Chlorine Measurement: Free and Total Chlorine Analyzers
Free and total chlorine measurement in water and process lines: DPD colorimetric online chlorine analyzers for drinking water, cooling water and treatment.
Measurement methods: DPD Colorimetry
What is chlorine and why is it measured?
Chlorine is the most widely used chemical in water disinfection. Free chlorine is the part present in water as hypochlorous acid and hypochlorite ion, which inactivates microorganisms; total chlorine also includes chloramines formed by free chlorine with nitrogen compounds. In drinking water networks, chlorine is monitored continuously to maintain an adequate disinfection residual and avoid overdosing. Keeping biological growth under control in cooling towers and checking chlorine residual after disinfection in wastewater discharge are also main purposes.
Where is chlorine measured?
- Drinking and domestic water treatment plants and networks: In disinfection control.
- Cooling water systems: In tracking biocide dosing.
- Wastewater treatment outlets: In chlorination and dechlorination control.
- Process and pool waters: In hygiene and quality monitoring.
How is chlorine measured?
The common method in online chlorine analyzers is DPD colorimetry. When DPD reagent reacts with chlorine, it forms a pink-red color; the color intensity is measured with a colorimeter to calculate chlorine concentration. The reagent combination is changed for free and total chlorine; additional sensor options are available to monitor the effect of pH and temperature in free chlorine measurement. The analyzer automatically doses the sample and reagent and transmits the output through 4–20 mA modules.
Choosing a chlorine analyzer
- Free chlorine or total chlorine
- Expected range and sample pH
- Reagent consumption, renewal time and maintenance
- Sample filtration on dirty lines such as cooling water
Chlorine measurement with Tetra
Tetra supplies the Thermo Orion Chlorine XP DPD chlorine analyzer in Türkiye and provides installation, supply of reagents and consumables, commissioning, calibration and service. Tetra panel systems combining conductivity, pH and chlorine analyzers for cooling water are prepared on a project basis.
Frequently asked questions
What is the unit of chlorine measurement?
Chlorine is generally expressed in mg/L Cl₂; at low concentrations, mg/L and ppm are used interchangeably in practice. Free chlorine and total chlorine are reported separately, and the difference is assessed as combined chlorine (chloramines).
What is the difference between the DPD method and an amperometric chlorine sensor?
The DPD method uses a reagent and measures color formation photometrically; it makes periodic measurements and is relatively little affected by pH changes. Amperometric sensors measure continuously without reagent, but sample pH, flow velocity and the condition of the sensor membrane affect the measurement; these sensors need checking regularly against a DPD reference measurement.
What errors are common in chlorine measurement?
In the DPD method, oxidizing substances such as manganese and chromium can falsely raise the reading. In free chlorine measurement, if the reading is delayed, chloramines can also react and raise the result. A long sample line or biofilm forming in the line can cause chlorine to be consumed before it reaches the measurement point, resulting in a low reading.
How is an online chlorine analyzer verified with a portable measurement?
A sample taken at the same time from the point the analyzer measures is measured with a portable DPD photometer, and the results are compared. If there is a systematic difference, the condition of the reagents and sample line is checked first, and calibration is then adjusted if necessary. Recording this comparison at regular intervals is good practice.
Products measuring this parameter
Let's choose the right product together
Tell us about your application; we will recommend the right model and configuration for the measuring point, process conditions and regulations.