Which component is essential for effective sampling to prevent false negatives for chlorine levels?

Sodium thiosulfate is essential for effective sampling when measuring chlorine levels because it acts as a dechlorinating agent. When chlorine is present in a sample, it can lead to false negatives in testing if there is a delay in analysis. This is particularly important when the sample will not be immediately analyzed, as free chlorine can dissipate over time, affecting the accuracy of the test results.

By adding sodium thiosulfate to the sample, the chlorine is neutralized, ensuring that any residual chlorine does not interfere with the sampling process. This allows for a more accurate representation of chlorine levels in the water, reflecting the true conditions at the time of sampling, and enhancing the reliability of the monitoring process.

The other substances mentioned do not serve this specific purpose. Hydrochloric acid, for example, can alter the pH of the water and could potentially lead to further chemical reactions that distort chlorine measurements. Calcium chloride and magnesium sulfate are also not relevant in this context as they do not modify or stabilize chlorine levels in a way that would prevent false negatives while sampling.