Sodium Thiosulfate - Structure, Application, Preparation, Uses, FAQs

The inorganic compound with the chemical formula Na₂S₂O₃.XH₂O is called sodium thiosulfate. XH₂O suggested that it is present in its hydrated form. Generally it is present in its a pentahydrate a form that is the chemical formula is Na₂S₂O₃.5H₂O. It is a white colorless compound and it loses its water easily and dissolves well in water.

The molecular mass of sodium thiosulfate is 158.11 gram per mole when it is present in the anhydrous former and if it is in the hydrated form that is pending hydrated form the value of molecular weight is 248.18 gram per mole. It is a colorless crystal and the density of Na₂S₂O₃ is 1.667 grams per centimeter cube. The melting point of sodium thiosulfate pentahydrate is 321.4k and its boiling point is 100 degrees Celsius.

The solubility in water is high but it is negligible soluble in alcohol. Sodium thiosulfate is prepared by heating sodium sulfite solution with sulfur or by heating sodium hydroxide solution with sulfur. The following equation represents the formation of sodium thiosulphate.

6NaOH+4S→Na₂S₂O₃+2Na₂S+3H₂O

Sodium thiosulphate has also possessed some other names like sodium hyposulphite, and hyposulphite of sodium, hypo, etc.

Sodium thiosulfate crystals

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Sodium thiosulfate has many applications; it is used extensively in gold mining, for the water treatment, for the development of a silver base as a photographic film and prints. One of the important applications of sodium thiosulphate is it can be used as a medicine for cyanide poisoning. In addition to that there are some other medical uses also it is used for the treatment of ringworm and tinea versicolor and also for treating the side effects caused during the chemotherapy treatment.

Thiosulphate has a wide application in the medical field just because it is a non-toxic material. But when sodium thiosulfate is decomposed it will produce sulfur dioxide fumes that are toxic and can cause irritation to skin eyes and even mucous membranes. Thiosulfate is used in analytical chemistry that is in the iodometric titration. Thiosulfate anion reacts with iodine to form iodide the following is the equation for the iodometric titration.

S₂O₃^2-+I₂→S₄O₆^2-+2I^-

In addition to the efficiency in this reaction with iodine thiosulfate also has an excellent shelf life so it is used as a titrant in iodometric titration.

Na₂S₂O₃ structure

From the chemical formula of sulfur we get to know that it contains two sodium cations and a thiosulphate anion i.e. S₂O₃^2-. The central atom of sulfur forms one bond with the sulfur atom and the other bonded to the remaining oxygen atoms. The central sulfur atom forms a single and double bond in a resonating way. And so the stability of sodium thiosulphate is high. The following figure shows the sodium thiosulfate structure.

Structure of sodium thiosulfate

As shown in the structure, it forms a double bond with one oxygen and a sulfur atom.

Uses of sodium thiosulphate

Due to its low toxicity and highly stable structure, it has a wide range of applications.

• Photography

Na₂S₂O₃.5H₂O is used in photography for the reduction of silver to metallic silver. That is for dissolving silver salt from the negatives sodium thiosulfate is used as a fixing agent.

• Industrial application

It is used for the extraction of gold from its ore. And also it is used as a dechlorinating agent for small water bodies like aquariums, ponds, etc. It also has applications in the leather industry that is for the tanning process.

• Medical application

For the preparation of pharmaceuticals, sodium thiosulfate is used. The most important application of sodium thiosulfate is used as an antidote for cyanide poisoning. And also it is used for reducing the side effects of chemotherapy.

Also, students can refer,

• NCERT solutions for Class 12 Chemistry Chapter 10 The S-Block Elements
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Chemical reactions of sodium thiosulphate

The solubility of thiosulfate in water is high and it produces sodium ions and thiosulfate ions. But the compound is insoluble in alcohol.

Na₂S₂O₃+H₂O→Na⁺+S₂O₃^-

The compound is a highly stable compound that we get from the structure itself but when it reacts with dilute acids it is decomposed to produce sulfur. The following equation shows the reaction of sodium thiosulphate with dilute hydrogen chloride acid.

Na₂S₂O₃+2HCl→2NaCl+S+H₂O +SO₂

Although it is a stable compound this decomposes when external heat is supplied to it. The following reaction shows the decomposition of sodium thiosulphate by the application of heat.

4Na₂S₂O₃→3Na₂SO₄+Na₂S₅

Preparation of sodium thiosulphate

Sodium thiosulfate, Na₂S₂O₃ can be prepared by using two methods, one by heating sulfur with sodium sulfite solution. The other method is by boiling sulfur with sodium hydroxide solution. It is generally found in its pentahydrate form and so it is also known as sodium hyposulfite. Sodium thiosulphate is a good reducing agent and so it is used in many reactions.

For iodometric titration sodium thiosulfate is the titrant with the starch as an indicator since starch contains high amounts of iodine. During iodometric titration before iodine forms, a complex with starch sodium thiosulfate reduces iodine to iodide. So it is a very important inorganic salt with very high industrial applications.

Hypo solution

The hypo solution formula is Na₂S₂O₃ and it is also known by the name of sodium thiosulphate solution. Hypo is present in liquid form and the color of the hypo solution is yellowish. Hypo solution is used for the photographic application i.e it is used as a photographic fixer for the development of the image. The hypo solution is used as the dechlorinating agent and also it is used in the iodometric titration as a titrant. It is the compound used for the treatment of cyanide poisoning. Hypochemical name is sodium thiosulfate.

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NCERT Chemistry Notes:

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