Which chemical is primarily used for internal treatment at higher pressures in boilers?

The use of coordinated phosphate for internal treatment in boilers operating at higher pressures is essential for controlling the chemistry of boiler water and preventing scaling and corrosion. At elevated pressure conditions, the solubility of certain chemicals changes, and coordinated phosphate plays a pivotal role in maintaining a stable pH and preventing carbonate formation.

Coordinated phosphate helps in the formation of a protective layer on the internal surfaces of the boiler. This layer can guard against the corrosion caused by acidic conditions that can develop if not properly managed. Furthermore, coordinated phosphate works by precipitating hardness and other unwanted ions, thus contributing to better water quality and reducing the risks of deposit formation.

The other options, while relevant in different contexts, do not serve the same purpose under high-pressure conditions. Sodium nitrate, for example, is typically used for oxygen scavenging, and while calcium hydroxide can affect pH levels, it is more suited to lower pressures. Hydrochloric acid is generally used for cleaning and not for continuous treatment in high-pressure boiler systems. Thus, coordinated phosphate is clearly the most effective chemical for internal treatment under these specific operating conditions.