Sodium thiosulfate

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General
Systematic name	Sodium thiosulfate (Sodium thiosulphate)
Other names	Sodium hyposulfite Hyposulfate of soda
Molecular formula	Na₂S₂O₃
Molar mass	158.09774 g/mol
Appearance	White crystals
CAS number	^*
Properties
Density and phase	1.667 g/cm³
Solubility in water	Very Soluble
Melting point	48.3 °C
Boiling point	N/A
Basicity (pK_b)	N/A
Structure
Coordination geometry	tetrahedral anion
Crystal structure	?
Dipole moment	? D
Hazards
MSDS	External MSDS
EU classification	Non-toxic.
R-phrases	R35
S-phrases	S1/2, S26, S37/39, S45
NFPA 704
Flash point	Non flammable
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

Sodium thiosulfate (Na₂S₂O₃) is a colorless crystalline compound that is more familiar as the pentahydrate, Na₂S₂O₃•5H₂O, 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.

Production and chemical synthesisHolleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5

Sodium thiosulfate is produced chiefly from liquid waste products of sodium sulfide or sulfur dye manufacture. It is also arises from the action of sodium sulfite on sulfur in aqueous solution. As such, the anion S₂O₃²⁻ represents a water-soluble form of elemental sulfur.

Principal reactions and applicationsHolleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5

Thiosulfate anion characteristically reacts with dilute acids to produce sulfur, sulfur dioxide and water:

S₂O₃²⁻(aq) + 2H⁺ (aq) → S(s) + SO₂(g) + H₂O(l)

This reaction has been employed to general colloidal sulfur. When the protonation is conducted at low temperatures, H₂S₂O₃ can be obtained. It is a strong acid pK_a = 0.6, 1.7.

Iodometry

Perhaps most notably in the laboratory, the thiosulfate anion reacts stoichiometrically with iodine, reducing it to iodide as it is oxidized to tetrathionate:

2S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2I⁻(aq)

Due to the quantitative nature of this reaction, as well as the fact that Na₂S₂O₃•5H₂O has an excellent shelf-life, it is used as a titrant in iodometry. Na₂S₂O₃•5H₂O is also a component of iodine clock experiments.

Photography

The "planetary" sulfur atom in S₂O₃^2- binds to "soft" metals with high affinity. Thus it dissolves silver halides, e.g. AgBr, which is a component of photographic emulsions:

2 S₂O₃^2- + AgBr → ^*^3-) + Br^-

In this application, discovered by John Herschel, sodium thiosulfate is known as a fixer and is used for both film and paper. It is also used in the tanning of leather, in chemical manufacture, in bacteriological water assessment, and in treating cyanide poisoning.

Other uses

This substance is also being used for automatic hand warmers. It is also used in photography as a fixing agent.
Sodium thiosulfate can also be used 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 dithionate.
Sodium thiosulfate can be used to dechlorinate tap water for aquariums or treat effluents 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 (crystaline) sodium thiosulfate per 10 liters of water.
It is used to lower chlorine levels in swimming pools and spas following super chlorination.
It can also be used to remove iodine stains, e.g. after the explosion of nitrogen triiodide.

References