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Sodium thiosulfate, also known as thiosulfuric acid or simply sodium sulphate, is an inorganic salt, with a chemical formula, Na2S2O3·(H2O)(x). It appears in either powder or white, odourless crystal pentahydrate form.
- Sodium thiosulfate is an efflorescent crystalline substance that dissolves quite well in water.
- The compound was found by Sir John Herschel in 1839.
- He wanted to create a photographic image on paper and prevent it from fading.
- The compound is highly soluble in water.
- The crystal formed in the compound is hygroscopic in nature.
- It is also known as disodium thiosulphate with the chemical formula Na2S2O3.
- Sodium thiosulfate has several medical uses and is also used in the dechlorination process.
Key Terms: Sodium thiosulfate, Ions, Sodium Sulphate, Dechlorination, Atoms, Oxygen, Ionic Compound, Resonance, Sodium Polysulfide, Air, Sodium Hydroxide, Sulphur
What is Sodium Thiosulfate?
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Sodium thiosulfate is an inorganic salt composed of sodium and thiosulfate ions in the ratio 2:1 ratio. It plays an important role as an antidote to cyanide poisoning.
- Sodium thiosulfate is available in pentahydrate form.
- When exposed to sulfide or sulfate in the air, the salt develops an alkaline nature.
- It is a crystalline solid in its solid form that has a tendency to lose water.
- The compound is highly soluble in water and gives thiosulfate ions.
- Sodium thiosulfate emits toxic fumes of sulfur oxides when heated.
- The compound becomes amorphous when the five water molecules are removed.
- It will undergo a redox reaction on heating with Copper (II) sulfate.
- The compound has been approved to prevent hearing loss in infant, child, and adolescent cancer patients.
Structure of Sodium Thiosulfate
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Sodium thiosulfate is an ionic compound with a chemical formula, Na2S2O3. It consists of two sodium cations (Na+) and a negatively charged thiosulfate anion (S2O3-).
- In the structure, the central sulfur atom forms bonds with three oxygen atoms and one sulfur atom.
- All atoms are bonded with single and double bonds possessing resonance characteristics.
- The solid is present in a monoclinic crystal structure.
- It forms a tetrahedral structure.
- The structure is formed by the replacement of an oxygen atom with one sulphur atom in a sulfate anion.
- The lone pair of electrons on oxygen are connected with a single-coordinate covalent bond.
Sodium Thiosulfate
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| Sulphur Cycle | Electrophoresis | Collision Theory |
| Second Order Reactions | Periodic Table | Ammonia |
Properties of Sodium Thiosulfate
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The chemical and physical properties of sodium thiosulfate are:
Physical Properties of Sodium Thiosulfate
The physical properties of Sodium Thiosulfate are as follows:
- The molar mass of Na2S2O3 in anhydrous form is 158.11 grams per mole.
- It is odorless and has a white, crystalline solid appearance.
- The molar mass of its pentahydrate form is 248.18 g/mol.
- The crystal structure of Na2S2O3 is monoclinic.
- The pentahydrate form has a melting point of 321.4 K and a boiling point of 373 K.
The physical properties of Sodium Thiosulfate are tabulated below:
| Properties of Sodium Thiosulfate | |
|---|---|
| Name | Sodium Thiosulfate |
| Another Name | Sodium Hyposulfite and Thiosulfuric Acid |
| Appearance of Compound | White crystals and Odourless Compound |
| Molecular Formula of Compound | Na2S2O3 |
| Boiling Point of Compound | 100 °C (Pentahydrate) |
| Melting Point | 48.3 °C (pentahydrate) |
| Molar Mass | (A)158.11 g/mol in anhydrous state (B) 248.18 g/mol in pentahydrate state |
| Density | 1.67 g/cm³ |
| Solubility in Water | Soluble |
Chemical Properties of Sodium Thiosulfate
The chemical properties of Sodium Thiosulfate are as follows:
Solubility
Sodium thiosulfate has a neutral charge. It dissociates in water and other polar solvents in order to yield Na+ and S2O32-. The reaction of dissociation is as follows:
Na2S2O3 + H2O→2Na++ S2O2−3
Reaction on Heating
At high temperatures, the salt decomposes and yields sodium sulfate along with sodium polysulfide. The reaction can be described as:
4Na2S2O3 +------> 3Na2SO4 + Na2S5
Reaction with Dilute Acids
When exposed to dilute acids, a decomposition reaction takes place, wherein the sodium thiosulfate yields sulphur and sulphur dioxide.
Na2S2O + 2HCl --------→ 2NaCl + SO2 + H2O + S
Reaction with Aqueous Solution
The alkylation of Na2S2O3 yields S-alkyl thiosulfates, which are commonly known as Bunte salts.
Preparation of Sodium Thiosulfate
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The preparation of sodium thiosulfate are as follows:
- In laboratory, sodium thiosulfate is prepared by heating the aqueous sodium sulfite solution along with sulphur.
- It can also be prepared by boiling of aqueous NaOH (sodium hydroxide) with sulphur.
6NaOH + 4S ------→ Na2S2O3 + 2Na2S + H2O
- Industrially, the preparation of sodium thiosulfate involves the use of liquid waste generated from the manufacture of sulphur dye.
