What substance must be present in plastic bottles that are used for collecting samples to test for coliforms?

The correct choice is sodium thiosulfate, which is essential in the context of collecting water samples for coliform testing. Coliform bacteria are indicator organisms used to evaluate water quality, particularly for fecal contamination. When sampling water, it is crucial to ensure that chemical agents do not inadvertently affect the test results.

Sodium thiosulfate serves a critical function by neutralizing chlorine or other oxidizing agents that may be present in the water sample due to disinfection processes. Chlorine can kill coliform bacteria, leading to false-negative results when testing. By including sodium thiosulfate in the sample bottle, you effectively preserve the integrity of the sample, allowing for an accurate assessment of the presence or absence of these indicator bacteria.

Other substances listed do not serve this specific purpose in water sample collection. For example, sodium bicarbonate might act as a buffer, but it does not neutralize chlorine. Potassium chloride and calcium sulfate have different applications and do not play a role in ensuring the viability of coliform bacteria during sampling. Thus, sodium thiosulfate is the substance required to maintain sample integrity for coliform analysis.