Sodium thiosulfate
1378708
225384755
2008-07-13T11:52:15Z
206.132.54.54
/* Iodometry */
{{Chembox new
| Name = Sodium thiosulfate
| ImageFile = Sodium-thiosulfate-3D-vdW.png
<!-- | ImageSize = 200px -->
| ImageName = Sodium thiosulfate
| ImageFile1 = Sodium-thiosulfate-pentahydrate-xtal-3D-balls.png
| ImageName1 = Crystal structure of sodium thiosulfate pentahydrate
| IUPACName = Sodium thiosulfate
| OtherNames = Sodium hyposulfite<br />Hyposulphite of soda
| Section1 = {{Chembox Identifiers
| CASNo = 7772-98-7
}}
| Section2 = {{Chembox Properties
| Formula = Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>
| MolarMass = 158.09774 g/mol
| Appearance = White crystals
| Density = 1.667 g/cm³, solid
| Solubility = Very Soluble
| MeltingPt = 48.3 °C
| BoilingPt = Decomposes
| pKb = N/A
}}
| Section3 = {{Chembox Structure
| Coordination = Tetrahedral anion
| CrystalStruct =
| Dipole =
}}
| Section7 = {{Chembox Hazards
| ExternalMSDS = [http://www.jtbaker.com/msds/englishhtml/S5230.htm External MSDS]
| EUClass = Non-toxic.
| RPhrases = [[R35: Causes severe burns]]
| SPhrases = {{S1/2}} {{S26}} {{S37/39}} {{S45}}
| NFPA-H = 1
| NFPA-F =
| NFPA-R =
| FlashPt = Non flammable
}}
}}
'''Sodium thiosulfate''' ([[Sodium|Na]]<sub>2</sub>[[Sulfur|S]]<sub>2</sub>[[Oxygen|O]]<sub>3</sub>) is a colorless [[crystal]]line compound that is more familiar as the [[pentahydrate]], [[Sodium|Na]]<sub>2</sub>[[Sulfur|S]]<sub>2</sub>[[Oxygen|O]]<sub>3</sub>•5[[Hydrogen|H]]<sub>2</sub>[[Oxygen|O]], an [[efflorescent]], [[monoclinic]] crystalline substance also called sodium hyposulfite or “hypo.”
The [[thiosulfate]] [[anion]] is tetrahedral in shape and is notionally derived by replacing one of the [[oxygen]] atoms by a [[sulfur]] atom in a [[sulfate]] anion. The S-S distance indicates a single bond, implying that the sulfur bears significant negative charge and the S-O interactions have more double bond character. The first protonation of thiosulfate occurs at sulfur.
==Industrial production and laboratory synthesis==
On an industrial scale, sodium thiosulfate is produced chiefly from liquid waste products of [[sodium sulfide]] or sulfur [[dye]] manufacture.<ref name="holleman">Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5</ref>
Small scale synthesis is by boiling an aqueous solution of [[sodium sulfite]] with [[sulfur]].
==Principal reactions and applications==
Thiosulfate anion characteristically reacts with [[dilution|dilute]] [[acid]]s to produce [[sulfur]], [[sulfur dioxide]] and [[water]]:<ref name="holleman" />
:S<sub>2</sub>O<sub>3</sub><sup>2−</sup><small>(aq)</small> + 2H<sup>+</sup><small>(aq)</small> → S<small>(s)</small> + SO<sub>2</sub><small>(g)</small> + H<sub>2</sub>O<small>(l)</small>
This reaction has been employed to generate colloidal sulfur. When the protonation is conducted at low temperatures, H<sub>2</sub>S<sub>2</sub>O<sub>3</sub> ([[thiosulfuric acid]]) can be obtained. It is a strong acid pK<sub>a</sub> = 0.6, 1.7.
===Iodometry===
In analytical chemistry, the most important use comes from the fact thiosulfate anion reacts [[stoichiometrically]] with [[iodine]], reducing it to [[iodide]] as it is oxidized to [[tetrathionate]]:
:2 S<sub>2</sub>O<sub>3</sub><sup>2−</sup><small>(aq)</small> + [[iodine|I]]<sub>2</sub><small>(aq)</small> → S<sub>4</sub>O<sub>6</sub><sup>2−</sup><small>(aq)</small> + 2 I<sup>−</sup><small>(aq)</small>
Due to the quantitative nature of this reaction, as well as the fact that Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>•5H<sub>2</sub>O has an excellent shelf-life, it is used as a [[titrant]] in [[iodometry]]. Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>•5H<sub>2</sub>O is also a component of ''iodine clock'' experiments.
