii) Action with acids : Sodium thiosulphate reacts with dilute hydrochloric acid or Sulfuric acid forming sulfur dioxide and sulfur. The solution turns milky yellow due to sulfur.

Na₂S₂O₃ + 2HCl —> 2NaCl + SO₂ + H₂O + S

(iv) Action with silver halides : Sodium thiosulfate forms soluble complex when treated with silver chloride or silver bromide,

This property of hypo is made use in photography.

Uses of sodium thiosulfate

(i) It is largely used in photography as a fixing agent.

(ii) It is used as a preservative for fruit products such as jams and squashes.

(iii) It is used as an antichlor in bleaching.

(iv) It is used as a volumetric agent for the estimation of iodine.

(v) It is used in medicine.