Thiosulfate is a sulfur-containing chemical anion that plays a diverse role across medicine, industry, and analytical chemistry. It is primarily known as a salt, such as sodium thiosulfate, a white crystalline solid that is highly soluble in water. This compound is used in applications ranging from life-saving medical treatments to developing photographs.
Chemical Identity and Properties
The thiosulfate anion has the chemical formula S2O3(2-) and is considered a sulfur oxyanion. Its structure can be visualized as a sulfate ion (SO4(2-)) where one oxygen atom has been replaced by a sulfur atom. This arrangement results in a tetrahedral shape with two chemically distinct sulfur atoms existing in different oxidation states.
Thiosulfate acts as a moderate reducing agent, readily donating electrons to other substances in a chemical reaction. This property is central to many of its practical uses, particularly in neutralizing oxidizing agents like halogens. In the presence of strong oxidizing agents, the thiosulfate ion is converted into tetrathionate (S4O6(2-)).
Thiosulfate stability depends on its environment. While solutions are stable in neutral or basic conditions, it rapidly decomposes upon acidification. When a strong acid is added, the thiosulfate ion breaks down into elemental sulfur, which appears as a cloudy precipitate, and sulfur dioxide gas.
Essential Roles in Human Health
Sodium thiosulfate (Na2S2O3), the most common form, is a long-established medication primarily recognized as an antidote for cyanide poisoning. It works as a sulfur donor, catalyzed by the enzyme rhodanese. This reaction converts the highly toxic cyanide ion (CN-) into thiocyanate (SCN-), which is approximately 200 times less toxic and can be safely excreted.
Thiosulfate is also used to mitigate the severe side effects of the chemotherapy drug cisplatin. Cisplatin causes damage to the kidneys (nephrotoxicity) and hearing loss (ototoxicity), often limiting the dose a patient can receive. Thiosulfate protects healthy cells through two mechanisms: it directly binds to and inactivates cisplatin, and it boosts antioxidant defenses to reduce oxidative stress. Concurrent administration allows for higher, more effective doses of cisplatin while protecting renal function and hearing.
Thiosulfate also treats calciphylaxis, a rare condition involving painful calcification of small blood vessels, often seen in patients with end-stage kidney disease. Though the exact mechanism is not fully understood, it is believed to chelate calcium ions. This forms a more soluble calcium thiosulfate complex that is easily cleared from the body, helping to dissolve deposits and reduce pain.
Common Industrial and Environmental Applications
Thiosulfate compounds have a long history of use outside of medicine, particularly in traditional film photography. Sodium thiosulfate, known as “hypo,” functions as the chemical “fixer.” After development, thiosulfate dissolves the remaining, unexposed silver halide crystals, such as silver bromide. This process stabilizes the image, making it permanent and preventing further darkening upon exposure to light.
In environmental and water treatment applications, thiosulfate is widely employed as a dechlorinating agent. Municipal water treatment plants and laboratories use it to neutralize residual chlorine or chloramine before discharging water or analyzing samples. It acts as a reducing agent to convert the chlorine disinfectant into harmless chloride ions. Aquarists also add sodium thiosulfate to tap water before using it in fish tanks to protect aquatic life.