This particular use can be set up to measure the oxygen content of water through a long series of reactions. It is also used in estimating volumetrically, the concentrations of certain compounds in solution ([[hydrogen peroxide]], for instance), and in estimating the chlorine content in commercial bleaching powder and water.
===Photographic processing===
The terminal sulfur atom in S<sub>2</sub>O<sub>3</sub><sup>2−</sup> binds to [[HSAB concept|soft metals]] with high [[affinity]]. Thus it dissolves [[silver]] halides, e.g. [[silver bromide|AgBr]], which is a component of photographic emulsions:
:2 S<sub>2</sub>O<sub>3</sub><sup>2−</sup> + AgBr → [Ag(S<sub>2</sub>O<sub>3</sub>)<sub>2</sub>]<sup>3−</sup> + Br<sup>-</sup>
In this application to [[photographic processing]], discovered by [[John Herschel]] and used for both [[film]] and paper processing, sodium thiosulfate is known as a ''[[photographic fixer]]''.
===Gold extraction===
Sodium thiosulfate is one component of an alternative [[lixiviant]] to [[cyanide]] for extraction of [[gold]].<ref name="aylmore">Aylmore, M. G.; Muir, D. M. "Thiosulfate Leaching of Gold - a Review", ''Minerals Engineering'', 2001, 14, 135-174</ref> It forms a strong complex with gold(I) ions, [Au(S<sub>2</sub>O<sub>3</sub>)<sub>2</sub>]<sup>3-</sup>. The advantage of this approach is that thiosulfate is essentially non-toxic and that ore types that are [[refractory]] to [[gold cyanidation]] (e.g. [[carbonaceous]] or Carlin type ores) can be leached by thiosulfate. Some problems with this alternative process include the high consumption of thiosulfate, and the lack of a suitable recovery technique, since [Au(S<sub>2</sub>O<sub>3</sub>)<sub>2</sub>]<sup>3-</sup> does not [[adsorb]] to [[activated carbon]], which is the standard technique used in [[gold cyanidation]] to separate the gold complex from the ore slurry.
==Other uses==
Sodium thiosulfate is also used:
*As a component in [[hand warmer]]s and other chemical [[heating pad]]s that produce heat by exothermic crystallization of a [[supercooling|supercooled]] solution.
*In Bleach
*In pH testing of bleach substances. The [[universal indicator]] and any other liquid pH indicator are destroyed by bleach, rendering them useless for testing the [[pH]]. If one first adds sodium thiosulfate to such solutions, it will neutralize the color-removing effects of bleach and allow one to test the pH of bleach solutions with liquid indicators. The relevant reaction is akin to the iodine reaction: thiosulfate reduces the hypochlorite (active ingredient in bleach) and in so doing becomes oxidized to sulfate. The complete reaction is:
:4 NaClO + Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> + 2 NaOH → 4 NaCl + 2 Na<sub>2</sub>SO<sub>4</sub> + H<sub>2</sub>O
*To dechlorinate tap water for aquariums or treat effluent from waste water treatments prior to release into rivers. The reduction reaction is analogous to the iodine reduction reaction. Treatment of tap water requires between 0.1 grams and 0.3 grams of pentahydrated (crystalline) sodium thiosulfate per 10 liters of water.
*To lower chlorine levels in swimming pools and spas following super chlorination.
*To remove iodine stains, e.g. after the explosion of [[nitrogen triiodide]].
*As an antidote to [[cyanide]] poisoning. Thiosulfate acts as a sulfur donor for the conversion for cyanide to thiocyanate (which can then be safely excreted in the urine), catalyzed by the enzyme [[rhodanase]].
*In [[bacteria|bacteriological]] water assessment.
*In the [[tanning]] of [[leather]].
*To demonstrate the concept of [[reaction rate]] in chemistry classes. The [[thiosulfate]] ion can decompose into the [[sulfite]] ion and a colloidal suspension of sulfur, which is opaque. The equation for this acid-catalysed reaction is as follows:<br />S<sub>2</sub>O<sub>3</sub><sup>2−</sup>(aq) → SO<sub>3</sub><sup>2−</sup>(aq) + S(s)
*To demonstrate the concept of [[supercooling]] in physics classes. Melted sodium thiosulfate is very easy to overcool to room temperature and when crystallization is forced, the sudden temperature jump to 48.3°C can be experienced by touch.
*As part of [[patina]] recipes for [[copper]] alloys.
*Often used in pharmaceutical preparations as an anionic surfactant to aid in dispersion.
*Treatment of calciphylaxis in hemodialysis patients with End-Stage Renal Disease
==References==
<references/>
[[Category:Thiosulfates]]
[[Category:Sodium compounds]]
[[Category:Photographic chemicals]]
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