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Sodium Thiosulphate

Last updated date: 29th May 2024
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Sodium Thiosulphate is a colorless, odorless salt that is inorganic in nature. It is a crystalline compound with five molecules of water in it. It is also referred to as disodium thiosulphate with the chemical formula of Na2S2O3. When it is decomposed by the application of heat it emits highly toxic fumes of sulfur oxides. It is often found in toxic wastes of dye industries. It is basically an ionic compound formed from two sodium cations with one thiosulfate anion where one sulfur atom is bonded to three oxygen atoms and one another sulfur atom. The oxygen atoms are bonded to the sulphur atom by double bonds and the other sulfur atm by a single bond. This compound has many applications in day-to-day use and its highly soluble nature serves as the basis of its importance. It becomes amorphous when the five water molecules are lost.

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. 

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. 

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 on Sodium Thiosulphate

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 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.

3. What are the medical uses of Sodium thiosulphate?

Sodium thiosulfate is an inorganic compound highly popular in medicine because it is available in the form of oral drugs. However it is still recommended to take it intravenously. Some of the important uses in medical history are listed below:

  • It is applied as intravenous injection in case of metal poisonings like arsenic, lead and mercury poisoning, cyanide poisoning and also in the cisplatin toxicity which is the most commonly used drug in cancerous tumors.

  • It is a well known anti-inflammatory agent and is used in several cases of inflammation.

  • A well known medicine for the treatment of the kidney disease called Calciphylaxis.

4. What are the future prospects of usage of Sodium thiosulphate?

Sodium thiosulphate is a very important compound and researchers are still going to explore many expected uses of this compound. Ongoing research on medicinal chemistry of this compound shows that patients of arthritis who used to take this compound in the form of medicine have a much reduced chance to develop Alzheimer disease in the long run. It has shown potential to be one of the best medicines for neurodegenerative treatment in the long run and research on this is going at present as a neuroprotective agent and may be in the recent future it  can be widely used in the treatment of Alzheimer disease and Parkinson disease.

5. How safe is it to use Sodium thiosulphate as medicine?

Sodium thiosulfate has immense use in the field of medicine but at the same time it can be toxic with side effects at times especially if the doses are not proper. The following are some common side effects of the drug:

  • During the treatment of kidney disease calciphylaxis, sodium thiosulphate has caused a very common side effect of nausea and vomiting

  •  Sodium thiosulphate is a hygroscopic compound and has a tendency to attract water hence in certain kidney, heart and liver disease patients this can not be used since retaining water can further worsen the situation.

  • It often causes disorientation and headache.

  • A common side effect is dermatitis and local skin irritations.

  • Intake of this drug often causes blood pressure to fall and become very low at times.

6. What is the detailed structure of Sodium thiosulphate ?

It is an inorganic crystalline compound with the thiosulfate anion being in a tetrahedral shape which is derived from replacing one oxygen atom in a sulphate anion by  a sulphur atom. In the thisosulphate anion the sulphur is bonded to three oxygen atoms by double bond and to the other sulfur atom by a single bond. This totally forms the anion of the ionic compound which is ionically bonded to two atoms of sodium as the cations. The crystal structure of the compound with five molecules of water is of tetrahedron shape.

7. What is the use of sodium thiosulphate in laboratories?

Sodium thiosulphate is an important compound in labs both chemical and pharmaceutical. It is a common reagent in pharmaceutical labs for its medicinal properties. It is also used in chemistry labs for iodine titration because it reacts in equimolar amounts with elemental iodine. It infact acts as a reducing agent in the titration. It is used as a reducing agent in many other reactions of chemistry labs. It is also used to dechlorinate water by reducing its property. It reacts with chlorine to form harmless chlorides which are removed from water.