Which chemical is commonly used in de-chlorinate tablets?

Sodium thiosulfate is commonly used in de-chlorinate tablets due to its effectiveness in neutralizing chlorine present in water. When sodium thiosulfate is added to chlorinated water, it reacts with chlorine to form harmless byproducts, effectively removing residual chlorine from the water. This process is essential in various applications, especially when treating drinking water to ensure that chlorine levels are reduced to safe limits before distribution or use.

Each of the other options has specific uses in water treatment but does not serve the same purpose as sodium thiosulfate. For instance, calcium hypochlorite is a source of chlorine used for disinfection, chlorine dioxide is used for disinfection but is not a de-chlorinator, and potassium permanganate serves as an oxidant in water treatment processes but is not effective for removing chlorine. By understanding the distinct roles of these chemicals, one can see why sodium thiosulfate is the correct answer when it comes to de-chlorination.