Shocking interference: how to accurately measure chlorine levels in water shocked with potassium monopersulfate.

The most popular sanitizers used in pools and spas--chlorine and bromine--function both as biocides (they kill bacteria and other potentially harmful microbes) and oxidizers (they "burn up" unpleasant organic contaminants such as bather wastes, dust and pollen). In a heavily used pool, as much as 90 percent of the chlorine or bromine may be working to eliminate organic impurities. This ratio is unwise. Periodic addition of a supplemental oxidizer--a "shock treatment"--can free up much of the sanitizer for its highest purpose: germ killing. A popular choice is a nonchlorine shock with potassium monopersulfate as the active ingredient. (The label may also call it potassium peroxymonosulfate.)

Potassium monopersulfate is a powerful oxidizer with several attractive properties. Properly applied, it will cleanse water in short order without raising the chlorine level or creating combined chlorine. Bathers can re-enter the water after waiting only one hour to allow proper mixing and circulation. The reaction byproducts are harmless sulfate salts.

However, monopersulfate does have one drawback when used in chlorinated pools: It can interfere with the combined chlorine reading obtained with DPD and FAS-DPD tests. Some pools even have been closed because of supposed high combined chlorine (chloramine) readings when, in tact, the high readings were the result of this test interference.

As you may know, commercial operators are generally required by regulatory authorities to use a DPD test to monitor chlorine. Kits for this purpose may employ liquids, tablets, a powder or a combination of these reagent forms, depending on the manufacturer. The test method can involve either color matching (the pink color that develops in the treated water sample is proportional to the amount of chlorine present; the reading is determined by matching the pink to a set of color standards), or counting drops (the treated water sample goes from pink to colorless upon the addition of a titrating reagent, and the number of drops used determines the amount of chlorine present). The reagent that all of the best-selling kits have in common is DPD No. 3. DPD No. 3 contains potassium iodide. The trouble is, monopersulfate shocks will react with the potassium iodide in DPD No. 3, making it seem as if there is a higher combined chlorine level in the water than there actually is.

Here are two typical scenarios:

In the standard color-matching DPD test, you first add DPD No. 1 and DPD No. 2 to your water sample to develop a pinkish-red color proportional to the level of free chlorine. After taking that reading, add DPD No. 3 to obtain the total chlorine level, again by comparing the result to a set of color standards. Then calculate the amount of combined chlorine by subtracting free from total chlorine.

Combined Chlorine = Total Chlorine--Free Chlorine