Thiosulfates

Thiosulfate anion

Thiosulfates are derivatives of thiosulfuric acid H ₂ S ₂ O _{3, which is} unstable in the free state . Their salts contain the thiosulfate anion S ₂ O ₃²⁻ . Thiosulfate occurs naturally and is produced in certain biochemical processes, e.g. B. during bacterial sulfate reduction .

properties

The prefix “ thio -” means that in comparison to the sulfate anion (SO ₄²⁻ ) in the thiosulfate anion, one oxygen atom has been replaced by a sulfur atom. The thiosulfate ion is tetrahedral and has a C _3v symmetry. The distance between the sulfur atoms corresponds approximately to a SS single bond, while the distance between the central sulfur atom and the oxygen atoms is comparable to the distances in the sulfate anion. It follows that although oxygen is more electronegative, the peripheral sulfur atom carries a negative charge. The second negative charge is distributed over the three remaining oxygen atoms. Correspondingly, in thiosulfuric acid, one proton is bound to the peripheral sulfur atom, the other to one of the oxygen atoms.

Aqueous solutions of thiosulphates are stable in a basic medium . In the presence of acids, due to the instability of the free thiosulfuric acid, they slowly disintegrate into sulfur , sulfur dioxide and water:

{\ displaystyle \ mathrm {S_ {2} O_ {3} ^ {2 -} + 2 \ H ^ {+} \ longrightarrow SO_ {2} + H_ {2} O + S \ downarrow}}

Complex formation occurs with iron (II) ions. This reaction is well suited to demonstrate the dependence of the reaction rate on the temperature and the (substance) concentration of the reactants with the help of a stopwatch and a sight .

use

The most important thiosulfate in chemistry and technology is the sodium salt and ammonium salt, also known as the fixing salt . In analog photography, it is used to fix the exposed materials (films, photo paper, color and black-and-white material) in the darkroom , as it dissolves the unexposed and thus undeveloped silver bromide as a soluble dithiosulfatoargentate complex. In the case of colored material, the reduced metallic silver is also removed by bleaching so that the emulsion is free of silver after processing.

Thiosulfate can u. a. (like sulfate) act as a monodentate as well as a bidentate chelating ligand :

{\ displaystyle \ mathrm {Na_ {2} S_ {2} O_ {3} + AgBr \ longrightarrow Na [Ag (S_ {2} O_ {3})] + NaBr}}

{\ displaystyle \ mathrm {2 \ Na_ {2} S_ {2} O_ {3} + AgBr \ longrightarrow Na_ {3} [Ag (S_ {2} O_ {3}) _ {2}] + NaBr}}

Thiosulfate can also be used as a reducing agent for halogens .

When thiosulphate reacts with the weak oxidizing agent iodine , the tetrathionate is formed with oxidative (SS) linkage :

{\ displaystyle \ mathrm {2 \ S_ {2} O_ {3} ^ {2 -} + I_ {2} \ longrightarrow 2 \ I ^ {-} + S_ {4} O_ {6} ^ {2-}} }

This reaction is used in iodometry , a method of determination in analytical chemistry .

Medical application

Sodium thiosulfate has been successfully tested to prevent side effects from cisplatin. Chemotherapy with cisplatin can lead to permanent hearing impairment in children with hepatoblastoma. If sodium thiosulfate was infused 6 hours after chemotherapy, hearing impairment occurred less frequently.

proof

Sequence of the course of the reaction

In the presence of an excess of silver ions , white silver thiosulphate is first precipitated. This then disproportionates to sulfide and sulfate. The sulfide ions form a black precipitate with the silver ions, so that the precipitate of silver thiosulfate changes color from white to yellow-orange and brown to black. This is used to detect thiosulfate ions. This reaction is also known as inorganic sunset , Abendrotreaktion or sunset reaction , referred to as the color change of the reaction is similar to that of the sunset.

{\ displaystyle \ mathrm {S_ {2} O_ {3} ^ {2 -} + 2 \ Ag ^ {+} \ longrightarrow Ag_ {2} S_ {2} O_ {3} \ downarrow}}

1st step: Thiosulfate is immediately precipitated as white silver thiosulfate by silver ions.

{\ displaystyle \ mathrm {Ag_ {2} S_ {2} O_ {3} + H_ {2} O \ longrightarrow Ag_ {2} S + H_ {2} SO_ {4}}}

Step 2: The silver thiosulphate slowly disintegrates (disproportionated) in the presence of water into silver sulphide and sulfuric acid .

If, however, thiosulfate is added in excess, the silver ions react immediately to form the above-mentioned dithiosulfatoargentate complex.

Web links

Wikibooks: Internship Inorganic Chemistry / Thiosulfate - Learning and teaching materials

Individual evidence

^ ^A ^b ^c ^d Wiberg, Egon., Wiberg, Nils ,: Textbook of Inorganic Chemistry . 102nd, heavily reworked and verb. Ed. De Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 .
↑ Blasius, Ewald, Jander, Gerhart: Jander / Blasius inorganic chemistry. 2 Quantitative Analysis and Preparations . 16., completely reworked. Edition Hirzel, Stuttgart 2012, ISBN 3-7776-2133-1 .
↑ Sodium Thiosulfate for Protection from Cisplatin-Induced Hearing Loss | NEJM. Retrieved December 17, 2018 .
↑ Jander, Gerhart, Blasius, Ewald: Jander / Blasius inorganic chemistry. 1 Introduction and qualitative analysis: with 21 formulas and 79 tables and poster "Pocket folder" . 17., completely reworked. Edition Hirzel, Stuttgart 2012, ISBN 3-7776-2134-X .

[:0-1] A ^b ^c ^d Wiberg, Egon., Wiberg, Nils ,: Textbook of Inorganic Chemistry . 102nd, heavily reworked and verb. Ed. De Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 .

[2] Blasius, Ewald, Jander, Gerhart: Jander / Blasius inorganic chemistry. 2 Quantitative Analysis and Preparations . 16., completely reworked. Edition Hirzel, Stuttgart 2012, ISBN 3-7776-2133-1 .

[3] Sodium Thiosulfate for Protection from Cisplatin-Induced Hearing Loss | NEJM. Retrieved December 17, 2018 .

[4] Jander, Gerhart, Blasius, Ewald: Jander / Blasius inorganic chemistry. 1 Introduction and qualitative analysis: with 21 formulas and 79 tables and poster "Pocket folder" . 17., completely reworked. Edition Hirzel, Stuttgart 2012, ISBN 3-7776-2134-X .