Which substance is commonly used to decrease pH in water treatment?

The substance that is commonly used to decrease pH in water treatment is citric acid. Citric acid is a weak organic acid that is effective in lowering the pH of water when added. It acts by donating hydrogen ions (H+) to the water, which results in a decrease in the overall pH level. This property makes citric acid beneficial in various water treatment processes where acidity adjustments are necessary to meet regulatory standards or to create optimal conditions for further treatment processes.

Citric acid's biodegradable nature and relative safety compared to stronger acids also contribute to its use in water treatment, making it an environmentally friendly option for pH adjustment. It is often preferred in situations where a gentle, controlled pH reduction is required.

In contrast, the other substances listed do not serve this specific purpose effectively. For example, calcium carbonate and sodium carbonate are typically used to increase pH rather than decrease it, making them unsuitable for this particular application. Vinegar, while it does contain acetic acid and can lower pH, is less commonly utilized in large-scale water treatment compared to citric acid, which is often chosen for its effectiveness and safety.