Water used to dilute sodium fluoride solutions should be softened if the hardness exceeds what level?

The correct choice is based on general guidelines regarding the reaction of calcium and magnesium ions found in hard water with certain chemicals, such as sodium fluoride. When the water hardness exceeds 75 mg/L as calcium carbonate, it can lead to the formation of precipitates when mixed with sodium fluoride. Such precipitates can affect the effectiveness of the fluoride treatment and reduce the overall quality of water being treated.

Softening the water before diluting sodium fluoride ensures that the potential for precipitation is minimized, which helps maintain the intended concentration of fluoride in the solution. This is critical in water treatment processes where precise chemical doses are necessary to achieve desired water quality standards while preventing unwanted side effects.

Understanding the specific hard water threshold is essential for operators in water treatment facilities to maintain compliance with health standards and optimize the effectiveness of added treatments like sodium fluoride. This helps ensure safe and effective consumption quality for the public.