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.

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

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

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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⁺ + 2S₂O₃^2- → 2Fe²⁺ + S₄O₆^2-

Uses Of Sodium Thiosulfate(Hypo)

Sodium thiosulfate works through various chemical reactions and applications due to its unique properties. Some common ways it works include:

Photography: In photography, sodium thiosulfate is used as a photographic fixer. It works by dissolving undeveloped silver halide crystals from photographic film or paper, leaving behind a permanent image.

Medical Uses: Sodium thiosulfate is employed in medicine as an antidote to cyanide poisoning. It reacts with cyanide to form thiocyanate, which is less toxic and can be excreted from the body.

Water Treatment: It is utilized as a dechlorination agent in water treatment processes. Sodium thiosulfate reacts with chlorine to form sodium chloride (table salt) and sulfur dioxide, effectively removing chlorine from water.

Chemical Industry: Sodium thiosulfate is used in various chemical reactions as a reducing agent and sulfur donor. It finds applications in gold extraction, analytical chemistry, and as a component in certain chemical processes.

Analytical Chemistry: In chemical analysis, particularly in titration procedures, sodium thiosulfate is used as a standard solution to determine the concentration of other substances, such as iodine or to analyze for various metals.

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

These are just a few examples of how sodium thiosulfate works in different contexts, showcasing its versatility and importance in various industries and applications.