- Physical and chemical properties
- Reactivity and hazards
- Applications
- Medicine
- Iodometry
- Water dechlorination
- Gold mining
- References
The sodium thiosulfate or sodium hyposulfite (Na 2 S 2 O 3) is an inorganic salt with several important medical uses. It is also available as its pentahydrated salt (Na 2 S 2 O 3.5H 2 O).
It is an ionic compound formed by two sodium cations (Na +) and the negatively charged thiosulfate anion (S 2 O 3 -), in which the central sulfur atom is attached to three oxygen atoms and another sulfur atom (hence the prefix uncle), through single and double bonds with resonance character. The solid exists in a monoclinic crystal structure.
Figure 1: structure of sodium thiosulfate.
Sodium thiosulfate can be prepared by heating sulfur with an aqueous solution of sodium sulfite or an aqueous solution of sodium hydroxide.
6NaOH + 4S → Na 2 S 2 O 3 + 2Na 2 S + 3H 2 O
It is a drug that is on the World Health Organization's Essential Drug List, the most effective and safe drugs needed in a healthcare system (Sodium Thiosulfate Formula, SF).
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. In addition, it has a density of 1,667 g / ml (National Center for Biotechnology Information., 2017).
Sodium thiosulfate are colorless monoclinic crystals without a characteristic aroma. Said solid is efflorescent, which means that it can be reduced to dust by itself by loss of water of crystallization when exposed to air. Its appearance is shown in figure 2.
Figure 2: appearance of sodium thiosulfate.
The compound has a melting point of 48 ° C for the pentahydrate form and starts to decompose at 100 ° C. Na 2 S 2 O 3 is very soluble in water, being able to dissolve 70 grams per 100 milliliters of solvent. The compound is practically insoluble in ethanol (Royal Society of Chemistry, 2015).
Sodium thiosulfate is a neutral salt that readily dissociates in water to give sodium and thiosulfate ions. Na 2 S 2 O 3 is a stable solid under normal conditions, but decomposes on heating to give sodium sulfate and sodium polysulfide:
4Na 2 S 2 O 3 → 3Na 2 SO 4 + Na 2 S 5
It also breaks down when treated with dilute acids to give sulfur and sulfur dioxide (called the clock reaction):
Na 2 S 2 O 3 + 2HCl → 2NaCl + S + SO 2 + H 2 O
It reacts stoichiometrically (in equimolar amounts) with aqueous iodine solutions, making it widely used in laboratories for iodine-based titrations.
Reactivity and hazards
Sodium thiosulfate single crystals.
Sodium thiosulfate is not a toxic material and is used for medical purposes. However, when it decomposes, it produces toxic sulfur oxide fumes, which can cause irritation to the eyes, skin, and mucous membranes.
The compound can irritate the eyes, the skin and the respiratory tract. The substance is toxic to the lungs and mucous membranes. Repeated or prolonged exposure to the substance can cause damage to these organs.
If compound comes into contact with eyes, contact lenses should be checked and removed. Eyes should be flushed immediately with plenty of water for at least 15 minutes with cold water.
In case of skin contact, the affected area should be rinsed immediately 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 the contact is severe, wash with a disinfectant soap and cover the contaminated skin with an antibacterial cream.
In case of inhalation, the victim should be moved to a cool place. If not breathing, artificial respiration is given. If breathing is difficult, give oxygen.
If the compound is ingested, vomiting should not be induced unless directed by medical personnel. Loosen tight clothing such as a shirt collar, belt, or tie.
In all cases, immediate medical attention should be obtained. (Material Safety Data Sheet Sodium thiosulfate anhydrous, 2013).
Applications
Medicine
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 drug in the emergency treatment of cyanide poisoning (US National Library of Medicine, SF).
In this treatment, sodium nitrite is injected intravenously to produce hemoglobin that combines with the cyanide ion, temporarily converting it to the cyanmethoglobin form. Subsequently, the sodium thiosulfate is injected.
Thiosulfate serves as a substrate for the enzyme rhodanase, which catalyzes the conversion of cyanide to the much less toxic thiocyanate, which is excreted in the urine (HSDB: SODIUM THIOSULFATE, 2003).
Sodium thiosulfate is also used as a treatment for calciphylaxis in people on hemodialysis with end-stage kidney disease. There is apparently a not fully understood phenomenon, whereby this causes severe metabolic acidosis in some patients.
Iodometry
Sodium thiosulfate reacts stoichiometrically with iodine to give iodide according to the reaction:
2Na 2 S 2 O 3 + I2 → S 4 O 6 2− + 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 powdered bleach and water.
Water dechlorination
Sodium thiosulfate is used to dechlorinate water, including lowering chlorine levels for use in aquariums and swimming pools and spas and within water treatment plants to treat established backwash water prior to release into the rivers.
The reduction reaction is analogous to the iodine reduction reaction, the thiosulfate reduces hypochlorite (active ingredient in bleach) and in doing so oxidizes to sulfate. The complete reaction is:
4NaClO + Na 2 S 2 O 3 + 2NaOH → 4NaCl + 2Na 2 SO 4 + H 2 O
Gold mining
Sodium thiosulfate is a component used as an alternative leaching agent to cyanide for gold extraction. However, it forms a strong soluble complex with the gold (I), 3- ions.
The advantage of this approach is that thiosulfate is essentially non-toxic and that types of ore that are refractory to cyanidation of gold can be leached by thiosulfate (MG Aylmore, 2001).
References
- EMBL-EBI. (2016, June 1). sodium thiosulfate. Recovered from ebi.ac.uk: ebi.ac.uk.
- HSDB: SODIUM THIOSULFATE. (2003, May 3). Recovered from toxnet: toxnet.nlm.nih.gov.
- G Aylmore, DM (2001). Thiosulfate leaching of gold-A review. Minerals Engineering Volume 14, Issue 2, 135-174. sciencedirect.com.
- Material Safety Data Sheet Sodium thiosulfate anhydrous. (2013, May 21). Recovered from sciencelab.com.
- National Center for Biotechnology Information.. (2017, March 18). PubChem Compound Database; CID = 24477. Recovered from pubchem.ncbi.nlm.nih.gov.
- Royal Society of Chemistry. (2015). Sodium thiosulfate. Recovered from chemspider.com.
- Sodium Thiosulfate Formula. (SF). Recovered from softschools: softschools.com.
- S. National Library of Medicine. (SF). Sodium Thiosulfate (Into a vein). Recovered from ncbi.nlm.nih.gov.