While circulating heated sulfuric acid through a feed water heater of a flash evaporator for the purposes of scale removal, what would be the indication of the continued presence of scale?

Correct Answer: Option C — Change in color (turning to orange with a pH of 4.0) and continued venting of carbon dioxide gas.

Chemical descaling of flash evaporators uses acid to dissolve calcium carbonate. The reaction produces carbon dioxide gas. A color change to orange (pH 4.0) indicates the acid has been neutralized by the scale. Continued gas evolution confirms that active chemical reaction is still occurring, meaning scale remains present.