Reader note: This article about how to accurately measure chlorine levels in water shocked with potassium monopersulfate originally appeared in Aquatics International and was reviewed before publication by Edward Lightcap of DuPont.
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 like bather wastes, dust, and pollen). In a heavily used pool, as much as 90% 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 the sanitizer for its highest purpose, germ killing. A popular choice is a non-chlorine 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 (see sidebar). 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. 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 fact, the high readings were the result of this test interference.
Chlorine test interferences
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 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 all the best selling kits have in common is DPD #3. DPD #3 contains potassium iodide. Monopersulfate shocks will react with the potassium iodide in DPD #3, making it seem 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 #1 and DPD #2 to your water sample to develop a pinkish-red color proportional to the level of free chlorine. After taking that reading, you add DPD #3 to obtain the total chlorine level. You then calculate the amount of combined chlorine by subtracting free from total chlorine.
Combined Chlorine = Total Chlorine – Free Chlorine
When monopersulfate is present in the sample, it reacts with DPD #3 in the total chlorine test, producing a dark pink/red color characteristic of a high total chlorine reading. However, monopersulfate will not react with the DPD #1 and #2 reagents used to measure free chlorine. Therefore, the combined chlorine level obtained doing the calculation above is artificially high.
In an FAS-DPD drop-count titration, you add DPD indicator powder to the water sample and it will turn pink if free chlorine is present. Next you add FAS-DPD titrating reagent drop by drop until the sample changes from pink to colorless. You then multiply the number of drops added by an equivalence factor (stated in the test instructions) to get the free chlorine reading. Finally, you add DPD #3 reagent to the treated sample, which will turn pink if combined chlorine is present. Once again, you titrate until the sample turns colorless and multiply the drop count by the given equivalence factor to get the combined chlorine reading. If monopersulfate is present in the sample it will react with DPD #3, artificially increasing the combined chlorine reading.
Preventive measures
To get an accurate combined chlorine reading, commercial operators have two options: 1) shock in the evening and wait at least eight hours afterward before testing or 2) if shocking is required before or during the hours of operation, use a test kit with reagents that can eliminate the interference. There are kits on the market that include a neutralizing agent for monopersulfate along with the standard chlorine test reagents, or the neutralizer can be purchased separately. You simply add the neutralizer as instructed then take the readings as you normally would.
Should you wish to measure monopersulfate concentrations, take a fresh sample and perform the chlorine tests a second time without masking the interference. The result will be the total amount of oxidizer in the water. Subtract the total chlorine reading obtained in the first test from this total oxidizer reading to find the level of monopersulfate. Note: This will give a monopersulfate reading in ppm as chlorine. To convert to ppm monopersulfate, multiply the result obtained by a factor of 5.
Monopersulfate Level = Total Oxidizer – Total Chlorine
Test strips are also available for analyzing monopersulfate itself. Be sure to check the strip manufacturer's test instructions to determine at what concentration chlorine or bromine will interfere with the monopersulfate test.
Sidebar: More on monopersulfate
Potassium monopersulfate was developed by the DuPont Company in the mid-1950s as an oxidizer for use in commercial products and processes. It is sold as a raw material to formulators under the trade name Oxone® and is marketed under various brand names as a shock treatment. (Monopersulfate is also used in some "two-part" bromine sanitizing systems to oxidize bromide salts to the active sanitizer hypobromous acid.) Since monopersulfate doesn't contain chlorine, it won't produce chloramines or chlorine odors and will not bleach vinyl liners. Moreover, monopersulfate dissolves quickly at normal pool temperatures without premixing, can be used day or night, and allows swimming to resume after a short waiting period for complete mixing and circulation. It is well suited for indoor use where odors from high doses of chlorine and persistent combined chlorine residuals are magnified. 

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