Which of the following is a common biocide used in cooling water treatment?

Sodium hypochlorite is commonly used as a biocide in cooling water treatment due to its effective antimicrobial properties. It acts as a chlorine compound that can oxidize and disinfect microorganisms, including bacteria, algae, and fungi, which often proliferate in cooling systems. By controlling these organisms, sodium hypochlorite helps prevent biofilm formation and ensures the efficiency and longevity of the cooling system.

The other substances listed do not serve as biocides. Potassium nitrate, for example, is often used as a fertilizer and does not have the germicidal properties required for effective microbial control in water systems. Calcium carbonate is primarily involved in scaling and hardness in water, rather than serving as a biocide. Sodium sulfate is typically used for different purposes, such as in chemical processes or as a drying agent, and does not function to eliminate or control microbial growth.