The factor used to determine the bromine residual with the chlorine test kit (DPD) is:

The correct approach to determine the bromine residual using a chlorine test kit, such as the DPD method, involves understanding the relationship between chlorine and bromine in water disinfection. Bromine is often used as a disinfectant in swimming pools and spa facilities, and its residual can be estimated based on the chlorine residual.

Bromine's effectiveness as a disinfectant is generally about 2.25 times that of chlorine, meaning that when you measure the chlorine residual, you can determine the bromine residual by multiplying the chlorine reading by this factor. This allows you to convert the chlorine residual measurement to an equivalent bromine level, since these two chemicals have different residuals due to their distinct chemical behaviors in water treatment processes.

This is why multiplying the chlorine residual by 2.25 is the correct method for determining the bromine residual when using a chlorine test kit like DPD. It not only provides an accurate estimation but also ensures that water treatment practices can be effectively monitored and managed for safety and health standards.