Sodium Thiosulphate


Sodium Thiosulphate also refers to disodium salt or thiosulfuric acid. It is an inorganic salt having a chemical formula of Na2S2O3. The salt is available in the pentahydrate and appears in powder form or as a white, odorless crystal. When decomposed to sulfate or sulfide in the air, thiosulphate salt possesses an alkaline nature. Check out the physical and chemical properties of sodium thiosulphate and explore how this chemical substance is used in real-life applications. 

Physical Properties of Sodium Thiosulphate

  • Appearance: Thiosulphate is a bright crystalline crystal, which is odorless and highly soluble in water.

  • Chemical formula: The chemical formula of thiosulphate is represented as Na2S2O3.

  • Density: The solid has a density of about 1.667 g/ ml.

  • Melting Point: The substance melts at a temperature of 48.3 degrees Celsius. 

The compound mostly exists in its hydrated form, written as Na2S2O3.xH2O. Naturally, this white crystalline compound is found in its pentahydrate form represented as Na2S2O3.5H2O.

Chemical Properties of Sodium Thiosulphate

  • Solubility: Thiosulphate is a neutral salt that dissociates in water readily and produces sodium and thiosulfate ions. The substance is also soluble in turpentine oil but insoluble in alcohol. 

Na2S2O3 + H2O → Na+ + S2O3-

  • Stability: From the structure of thiosulfuric acid, it’s clear that this inorganic solid is highly stable. It is highly compatible with some of the strong acids and strong oxidizing agents. However, when reacted with dilute acids, the substance gets decomposed to produce sulfur. 

  • Reaction Upon Heat: Though thiosulphate is a stable substance under normal conditions. However, if some heat is provided, then it decomposes to give sodium polysulfide and sodium sulfate. 

4Na2S2O3 → 3Na2SO4 + Na2S5

  • Reaction With Dilute Acids: When a solid is treated with dilute acids, it gets decomposed to give sulfur plus sulfur dioxide. The reaction below is termed as the Clock reaction.

Na2S2O3 + 2HCl (Dilute Hydrochloric Acid) → 2 NaCl + S + SO2 + H2O

  • Reaction With Aqueous Solutions of Iodine: Sodium Thiosulphate reacts in equimolar amounts (stoichiometrically) with aqueous solutions of iodine. That’s why the solid is widely used in laboratories for titrations based on iodine. 

Structure of Sodium Thiosulphate

Having a chemical formula as Na2S2O3, thiosulphate is an ionic compound that consists of two cations of a sodium atom, that is, Na+ and a negatively charged anion of thiosulfate, that is, S2O3-. From the thiosulphate structure, it’s clear that the central atom (S) forms one bond with sulfur and other bonds to three oxygen atoms. The central atom forms two bonds with each atom, single and double possessing resonance character. 

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The above diagram represents the structure of sodium thiosulfate. In this, the central atom (S) forms one single and one double bond with three oxygen atoms and one sulfur atom. 

In the structure of thiosulfuric acid, the thiosulfate anion is obtained by the replacement of one of the oxygen atoms by the use of sulfur (S) atom in the sulfate anion. Thus, it forms a tetrahedral structure.  

Preparation of Sodium Thiosulphate

Sodium Thiosulphate is usually prepared by heating sulfur along with aqueous sodium hydroxide solution or by using aqueous sodium sulfite solution. 

6NaOH + 4S → Na2S2O3 + 2Na2S + 3H2O

The structure of thiosulfuric acid shows that the solid also occurs in a monoclinic crystalline structure. Thiosulphate is not a toxic material, and thus commonly possesses wide medical applications. However, when decomposed, this chemical compound produces toxic sulfur oxide fumes. The fumes formed can cause irritation to skin, eyes, and even mucous membranes. 

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The above diagram shows sodium thiosulphate contains two types of ions. Two cations of sodium and an anion of thiosulphate. 

Applications of Thiosulphate in Real-Life

Due to stable thiosulphate structure, the substance is widely used in different fields such as gold extraction, photography, medicine, and other areas.

  • Photography: This chemical substance is often used as a fixing agent in photography. It helps in dissolving the silver salts from the negatives. 

  • Cleansing Agent: When dissolved in an enormous quantity of warm water, this chemical substance can be used as a cleansing agent.

  • Industries: Thiosulphate is popularly used for the dechlorinating of small water bodies such as aquariums, ponds, and so on. This inorganic substance is highly used in the production of patinas. It is widely used in gold extraction from its ores. 

  • Medical Field: It is used in pharmaceutical preparations, including anionic surfactant helping in dispersion. Additionally, it is a crucial antidote to cyanide poisoning. It aids in treating ringworm plus overcoming the side-effects of chemotherapy. 

Apart from the above applications, the thiosulphate structure shows that the compound is preferably used in water treatment, neutralizing bleach, leather tanning, photographic film processing, chemical heating pads, and so on. 

FAQs (Frequently Asked Questions)

1. What are the Side Effects of Sodium Thiosulphate?

Basically, salt is used in medicines. However, before you use them, you need to understand to use this compound in the proper amount. Before you intake this as medicine, you must consider its side effects too. This medicine might cause allergies. Thus it must be taken under proper prescription. If you intake the overdose, it might lead to agitation, blurred vision, mental changes, hallucinations, muscle pain, cramps, vomiting, pain in your joints, and also there could be many mental changes. Usually, this salt affects adults as per studies. It does not cause many problems among children. Thus use this with a proper dosage.

2. What Happens When Mixing Sodium Thiosulphate with Water?

Mixing of Sodium Thiosulphate with water is an endothermic reaction. In such reactions to take place, these take energy from surroundings. Thus, this compound needs the energy to dissolve in the water. Thus water warms up. Later heat is absorbed into sodium thiosulphate, and thus water turns cold. Also, being an organic compound, then also this white or colorless substance when mixed with water. In the solid form (without water), it is in the crystalline form which is also efflorescent. However, when water is added to it, the compound gets dissolved in water quickly.