Definition - What does Combined Chlorine mean?
Combined chlorine refers to the mixture of chlorine with organic amines or ammonia. Combined chlorine forms when free chlorine has combined with other chemicals in the water, and is therefore unable to sanitize or purify the water. Essentially, the free chlorine has combined with water pollutants to form combined chlorine, ultimately losing its disinfectant properties.
Corrosionpedia explains Combined Chlorine
Combined chlorine is formed when chlorine is added to water, forming hypochlorous acid (HOCl) and hypochlorite ions (OCl-). Hypochlorous acid is a strong bacterial killing form of chlorine while hypochlorite ion is a weaker form of chlorine in water. When substances such as ammonia are added to a chlorinated water supply to provide inorganic chloramines, they react with both forms of chlorine to generate combined chlorine.
When combined chlorine rates are too high then negative effects such as irritation of the eyes, skin and mucous membranes may occur.
Combined chlorine is a source of pH imbalance in the water. This imbalance can create an environment for increased ion exchange, which promotes corrosion of any metallic components. Corrosion due to combined chlorine can be remediated by the addition of free chlorine.