Free Chlorine vs Total Chlorine: What Is the Difference?
Quick answer: what is the difference between free and total chlorine?
Free chlorine is the chlorine available mainly as hypochlorous acid and hypochlorite ion, while total chlorine includes free chlorine plus combined chlorine. Combined chlorine is chlorine that has reacted with substances such as ammonia to form chloramines.
Basic relationship
Total chlorine = free chlorine + combined chlorine.
Why free chlorine matters
Free chlorine is commonly monitored because it represents the more immediately available disinfecting residual in many chlorinated-water systems.
Why total chlorine matters
Total chlorine is useful when chloramines may be present or when a process needs to know the complete measurable chlorine residual.
Why the difference matters for RO pretreatment
Many polyamide RO membranes are sensitive to oxidants. Chlorine control upstream of the membranes should therefore be based on the membrane supplier’s limits and actual oxidant chemistry. A single total-chlorine or free-chlorine reading should be interpreted in the context of the disinfectant being used.
Testing considerations
- Use the correct test method for free or total chlorine.
- Check the kit’s range and resolution.
- Test promptly because chlorine residual can change after sampling.
- Follow the reagent and timing instructions exactly.
Frequently asked questions
Can total chlorine be lower than free chlorine?
Under correct measurement, total chlorine should not be lower than free chlorine because total includes the free fraction.
Is combined chlorine simply total minus free?
Yes, operationally it is commonly estimated as total chlorine minus free chlorine.
Which chlorine test should an RO plant use?
Use the parameter and method required by the membrane supplier and pretreatment design, and verify whether free chlorine, total chlorine or another oxidant measure is relevant.
Related ABHIRO resources
See ABHIRO’s Free Chlorine Test Kit, Total Chlorine Test Kit, Chloroscope Test Kit and Knowledge Centre.
