15 julio, 2024

Sodium thiosulfate: what it is, formula, properties, dangers and uses

What is sodium thiosulfate?

He sodium thiosulfate or sodium hyposulfite (Na2S2O3) is an important inorganic salt with various medical uses. ANDIt is also available as its pentahydrate salt (Na2S2O3.5H2O).

It is an ionic compound formed by two sodium cations (Na+) and the negatively charged thiosulfate anion (S2O3–), in which the central sulfur atom is bonded to three oxygen atoms and another sulfur atom (hence the prefix -thio). ), through single and double bonds with resonance character. The solid exists in a monoclinic crystal structure.

Sodium thiosulfate can be prepared by heating sulfur with aqueous sodium sulfite solution or aqueous sodium hydroxide solution.

6NaOH + 4S → Na2S2O3 + 2Na2S + 3H2O

It is a drug that is on the World Health Organization’s List of Essential Medicines, the most effective and safe medicines that are needed in a health system.

Physical and chemical properties

– Sodium thiosulfate has a molecular weight of 158.11 g/mol for its anhydrous form, and 248.18 g/mol for the pentahydrate form.

– It has a density of 1.667 g/ml.

– Sodium thiosulfate is a colorless monoclinic crystal with no characteristic aroma. Said solid is efflorescent, which means that it can be reduced to dust by itself due to loss of water of crystallization when exposed to air.

– The compound has a melting point of 48 °C for the pentahydrate form, and starting at 100 °C it begins to decompose.

– Na2S2O3 is very soluble in water, being able to dissolve 70 grams per 100 milliliters of solvent. The compound is practically insoluble in ethanol.

– Sodium thiosulfate is a neutral salt that readily dissociates in water to give sodium and thiosulfate ions. Na2S2O3 is a stable solid under normal conditions, but decomposes on heating to give sodium sulfate and sodium polysulfide:

4Na2S2O3 → 3Na2SO4 + Na2S5

– Also decomposes when treated with dilute acids to give sulfur and sulfur dioxide (called clock reaction):

Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H₂O

– It reacts stoichiometrically (in equimolar amounts) with aqueous iodine solutions, which is why it is widely used in laboratories for iodine-based titrations.

Reactivity and hazards

Sodium Thiosulfate is a non-toxic material and is used for medical purposes. However, when decomposed, it produces toxic sulfur oxide fumes, which can cause irritation to the eyes, skin, and mucous membranes.

The compound can irritate the eyes, skin and respiratory tract. The substance is toxic to the lungs and mucous membranes. Repeated or prolonged exposure to the substance may cause damage to these organs.

If the compound comes into contact with the eyes, check and remove contact lenses, if worn. Eyes should be immediately flushed with plenty of water for at least 15 minutes with cold water.

In case of skin contact, the affected area should be immediately rinsed with plenty of water for at least 15 minutes, while removing contaminated clothing and shoes.

Cover irritated skin with an emollient. Wash clothing and shoes before reuse. If contact is severe, wash with disinfectant soap and cover contaminated skin with antibacterial cream.

In case of inhalation, the victim should be moved to a cool place. If he is not breathing, artificial respiration is given. If breathing is difficult, oxygen must be provided.

If the compound is swallowed, do not induce vomiting unless directed to do so by medical personnel. Loosen tight clothing, such as a shirt collar, belt, or tie.

In all cases immediate medical attention should be obtained.



– Sodium thiosulfate can be used to decrease some of the side effects of cisplatin (a cancer medicine).

– It is used in the management of extravasations during chemotherapy. Sodium thiosulfate prevents alkylation and tissue destruction by providing a substrate for alkylating agents that have invaded subcutaneous tissues.

– It is also used with another medicine in the emergency treatment of cyanide poisoning.

In such treatment, sodium nitrite is injected intravenously to produce hemoglobin that combines with the cyanide ion, temporarily converting it to the cyanemetoglobin form. Subsequently, sodium thiosulfate is injected.

– Serves as a substrate for the enzyme rhodanese, which catalyzes the conversion of cyanide into the much less toxic thiocyanate, which is excreted in the urine.

– It is also used as a treatment for calciphylaxis in people on hemodialysis with end-stage renal disease. There is apparently a not fully understood phenomenon whereby this causes severe metabolic acidosis in some patients.


– Sodium thiosulfate reacts stoichiometrically with iodine to give iodide according to the reaction:

2Na2S2O3 + I2 → S4O62− + 2I−

This property makes the compound used as a titrant in the determination of iodine.

– This particular use can be established to measure the oxygen content of water through a long series of reactions in the Winkler test for dissolved oxygen.

– It is also used in the volumetric estimation of the concentrations of certain compounds in solution (hydrogen peroxide, for example) and in the estimation of the chlorine content in commercial powder and water bleach.

water dechlorination

– Sodium thiosulfate is used for dechlorination of water, including the reduction of chlorine levels for use in aquariums and pools and spas, and within water treatment plants to treat established backwash water prior to its use. release into rivers.

The reduction reaction is analogous to the reduction reaction of iodine, thiosulfate reduces hypochlorite (active ingredient in bleach) and in doing so oxidizes to sulfate. The complete reaction is:

4NaClO + Na2S2O3 + 2NaOH → 4NaCl + 2Na2SO4 + H2O

gold extraction

Sodium thiosulfate is a component used as an alternative leaching agent to cyanide for gold extraction. However, it forms a strong soluble complex with gold(I) ions, [Au (S2O3)2]3-.

The advantage of this approach is that thiosulfate is essentially non-toxic and ore types that are refractory to gold cyanidation can be leached by thiosulfate.


HSDB: SODIUM THIOSULFATE. Retrieved from toxnet.nlm.nih.gov.
Material Safety Data Sheet Sodium thiosulfate anhydrous. Retrieved from sciencelab.com.

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