Sodium Thiosulfate
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Uses of Sodium Thiosulfate
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Due to its stable structure, the substance is used in different fields. Some of the uses of sodium thiosulfate are as follows:
- Sodium Thiosulfate is mostly used in the extraction of gold from its ores.
- Thiosulfate is also used in the dechlorination process of small water bodies like ponds and aquariums.
- It is used in pharmaceutical preparations, like anionic surfactants, that help in dispersion.
- Moreover, it aids in treating ringworm and overcoming the side effects of chemotherapy.
- Thiosulfate helps in dissolving the silver salts from the negatives and thus works as a fixing agent in photography.
- The substance can also work as a cleansing agent when dissolved in a good quantity of warm water.
Sodium Thiosulfate is also used in chemical heating pads, neutralizing bleach, water treatment, leather tanning, photographic film processing and much more.
Things to Remember
- Sodium thiosulfate is an ionic compound that occurs in a monoclinic crystalline structure.
- Chemical formula and molecular mass of sodium thiosulfate is Na2S2O3 and 248.18 g/mole.
- It is white, odorless and highly soluble compound.
- Sodium thiosulfate is prepared by heating the aqueous sodium sulfite solutions along with sulphur in laboratory.
- It is used in various fields like, medicine, photography etc.
Sample Questions
Ques. What is the molecular weight and formula of sodium thiosulfate? (1 mark)
Ans. The chemical formula and molecular mass of sodium thiosulfate is Na2S2O3 and 248.18 g/mole respectively.
Ques. Is sodium thiosulfate a reducing agent? (2 marks)
Ans. Sodium thiosulfate is also known as a reducing agent and is used for controlling the reductive-oxidative properties of the sulphide mineral during flotation. It is basically a moderately strong reducing agent that is used through an indirect process, wherein, iodine is an intermediate agent for determining the oxidizing agents.
Ques. Explain (A) What happens when sodium thiosulfate reacts with iodine
(B) What is the ph of sodium thiosulfate? (2 marks)
Ans. (A) Tetrathionate sodium and sodium iodide are formed through the reaction of sodium thiosulfate and iodine.
(B) The ph of sodium thiosulfate lies between 7.0-9.0
Ques. What is the normality formula? (2 marks)
Ans. The number of equivalents multiplied by the molarity gives the normality of a solution. Standard(N) = Molarity(M) x equivalent number. It can be defined as the gram weight equivalent of the solution per liter.
Ques. Is sodium thiosulfate soluble in water? (2 marks)
Ans. Sodium thiosulfate is an inorganic salt that is soluble in water and is a reducing agent that reacts with oxidising agents. Thiosulfate also provides an exogenous source of sulfur, thus, boosting the detoxification of cyanide through enzyme rhodanese.
Ques. How does sodium thiosulfate reacts with hydrochloric acid? (2 marks)
Ans. Sodium thiosulphate reacts with dilute hydrochloric acid to produce sulphur, water, sodium chloride and sulphur dioxide. The sulphur that results in the reaction is insoluble and appears in the mixture as a pale-yellow or white precipitate that has a milky appearance and makes the mixture opaque.
Ques. What does sodium thiosulfate do to chlorine? (2 marks)
Ans. Sodium thiosulfate works to remove free chlorine and combined chlorine, also known as chloramines from the pools. This is quite an easy and inexpensive way to balance the chemical levels in the pool. In order to use this reducer, you just need to add around one cup of the dry material for every 5,000 gallons of water.
Ques. Explain the properties of sodium thiosulfate? (4 marks)
Ans. The properties of sodium thiosulfate are as follows:
- Sodium thiosulphate is an inorganic chemical that crystallizes into a white, translucent, colorless substance.
- It is a material that dissolves in water and turpentine oil, but not in alcohol.
- The material melts between 48 and 52 degrees Celsius.
- This chemical compound has great stability and is said to be incompatible with certain potent acids and oxidizers.
- With the help of the thiosulfate anion, diluted acids quickly react to produce sulfur, sulfur dioxide, and water.
- The density of the substance is approximately 1.667 g/mL.
Ques. States the uses of sodium thiosulfate? (3 marks)
Ans. The uses of sodium thiosulfate are as follows:
- The production of patinas requires the use of sodium thiosulfate.
- The chemical is used in industry to dechlorinate small water bodies, such as aquariums and ponds.
- The chemical is used as a fixing agent in photography to remove the silver salts from the negatives.
- When the substance is dissolved in a large amount of warm water, it can be utilized as a cleaning agent.
- It works effectively as an antidote for cyanide poisoning.
Ques. What are the harmful effects of sodium thiosulfate? (2 marks)
Ans. The harmful effects of sodium thiosulfate are as follows:
- Heart Diseases
- Hypertension
- Edema
- Liver Disease
- Water retention in the body
- Vision Loss
- Kidney ailments
- Nausea and vomiting
- Cramps
- Joint pains
Ques. How sodium thiosulfate is used to neutralise chlorine? (3 marks)
Ans. The pH of the water determines how much sodium thiosulfate is required to neutralize chlorine. It is usually recommended to use a solution of two to seven parts sodium thiosulfate to one part chlorine.
- Similarly, 1.5–5.3 grams might be needed for a gallon of a 200 ppm chlorine solution.
It is advisable to employ a chlorine test kit to precisely ascertain the ultimate amounts of chlorine, guaranteeing precise dose and efficient neutralization during water treatment procedures.
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