Question 367Steam Plants - Assistant Engineer

The correct answer is C) Sodium phosphate. Sodium phosphate is commonly used to precipitate calcium minerals, such as calcium carbonate and calcium sulfate, out of boiler water. These minerals can form scale on the inside of boiler tubes, reducing heat transfer efficiency and potentially leading to damage. By adding sodium phosphate to the boiler water, it reacts with the dissolved calcium ions to form an insoluble calcium phosphate precipitate that can be removed, preventing scale buildup. The other options are incorrect because: A) Sodium hydroxide is used to raise the pH of boiler water, but does not specifically precipitate calcium minerals. B) Caustic soda is another name for sodium hydroxide. D) Phenolphthalein is an indicator dye used to determine pH, not a chemical used for water treatment.