Potassium thiosulfate

Potassium thiosulfate is an inorganic compound with the formula K₂S₂O₃. This salt can form multiple hydrates, such as the monohydrate, dihydrate, and the pentahydrate, all of which are white or colorless solids.^[1] It is used as a fertilizer.

Potassium thiosulfate

Names
Other names Dipotassium monothiosulfate
Identifiers
CAS Number	10294-66-3
3D model (JSmol)	Interactive image
ChemSpider	55421
ECHA InfoCard	100.030.593
EC Number	233-666-8
PubChem CID	61501
UNII	UK1TD58L5O
CompTox Dashboard (EPA)	DTXSID70893108
InChI InChI=1S/2K.H2O3S2/c;;1-5(2,3)4/h;;(H2,1,2,3,4)/q2*+1;/p-2 Key: FGRVOLIFQGXPCT-UHFFFAOYSA-L
SMILES [O-]S(=O)(=S)[O-].[K+].[K+]
Properties
Chemical formula	K2S2O3
Molar mass	190.32 g/mol
Appearance	White solid
Density	2.37 g/cm3
Solubility in water	96.1 g/100 ml (0 °C) 155.4 g/100 ml (20 °C) 165 g/100 ml (25 °C) 175.7 g/100 ml (30 °C) 204.7 g/100 ml (40 °C) 215.2 g/100 ml (50 °C) 238.3 g/100 ml (60 °C) 255.2 g/100 ml (70 °C) 293.1 g/100 ml (80 °C) 312 g/100 ml (90 °C)[1]
Hazards
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319
Precautionary statements	P264, P280, P302+P352, P305+P351+P338, P321, P332+P313, P337+P313, P362
Related compounds
Other anions	Potassium sulfite; Potassium sulfate
Other cations	Sodium thiosulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Formation and reactions

Main article: Thiosulfate

Thiosulfate salts are produced by the reaction of sulfite ion with elemental sulfur, and by incomplete oxidation of sulfides. For example, this salt is produced by reacting potassium hydroxide with ammonium hydroxide, sulfur dioxide, and elemental sulfur.^[2] Thiosulfates are stable in neutral or alkaline solutions, but not in acidic solutions, due to disproportionation to sulfur dioxide and sulfur:^[3]

S
₂O²⁻
₃ + 2 H⁺ → SO₂ + "S" + H₂O

Due to this property, it can sequester metals, especially iron.^[2]

Thiosulfate reacts with iodine to give tetrathionate, in this case potassium thiosulfate reacts with iodine to produce potassium tetrathionate and potassium iodide:

2 K₂S
₂O
₃ + I₂ → K₂S
₄O
₆ + 2 KI

Thiosulfate extensively forms diverse complexes with transition metals. In the era of silver-based photography, thiosulfate was consumed on a large scale as a "stop" reagent. This application exploits thiosulfate's ability to dissolve silver halides. Thiosulfate is also used to extract or leach gold (sodium thiosulfate) and silver from their ores as a less toxic alternative to cyanide.^[3]

Uses

Potassium thiosulfate is commonly used as a fertilizer alone or with urea and/or urea ammonium nitrate^[4] due to its ability to delay nitrification.^[2] It thus has the ability to reduce the emission of nitrous oxide.^[5] It can also reduce the amount of fumigants being released from the soil.^[6] If used alone it is used in very dilute solution due to its ability to cause phytotoxicity symptoms. This is caused by the elemental sulfur being oxidized to produce sulfuric acid.^[4]

References

1. Atherton Seidell (1919). Solubilities of inorganic and organic compounds c. 2. D. Van Nostrand Company. p. 568.
2. Sulewski, Gavin; Thompson, Michael; Mikkelsen, Robert; Norton, Robert; Scott, T., eds. (2020). Improving Potassium Recommendations for Agricultural Crops (Ebook). Springer International Publishing. p. 60. ISBN 978-3-030-59197-7. Retrieved 6 October 2021.
3. Barbera, J. J.; Metzger, A.; Wolf, M. (2012). "Sulfites, Thiosulfates, and Dithionites". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a25_477. ISBN 978-3-527-30673-2.
4. Western Plant Health Association (2018). Barlow, Dave; Pier, Jerome (eds.). Western Fertilizer Handbook (Ebook) (Third Horticulture ed.). Waveland Press. p. 186. ISBN 978-1-4786-3884-1. Retrieved 6 October 2021.
5. Zejiang Cai; Suduan Gao; Minggang Xu; Bradley D Hanson (2017). "Evaluation of potassium thiosulfate as a nitrification inhibitor to reduce nitrous oxide emissions". Science of the Total Environment. 618: 243–249. doi:10.1016/j.scitotenv.2017.10.274. PMID 29128773.
6. Ruijun Qin; Suduan Gao; Jason A McDonald; Husein Ajwa; Shachar Shem-Tov; David A Sullivan (2008). "Effect of plastic tarps over raised-beds and potassium thiosulfate in furrows on chloropicrin emissions from drip fumigated fields". Chemosphere. 72 (4): 558–563. Bibcode:2008Chmsp..72..558Q. doi:10.1016/j.chemosphere.2008.03.023. PMID 18440581.