What unit is commonly used to express chlorine residual in water?

The unit mg/L, which stands for milligrams per liter, is commonly used to express chlorine residual in water. This unit indicates the concentration of chlorine present in the water and is equivalent to parts per million (ppm) in the context of water quality, making it particularly suitable for measuring low concentrations of contaminants. Chlorine residual is critical for ensuring drinking water safety, and expressing it in mg/L allows for precise monitoring and regulation of disinfection levels in water distribution systems.

While other options may be relevant in different contexts—such as grams per liter for more concentrated solutions or milliliters for volume measurements—they are not standardized for reporting chlorine residuals in water. Therefore, mg/L is the most appropriate and widely recognized unit for this purpose.