Expand Your Water Testing Options


Water chemistry in public pools and spas has regained focus in recent years as more relevant data becomes available. According to a recent report from the Centers for Disease Control & Prevention (CDC), pool and hot tub inspection data from 16 jurisdictions across the United States at various aquatic venues indicated that 12.3 percent of inspections resulted in immediate closure because of the seriousness of identified violations. Violations regarding the following issues are frequently identified: Free Chlorine level (11.9 percent of inspections), proper pH level (14.9 percent), other water chemistry (12.5 percent). This highlights the risk that improperly maintained water can quickly lead to infections and recreational water illnesses.

Why DPD?

Regular water testing is critical to ensure there is an adequate level of sanitizer in the water, and that other parameters are in balance to ensure its 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 (FAC, or Free Available Chlorine) 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 tests, in which a color comparison is done to obtain results. However, the DPD chemistry is now available as a test strip. We'll 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 water sample in a vial, which trigger a color reaction or lack thereof. For Free Chlorine, reagents from different bottles are added and then mixed before the color of the sample water is compared to a color standard on the comparator. For Total Chlorine, a similar process is repeated to obtain results. 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, and not under fluorescent lights.

The typical range for a liquid DPD test using the visual method is 0ppm to 10ppm without dilution.

DPD Tablet Method

Tablets are another method for determining Total and Free Chlorine levels using the DPD chemistry, though 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 0ppm to 6ppm. More recently colorimeters have been developed to read liquid or tablet DPD reagents, allowing for more precise readings. However, they have done little to improve on the user requirements highlighted above. Regardless of how the liquid or tablet test is being completed, the average user spends 6 to 10 minutes completing a full panel of tests.

DPD Test Strip Method

Although DPD test methods 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 testing with DPD. 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, an NSF Certified Free Chlorine and pH test strip is available, providing the credibility needed to consider an alternative DPD method.Each test pad has the right amount of reagent on it-the user just dips the test strip directly in the pool or spa and compares 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, it indicates that Combined Chlorine is present.

The test strip method also offers extended testing ranges for chemistries without the need for diluting high-range samples, giving end users a broader perspective of sanitizer 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. This is a real advantage when there are circumstances, such as fecal accidents, that requires the Free Chlorine level be brought up to a minimum of 20 ppm and sustained for a period of time. The DPD test strip does not bleach out when chlorine levels reach 25 ppm, 30 ppm or even higher. The DPD test strip method provides a new testing option while meeting regulations that require the use of DPD for detecting chlorine.

Now there are more options for aquatic professionals and staff who conduct millions of tests each year, and are seeking accuracy and reliability. The DPD test strip technology assures professionals of a fast, easy, accurate and reliable testing method.

Caption: Caption: Hach has pioneered a new testing solution for aquatic professionals who are responsible for maintaining healthy water


Lara Winter is a product manager with Hach Company/ETS and has been in the pool & spa industry for more than seven 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.

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. 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. Joe is also a member of NSF Task Group on Test Strips, Kits and Devices. He has presented seminars at National and Regional Pool and Spa trade shows on various water chemistry topics.

For more information, visit www.dpdproteststrips.com.