Free chlorine vs total chlorine vs monochloramine
What is the difference between free chlorine, total chlorine and monochloramine? Understand the measurements before selecting a drinking-water sensor.
Free chlorine and total chlorine are different measurements. Total chlorine includes free chlorine and combined chlorine; monochloramine is one combined-chlorine species. A total-chlorine reading alone does not identify monochloramine, so specify the disinfectant measurement your treatment process needs before choosing an online sensor.
Why the species matters
A residual-disinfectant result is useful only when you know what it represents. Free chlorine, total chlorine and monochloramine describe different aspects of the water chemistry. A sensor selected for one measurement should not be assumed to provide the others. Begin with the treatment process and the question the operator needs the measurement to answer.
What does a free chlorine measurement tell you?
Free chlorine refers to hypochlorous acid and the hypochlorite ion in water. Their relative proportions change with pH, which is one reason water chemistry matters to the sensor response. Review the instrument’s compensation approach and operating limits when comparing its installed reading with a reference measurement.
What does a total chlorine measurement include?
Total chlorine combines free and combined chlorine. It is useful to understand that aggregate result, but it does not by itself show the distribution of combined-chlorine species. Check what the instrument measures directly and what it calculates from other signals, and use those definitions consistently when reviewing site trends.
Why measure monochloramine separately?
A chloraminated supply may need species-specific information rather than an undifferentiated total-chlorine result. A sensor designed for monochloramine can help operators review that residual alongside the wider process and reference data. It should be selected for the actual water chemistry and used within the site’s broader monitoring programme.
Can you obtain monochloramine by subtracting free from total chlorine?
Do not assume that a difference between two chlorine results is a selective monochloramine measurement. Other combined species and method interference can affect the interpretation. Hach’s technical guidance specifically advises against using the difference between its total and free DPD tests to measure monochloramine. Use an appropriate species-specific method and its instructions when checking a monochloramine sensor.
Why product generations must stay separate
The supplied legacy MP6 guide describes free chlorine and monochloramine alongside a particular set of water-quality outputs. Current MP-TOTAL material describes a different parameter set. Product names alone do not establish equivalent performance, output availability or sensing architecture. PMA should confirm the exact model and revision against the measurements you need before supply.
Prepare the specification discussion
Tell PMA which disinfectant is used, the expected residuals, water source, pH and conductivity, and the purpose of the measurement. Include the current instrument and reference method, any treatment transitions and the required PLC or telemetry interface. That gives the selection discussion a practical basis and helps keep the sensor’s role clear.
Which page should you read next?
If the treatment process uses a free-chlorine residual, review the free chlorine sensor page. For a chloraminated supply, review the monochloramine analyser and application guidance. In either case, define the measurement first, then confirm the current sensor generation, operating envelope and installation with PMA.
