What Is Thiosulfuric Acid?
Thiosulfuric acid is a sulfur oxoacid, composed of hydrogen and thiosulfuric ions.
With the chemical formula H2S2O3, this divalent inorganic acid has a molecular weight of 114.14. Its acid dissociation constants are pKa1 = 0.6 and pKa2 = 1.6. The salts of thiosulfuric acid, including those of ammonia, alkali, and alkaline earth metals, are highly soluble in water and relatively stable in neutral or alkaline solutions.
Isolating free thiosulfuric acid is challenging, as it decomposes into colloidal sulfur and sulfur dioxide in strongly acidic solutions. While it has been isolated as a diethyl ether complex at low temperatures and in liquid carbon dioxide, these are specialized conditions.
The free acid is highly unstable, decomposing into hydrogen sulfide and sulfur trioxide, even at low temperatures. Higher temperatures accelerate its decomposition into sulfur, sulfur dioxide, and water. In air, its decomposition products are more complex, reacting with water and oxygen to form additional compounds like sulfuric acid and polythionic acid.
Uses of Thiosulfuric Acid
Due to its instability, thiosulfuric acid is not used in its raw form industrially but rather as a salt.
For instance, anhydrous sodium thiosulfate is utilized as a dechlorinating agent, dyeing aid, chrome tanning reducer for leather, neutralizer for ballast water chemicals, bathing agent, and disposable body warmers. Both crystalline and aqueous solutions of sodium thiosulfate serve similar purposes.
Thiosulfuric acid is also used in synthesizing high-value amino acids like cysteine in bacterial bodies.
Principle of Thiosulfuric Acid
Thiosulfuric acid and its ions often act as redox and chelating agents for metals. A typical redox reaction involves the reduction of halogens, where the thiosulfuric acid ion converts to a sulfate ion, making sodium thiosulfate a useful titrant in iodine titration.
As a chelating agent, it solubilizes poorly soluble silver halides into silver thiosulfate complexes, aiding in the photo fixation process. In acidic solutions, thiosulfuric acid decomposes into sulfur dioxide, combining sulfur with sulfurous acid.
Types of Thiosulfuric Acid
Typically sold in salt form due to the difficulty of isolating the free acid, common types include sodium thiosulfate, magnesium thiosulfate, ammonium thiosulfate, and barium thiosulfate. These are available in various laboratory-friendly volumes like 25 g, 100 g, and 500 g.
The most prevalent product is sodium thiosulfate, available both as a solid and in aqueous solutions. For industrial use, “anhydrous sodium thiosulfate” is provided in larger quantities, such as 25 kg.