To drop the pH during the softening process, what should be added?

During the softening process, the goal is often to reduce the pH to facilitate the removal of certain dissolved minerals, such as calcium and magnesium. Adding carbon dioxide is effective for this purpose because it reacts with water to form carbonic acid, which lowers the pH of the water. This mild acidification can help maintain the desired chemical balance during the softening process, especially in processes involving precipitation.

When carbon dioxide is introduced, it will decrease the alkalinity of the water, allowing for better control of the pH levels. This is particularly useful because a lower pH can improve the efficiency of softening agents and aid in the overall treatment process.

In contrast, lime, sodium hydroxide, and hydrochloric acid have different effects on pH. Lime and sodium hydroxide would raise the pH, while hydrochloric acid would also lower pH but is not typically used in softening as a first approach due to the aggressive nature of strong acids compared to the gentler adjustment that carbon dioxide provides. Thus, for effectively lowering pH in the context of water softening, carbon dioxide is the appropriate choice.