Public Pool Operators Now Have More Options for DPD Testing

Guest Blog by Joe Sweazy and Lara Winter, Hach Company - WAHC Sponsor

HACH/DPD ProWater chemistry in public pools and spas has regained focus in recent years as more relevant data has become available. According to a recent CDC
report, inspection data from pools and hot tubs in 16 jurisdictions across the United States at various aquatic venues indicated that 12.3% of Minseptaections resulted in immediate closure because of the seriousness of identified violations. Violations regarding the following issues are frequently identified: free chlorine level (11.9% of inspections), proper pH level (14.9%), other water chemistry (12.5%).1 This highlights the risk that improperly maintained water can quickly lead to infections and recreational water illnesses.


Regular water testing is critical to ensure an adequate level of disinfectant in the water, and to ensure that other parameters are in balance enabling the disinfectant’s effectiveness. For years N, N-Diethyl-p-Phenylenediamine (more commonly referred to as DPD) has been preferred over orthotolidine (or OTO) and other methods for detecting chlorine. DPD detects both total chlorine and free chlorine, providing assurance that the ideal form of chlorine, free available chlorine (FAC) is present. Furthermore, many public pool regulations specifically call out the DPD test method as a requirement for reporting chlorine levels in public pools. DPD has been available as liquid or tablet visual colorimetric test, in which a color comparison is done to obtain results. However, the DPD test method is now also available as a test strip.
Let’s explore these different types of DPD test methods, highlighting key advantages as we go along.

DPD Liquid Method

The DPD liquid method requires the addition of liquid reagents into a vial of sample water, which triggers a color reaction, or lack thereof. For free chlorine testing, reagents from different bottles are added and then mixed; the color of the sample water is compared to a color standard on the comparator. For total chlorine testing, a similar process is repeated. The liquid DPD method is commonly used, but users should follow directions carefully to get the best results:
  • Ensure the vial is clean before testing
  • Fill exactly to the vial “fill line” 
  • Hold reagent bottles straight up and down when dispensing drops 
  • Mix samples thoroughly
  • Make color comparisons in outdoor light, not under fluorescent lights
The typical range for a liquid DPD test using the visual method is 0 ppm to 10 ppm without dilution.

DPD Tablet Method

HACH Guest Blog POOLAs with the DPD liquids, tablets can be used to determine total and free chlorine levels, although tablets are not as widely used. Instead of adding drops, tablets are crushed and added to a sample vial. Then a color comparison is made to obtain the reading. With tablet kits, there is no dispensing or counting of drops, but it’s still necessary to add reagents separately to obtain both free chlorine and total chlorine results. The typical range for DPD tablet kits using the visual method is 0 ppm to 6 ppm. More recently, colorimeters have been developed to read liquid or tablet DPD reagents, allowing for more precise readings, but users must still exercise care in following directions. Regardless of how a liquid or tablet test is performed, the average user spends 6 to 10 minutes completing a full panel of tests.

DPD Test Strip Method

Although the DPD test methods discussed so far have been in use for years, individuals responsible for testing the water chemistry in public pools have expressed interest in a faster, easier DPD method. Now operators have another option when it comes to DPD testing. Test strip technology has advanced to include the DPD chemistry for free and total chlorine, making it the first of its kind for rapid, dip-and-read commercial testing. In fact, NSF Certified test strips for free chlorine and pH are available, providing the credibility needed to consider an alternative DPD method.
Each test pad has just the right amount of reagent on it, so no further preparation is required. The user simply dips the test strip directly in the pool or spa and compares the strip to the provided color chart at the specified time. Testing is completed for free chlorine, total chlorine and pH in less than a minute. The DPD test strip method also offers something new for pool operators: a side-by-side comparison of total chlorine and free chlorine. Both chemistries are compared to the color chart at the same time, enabling users to immediately determine if combined chlorine is present. If the total chlorine pad is darker than the free chlorine pad, this result indicates the presence of combined chlorine.

The test strip method also offers extended testing ranges for chemistries, which eliminates the need for diluting high-range samples, giving end- users a broader perspective of disinfectant levels with less chance for the introduction of error. For free chlorine and total chlorine, the range is from 0 to 20 ppm, and for bromine the test measures up to 40 ppm.

With today’s technology, options have expanded for aquatic professionals and staff who collectively conduct millions of tests each year and seek methods that offer accuracy, simplicity, and reliability.

Click here to learn more about the DPD Pro test strip kit  
1CDC Immediate Closures and Violations Identified During Routine Inspections of Public Aquatic Facilities — Network for Aquatic Facility Inspection Surveillance, Five States, 2013. MMWR Morbidity and Mortality Weekly, Volume 65, Number 5, May 20, 2016. 

About the Authors

Lara Winter is a product manager with Hach Company/ETS and has been in the pool & spa industry for over 7 years. In 2015 she also received CPO certification from the National Swimming Pool Foundation. With previous experience as a lifeguard, she brings a unique perspective and passion to improving water quality in public pools and spas. She has a BA in English Writing from the University of Pittsburgh and an MBA from University of Wisconsin - Whitewater.  She may be reached via email at
Joe Sweazy is National Sales Manager for Hach Company/ETS.  Joe has been with Hach since 1998. Joe brings years of experience and insights that have helped guide the industry in improving water quality for residential and public pool users.  He is holds a degree in Chemistry and is a member of the Monitoring and Testing Technical Committee of the Model Aquatic Health Code which examines water quality issues, including: sample collection, sampling locations, testing equipment, and sample analysis for the various disinfection methodologies, microbial quality, chemical quality, water clarity, temperature, saturation index, recirculation effects, and other special requirements. He has presented seminars at National and Regional Pool and Spa trade shows on various water chemistry topics. He may be reached via email at
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