Final answer:
Chlorine residual at the point of production and distribution involves free and combined chlorine species, measured to ensure adequate water disinfection. The total chlorine residual is determined by specific tests to maintain water quality through production and distribution.
Step-by-step explanation:
When discussing chlorine residual at the point of production and distribution, two types of residuals are considered: the free chlorine residual and the combined chlorine residual. The former includes disinfecting species like Cl2, HOCl, and OCl-, while the latter consists of reduced forms such as NH2Cl, NHCl2, and NCl3 that no longer have disinfecting capability. The total chlorine residual is measured by the oxidation of I- to I3- and subsequent titration. This measurement is crucial for ensuring that the correct amount of chlorine is added during water treatment, which is vital for maintaining water quality through the production and distribution processes.
In the context of water treatment, chlorine is added to a water supply to disinfect it. The amount of chlorine added must be sufficient to react with any oxidizable substances and produce the desired residual for effective disinfection. Various chlorine-containing compounds, such as hypochlorite solutions, dichlor, and trichlor, are often used as alternatives to chlorine gas for practical and safety reasons. This treatment ensures safe water for various uses, including in pools where a maximum of 3.0 ppm is recommended to maintain sanitary conditions.