For which sampling is sodium thiosulfate usually added?

Sodium thiosulfate is commonly added during sampling for heterotrophic plate count (HPC) analyses to neutralize any residual chlorine present in the water sample. Chlorine is a disinfectant that can inhibit the growth of bacteria, which would lead to an inaccurate representation of the microbial population present in the sample. By adding sodium thiosulfate, the potential negative effect of chlorine on microbial growth is mitigated, allowing for a more accurate assessment of the heterotrophic microorganisms in the water being tested.

In contrast, for other options such as UV254 and turbidity, the preservation methods and reagents used focus on maintaining the integrity of the samples for different purposes, and they do not require the neutralization of chlorine. For the 524 VOCs sampling, specific preservation techniques involving refrigeration and the use of specialized containers are utilized, rather than sodium thiosulfate.