Sodium thiosulfate

What is the formula of sodium thiosulphate?

  • Sodium sulfate – Na2SO4
  • Sodium pentasulfide – Na2S5
  • Sodium tetrathionate – Na2S4O6
  • Silver nitrate – AgNO3
  • Silver Thiosulfate – Ag2S2O3
  • Sodium sulfate – Na2SO4
  • Sodium sulfide – Na2S
  • Sodium silver thiosulfate – AgNa3O6S4
  • Ferric thiosulfate – Fe2(S2O3)3

Sodium thiosulfate: At the beginning of the development of the chemical, the formula of this additive was considered Na2S2O4 and was named sodium hypo-sulfate. The short name is called Hypo. Later its formula Na2S2O3 was established. Due to its acronym hypo becoming more popular, its common name is still hypo today.

Sodium thiosulfate


Sulfur is boiled with caustic soda solution.

[S + 2NaOH → Na2S + H2O + O] X 3

Na2S + 3O → Na2SO3

Na2SO3 + S → Na2S2O3


4S + 6NaOH → Na2S2O3 + 2Na2S + 3H2O

When an aqueous solution of sodium sulfide is boiled with sulfur, sodium thiosulfate is obtained. After filtering the solution and cooling it, monoclinic shaped crystals of Na2S2O3.5H2O are obtained.

Na2SO3 + S → Na2S2O3

Hypo is obtained when a mixture of sodium sulfate and sodium sulfide is reacted with iodine.

Na2S + I2 → 2NaI + S

Na2SO3 + S → Na2S2O3


Na2SO3 + Na2S + I2 → Na2S2O3 + 2NaI

In this reaction, iodine is used in moderation. Otherwise, it reacts with the product to make another product.

When sulfur dioxide gas flows in a solution of sodium carbonate and sodium sulfide

2Na2S + 2H2O + SO2 → 4NaOH + 3S

Na2CO3 + SO2 → Na2SO3 + CO2

Na2SO3 + S → Na2S2O3

When flow of SO2 in sodium carbonate solution in the presence of sulfur

Na2CO3 + SO2 → Na2SO3 + CO2

Na2SO3 + S → Na2S2O3

By the reaction of sodium hydrogen sulfide and sodium sulfate

2NaHS + 4NaHSO3 → 3Na2S2O3 + 3H2O

Sodium Thiosulphate Structure

Physical and Chemical Properties:

It is a colorless crystalline solid. Its melting point is 48°C. It is soluble in water.

Thermal Decomposition: By heating to 215°C, its crystallization water is separated.

Na2S2O3.5H2O → Na2S2O3 + 5H2O

At 220°C or higher temperature, it is decomposed into sodium sulfate and sodium pentasulfide.

4Na2S2O3 → 3Na2SO4 + Na2S5

Reaction with dilute HCl: It reacts with dilute acids to produce SO2 gas and the sulfur precipitate is obtained.

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

Action with Chlorin: By acting with chlorine, it forms hydrochloric acid. For this reason, it is called AntiChlor.

Na2S2O3 + Cl2 + H2O → Na2SO4 + 2HCl + S

Action with iodine: It reacts with iodine to make NaI and sodium tetrathionate. Both these compounds are colorless, so the violet color of iodine disappears. This reaction is used in the quantitative analysis of iodine.

2Na2S2O3 + I2 → 2NaI + Na2S4O6

Reaction with silver nitrate: By reacting with silver nitrate it first forms a white precipitate of silver thiosulfate. Whose color eventually turns black due to reduction.

Na2S2O3 + 2AgNO3 → Ag2S2O3 + 2NaNO3

Ag2S2O3 + H2O → Ag2S + H2SO4

If sodium is taken in excess, the white precipitate of silver thiosulphate forms sodium agents by dissolving in excess of Sodium silver thiosulfate and black precipitate of Ag2S is not obtained.

Ag2S2O3 + 3Na2S2O3 → 2Na3[Ag(S2O3)2]

Action with silver bromide: Silver chloride, silver bromide, and silver iodide are insoluble in water but dissolve in an aqueous solution of sodium thiosulfate. The reason for this is that the reaction of silver halide and sodium Thiosulfate forms a hybrid salt called Sodium Argento Thiosulphate which is soluble in water.

AgBr + 2Na2S2O3 → Na3[Ag(S2O3)2] + NaBr

Action with ferric chloride: The reaction of ferric chloride and sodium thiosulfate gives pink color due to the formation of ferric thiosulfate.

2FeCl3 + 3Na2S2O3 → 6NaCl + Fe2(S2O3)3

After some time the pink color disappears due to the following reaction.

2Fe3+ + 2S2O32- → 2Fe2+ + S4O62-

Uses Of Sodium Thiosulfate(Hypo)

To make negatives in photography and to make positive

To overcome the excess of chlorine as Antichlor.

In quantitative analysis, titrations with iodine determine their amounts in solutions of oxidizers such as CuSO4, KMnO4, K2Cr2O7 etc.

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