What chemical is always present in the sample bottle for the coliform test?

In the coliform test, sodium thiosulfate is used to neutralize any residual chlorine in the water sample. This step is crucial because chlorine is a disinfectant that can kill coliform bacteria and other microorganisms present in the sample, leading to false-negative results. By adding sodium thiosulfate, you ensure that any chlorine does not interfere with the growth of coliform bacteria in the incubation stage of the test.

The other chemicals listed do not serve this specific purpose in the context of the coliform test. Chlorine is the substance that is being neutralized, calcium chloride is typically used for other purposes in water treatment and does not relate directly to the coliform testing process, and potassium dichromate is commonly used in analytical chemistry for different types of testing, such as measuring chemical oxygen demand, but is not a standard part of coliform testing. Hence, identifying sodium thiosulfate as the chemical that must be present for accurate coliform testing is the key to understanding this procedure.