The testing procedures adopted internationally for free and combined chlorine residuals are versions of the DPD procedure developed and refined in the 1940s and ’50s by Dr. Tom Palin in the United Kingdom.

While the practical benefits of breakpoint chlorination in giving better and faster disinfection were already recognized, little was known about the chemical reactions of the breakpoint phenomenon. Also, at a practical level, reliable control tests were needed. Palin’s classic work on chlorination chemistry led to the development of new analytical procedures: the DPD methods.

The tests were first formatted as titration/drop-count tests, with ferrous ammonium sulphate titrant (which is not stable) and DPD/buffer powder to detect the endpoint. The development of the tablet reagent system by Palin was a natural progression, providing a simple, stable and reliable system of routine testing for waterworks and pool operators.

Since Palin’s work, much more research and development has been conducted to explore the limitations of such testing and create better ways of performing it.

Interference by turbidity

A major change in testing came in response to the increasing popularity of calcium hypochlorite as a disinfection chemical. This causes buildup of calcium hardness in the pool. Calcium reacts with the buffer chemicals used to control the pH of the DPD test, which causes cloudiness to form in the test solution. That makes it difficult to visually assess the color and impossible to get a correct photometric reading.

Reformulation of the reagents with a sufficient chelating agent to deactivate the calcium up to 1,000 ppm calcium or 2,500 ppm hardness prevented this effect and restored accurate results.

Interference by shock dosing with potassium monopersulphate Nonchlorine oxidizers became popular as superchlorination substitutes for use as chlorine or bromine enhancers, with brand names often including the term “oxy” (such as Oxybrite). These generally consist of an oxidizing agent called potassium monopersulphate combined with a silver catalyst.

This is effective at regenerating free bromine from bromide or free chlorine from spent chlorine residual. It can oxidize bather waste and prevent the buildup of unwanted chloramines in the pool.

When the method first became popular, we received many queries from users who were experiencing a rise in combined chlorine due to the addition of a product to reduce combined chlorine.

This occurred because monopersulphate does not react in the DPD No. 1 test for free chlorine. However, it does raise the result of the total chlorine reading, which is obtained when the DPD No. 3 tablet is added. Since combined chlorine is the difference between total and free chlorine, any monopersulphate residual in the water raises the apparent level of combined chlorine.

Laboratory studies show that it is actually the catalyst in the shock dosing chemicals that causes these products to interfere in the total chlorine test. Eliminate that and the results are corrected. A new DPD reagent had to be formulated to stop the effect. It must be added after the DPD No. 1 tablet and be completely dissolved so that it deactivates the silver catalyst before the DPD No. 3 is added. Then you will achieve correct results for total and combined chlorine. It is critical to get that reagent dissolved into the sample before adding the DPD No. 3 reagent.

DPD extended range

This recent development in DPD testing was prompted by the needs of swimming pool professionals in the United States and Australia. In these regions, pools tend to be controlled with a higher level of free chlorine than would be found in Europe, where we typically work between 1.0- and 1.5 ppm. Also, there is much greater emphasis on tight control of combined chlorine residual.

The traditional range of the DPD test for chlorine is up to 5 ppm, and if this is exceeded, a dilution is required. This introduces undesirable complexity and opportunity for error. This range was believed to be the upper limit at which we could be confident that bleaching does not occur.

However, it was discovered that the range could be significantly extended,

increasing the amount of color produced and delaying bleaching until above at least 100 ppm chlorine. Thus, new tablet ranges were developed. These make it possible to avoid sample dilution while retaining all the benefits of instrumental measurement. Optics were carefully selected to give the best performance in the most useful range between approximately 3- and 6 ppm.

Timing inconsistencies

Different DPD test kits recommend different times for results. The kits all agree on taking the free chlorine reading immediately, but the recommended time to the total chlorine reading is either instant for drop count kits or two minutes for tablet reagents.

We launched an investigation to see, in the particular environment of the controlled pool, how fast the total chlorine reading can be taken without compromising the accuracy of the result, despite what test kit instructions indicate.

The findings showed that the total chlorine test must be allowed to develop for two minutes after addition of all reagents, or a substantial underestimate of the combined chlorine fraction may occur. Additionally, the tablet test is always complete after two minutes, and the FAS-DPD drop count method should be allowed four minutes for the most accurate result.