Which chemical is used to control alkalinity in boiler feedwater?

Caustic soda, or sodium hydroxide, is the correct choice for controlling alkalinity in boiler feedwater. It functions by raising the pH level of the water, which helps to mitigate the corrosion potential of the boiler system. Maintaining proper alkalinity is vital for preventing issues such as scaling and corrosion, both of which can significantly affect the efficiency and longevity of boiler operation.

Calcium carbonate, while it can be part of the water treatment process, primarily serves as a source of calcium and is used more for hardness control rather than alkalinity. Sodium chloride is mainly used for water softening and does not directly influence alkalinity. Hydrochloric acid is used to lower pH and can combat high alkalinity, but it creates opposite conditions that are not suitable for the purpose of maintaining alkalinity in boiler systems. Therefore, caustic soda is employed for effective control of alkalinity, promoting a stable operating environment for the boiler.