Free and Total Chlorine Measurement by DPD Colorimetry

How does the DPD method work?

In the DPD (N,N-diethyl-p-phenylenediamine) method, DPD reagent is added to the sample. Free chlorine reacts rapidly with DPD to form a pink-red color. The color intensity is proportional to the chlorine concentration and is measured by a colorimeter at a specific wavelength.

For total chlorine measurement, iodide is added to the reagent, so chloramines also take part in the reaction and total active chlorine is measured. The difference between free and total chlorine gives the combined chlorine value. In online analyzers, the sample, reagents and measurement cell are managed automatically; reagent pumping, mixing and measurement are repeated periodically.

What the DPD method measures and where it is used

The method is used for online monitoring of free or total chlorine. Typical applications:

• Drinking water distribution and treatment plants
• Pools and water parks
• Process water in the food and beverage industry
• Cooling water and process water disinfection

Where the DPD method works well and where it struggles

Because the DPD method is widely used in laboratories, online results can be compared with hand tests. Separating free and total chlorine is important for managing the disinfection process and chloramine formation. It is based on a selective color reaction.

Limitations: the method consumes reagent; reagent and tubing must be replaced regularly. Oxidizing compounds such as manganese and chromium in the sample and high concentrations of other disinfectants can interfere. Because measurement runs in intermittent cycles, results are not instantaneous but are renewed at each measurement period. The sample's pH must be within a suitable range for the DPD reaction; a buffer is used for this purpose.

Choosing an instrument for the DPD method

When selecting, the parameter to be measured (free or total chlorine), sample pH, measurement range and output modules are assessed together. Reagent consumption and tubing replacement interval should be included in the operating plan. During commissioning, Tetra configures the sample line, reagent feed arrangement and alarm outputs according to site operating conditions, and it also takes on reagent and tubing replacement during regular maintenance.

Tetra and the DPD chlorine analyzer

Tetra supplies, installs, commissions, calibrates and services online chlorine analyzers in Türkiye. The product range includes the Thermo Orion Chlorine XP DPD analyzer, which measures free or total chlorine and offers pH and temperature options and 4–20 mA output modules. Tetra can also prepare pH and conductivity analysis systems that include chlorine measurement.