DPD as a Reference Measurement

The most common and international recognised reference measurement for disinfectants is DPD (ISO 7393-2:2017).

DPD (N,N Diethyl-1,4 Phenylenediamine) is a reagent that reacts directly with disinfectants (e.g. chlorine, chlorine dioxide, ozone, etc.) to produce a magenta colored solution. The intensity of this colored solution is proportional to the concentration of disinfectant in the sample. The more disinfectant present, the darker the magenta color produced. This type of measurement is commonly called a colorimetric measurement.

DPD measurements require a "grab sample" of process water, collected in a sample vial. The DPD reagent is then added to the vial, mixed, and inserted into a colorimeter for measurement.

One drawback of online DPD measurements is that the sample collected for measurement cannot be returned back to the process due to the addition of reagents needed. This requires the need for a chemical waste stream, adding to the lifecycle cost of colorimetric measurements.

DPD measurements are also subject to several chemical and physical interferences that are commonly found in process water. If the sample contains particulate matter, or is not colorless, the DPD measurement may not be accurate since it depends on a light with a specific wavelength passing through the sample and being detected by the photometer/colorimeter.

The DPD reaction is also non-specific; the colored product will form when DPD reacts with free chlorine,  chlorine dioxide, ozone, and peroxides. Thus, it can be difficult to interpret DPD measurement results if there is more than one disinfectant species present. DPD will also react with any oxidising component (light, oxygene etc.) that may be present in the testing solutio. Manganese or iron, for example, will produce a false positive result. These chemical and physical interferences can be difficult to control or correct for, limiting the use of DPD in many applications.

For most users, DPD measurements are used as a reference measurement that provide the "starting point" for the calibration of their amperometric sensors. A poor calibration caused by an erroneous DPD measurement will impact the quality of the amperometric measurement until the next calibration.

Thus, it is important to be aware of the limitations of DPD as a reference measurement, and how to correct for them if possible. Consider the conditions of your process: is your water cloudy? Does it contain any other disinfectants or interfering species? If there is a commercially-available method to eliminate an interference (e.g. a different reagent set that does not react with the interference), consider using that as your reference measurement instead. Even seemingly insignificant errors can turn into large errors in the long run. Pleae read thoroughly the manufacturer´s manual of your photometer choice before measurement. 

If you aren't sure whether something in your process could be impacting your reference measurement, please contact a Kuntze representative.