CWEA Grade 3 Lab Analyst Practice Exam 2026 – Complete Prep Guide

The preservation requirement for coliform sampling is that sodium thiosulfate is added to the samples. This is crucial because sodium thiosulfate serves to neutralize any residual chlorine that may interfere with the analysis of coliform bacteria. Chlorine is a common disinfectant used in water treatment, and its presence can kill bacteria, leading to false negative results in coliform tests. By adding sodium thiosulfate, the integrity of the sample is preserved, ensuring that the bacterial population is not altered prior to analysis.

In the context of ensuring accurate coliform counts, this preservation step is essential for obtaining reliable data, as it allows for the detection of the true microbial content of the water sample. Other methods, like adjusting pH or using amber glass, do not address the critical issue of chlorine interference, and therefore do not fulfill the preservation needs required for accurate coliform sampling.