What is a precaution for the use of water baths?

Adding a phenolic detergent to the water bath is a precaution that helps prevent contamination. Water baths are often used to maintain consistent temperatures for samples, and during this process, they can also become breeding grounds for microorganisms if not properly maintained. The inclusion of phenolic detergents can help neutralize potential contaminants and provide a more sterile environment for the samples, ensuring accurate and safe laboratory results.

The other options present practices that either do not contribute positively to safety or standard operational procedures. For instance, using sodium azide, which is toxic, in the water bath poses serious health risks, while leaving the water bath plugged in when filling or emptying can create electrical hazards and increase the potential for accidents. Therefore, the precaution of adding a phenolic detergent is the most appropriate and safety-conscious choice.