Why is sodium thiosulfate added to total coliform sample bottles?

Sodium thiosulfate is added to total coliform sample bottles primarily to dechlorinate the sample. In many water sources, especially in treated municipal water, chlorine is used as a disinfectant. However, chlorine can be toxic to certain microorganisms, including coliform bacteria, and can inhibit their growth. When analyzing samples for total coliforms, it is critical to ensure that any chlorine present in the sample does not interfere with the growth of these bacteria during incubation.

By adding sodium thiosulfate, the free chlorine in the sample is neutralized, allowing for a more accurate assessment of the coliform bacteria present. This step is crucial, as it ensures that any coliforms in the water sample can proliferate, giving a true representation of their levels. This practice is vital for accurate water quality testing and public health safety.