What preservative is commonly added to sample containers for total coliform samples?

The preservative commonly added to sample containers for total coliform samples is sodium thiosulfate. This compound plays a crucial role in ensuring the accuracy of microbiological testing by neutralizing chlorine present in the water samples. Chlorine is often used in water treatment to kill pathogens, but it can also inhibit the growth of bacteria in the sample, potentially leading to false negatives in laboratory analysis.

Adding sodium thiosulfate helps to eliminate the chlorine's toxic effects on coliform bacteria, allowing them to remain viable for testing. This is essential for determining the presence of coliforms, which are indicator organisms used to assess the microbiological quality of drinking water. When testing for total coliforms, maintaining sample integrity is vital, and sodium thiosulfate is effective in preserving that integrity under typical sample storage conditions.

In contrast, the other options don't serve this specific purpose. Potassium permanganate acts as an oxidizing agent, which could interfere with the detection of bacteria. Calcium chloride does not have preservative properties relevant to microbiological samples, while sodium chloride does not neutralize chlorine and would not protect the sample during transit to the laboratory.