Which of the following chemicals is commonly used in a phosphate program?

The correct response focuses on the significance of orthophosphate in water treatment processes, particularly in phosphate programs. Orthophosphate is a key component because it is utilized to control scale formation and corrosion in boilers and cooling systems. By adding orthophosphate to the water, it helps in the formation of a protective layer on the surfaces of the equipment, thereby minimizing corrosion and extending the lifespan of the systems. This ability to chelate with metal ions also aids in preventing the precipitation of hardness salts, promoting more efficient water use.

In auxiliary to the primary function, orthophosphate contributes to maintaining the desired phosphate levels in the water, ensuring optimal performance of the phosphate program. It's essential for maintaining the balance against iron and other metal ions that otherwise would cause scaling problems.

Other choices do not serve the same function in a phosphate program. Hydrochloric acid is typically used for pH adjustment or as a cleaning agent, calcium carbonate primarily serves as a buffering agent and for controlling scale, while sodium chloride is usually associated with water softening and not specifically for phosphate programs. Each of these chemicals has its own application, but none performs the unique functions that orthophosphate does within a phosphate program.