August 2008 Issue
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Testing High Chlorine Concentrations
The trend over recent years has been to increase chlorine and bromine residuals in recreational waters. The main reason is concern for pathogens such as Cryptosporidium. During normal facility operation, these residuals are usually below 4 ppm for chlorine and 6 ppm for bromine.
A superchlorination or shock treatment can require from 5-30 ppm of chlorine. Hyperchlorination is a term referring to the maintenance of a high chlorine concentration for an extended time due to a fecal accident. The current CDC Ct value for Crypto inactivation is 15,300. The example they use is 15 ppm for 1020 minutes, or 17 hours.
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The DPD test is the preferred way to measure free and total chlorine residuals. Many facilities use color comparator tests. With these tests, the operator adds liquid, powder, or tablet reagents to a sample and then compares the resulting pink color to standard colors on a dip cell or a slide to determine the chlorine concentration. Chlorine concentrations above 6 ppm will partially bleach the DPD. Complete bleaching usually occurs above 10 ppm. Higher concentrations can be diluted and the results multiplied to get the correct value. Usually the most practical way to dilute the sample is by using tap water, but this contains chlorine, which will skew the results. Another option is to increase the amount of reagent added to the sample, usually twice as much, to get a reading, but this reading will be off scale on a normal comparator.
So how can one test higher chlorine concentrations? There are three test methods.
FAS DPD Titration
Ferrous Ammonium Sulfate (FAS) is a reducing agent used with a powdered DPD indicator to titrate free and total chlorine. It is used by many municipal water treatment operators. FAS is only stable for about 1-3 weeks depending on storage. The municipal operator must mix the reagent regularly. However, a stable version, Ferrous ethylenediammonium sulfate is available for the recreational water area. This is stable for one year.
FAS DPD titration kits are available from many manufacturers. The test procedure is a bit more involved than a colorimetric test, but the features of this method more than compensate for this. The operator adds DPD powder to the sample. It will turn pink if free chlorine is present. The operator then adds the FAS reagent until the pink color disappears. The volume of reagent is then used to calculate the concentration. If a second reagent containing potassium iodide is added to the same sample, and the pink color returns, this is due to combined chlorine. The FAS is again added to determine this.
The FAS method gained its popularity for use in everyday operations for testing residuals between 1 and 10 ppm. Color comparator tests lose precision above 1 ppm, so it is difficult for operators to read 0.2 ppm chloramines at higher concentrations. FAS DPD titrations solve this problem because they can detect as low as 0.2 ppm combined chlorine. They also can be used for bromine with a 0.5 ppm detection limit. Since the test uses a titration whereby a color disappears, there is no need to judge a shade of color, a feature that color-blind users (and many other operators) like. A key feature that is pertinent here is that the method is also very suitable for high chlorine concentrations.
With colorimetric DPD tests, the analyst is limited to the amount of DPD in the tablet or liquid. As stated above, one can increase the amount of reagent, but the resulting dark color will be difficult to quantify. Since a DPD powder is used for this test and the test is a titration, one can add more reagent powder and the FAS will still titrate the darker color to the clear endpoint. Some kits use a pipet to take a 1 mL sample instead of the usual 10 mL. In both cases, one can expand the range of chlorine testing to 100 ppm. This method will determine free and total chlorine even at higher concentrations.
Iodometric Titration
As the name indicates, this test is based on iodine. This method is used for a number of oxidation-reduction titrations. Dissolved oxygen, sulfite, hydrogen peroxide, and chlorine are some of the typical iodometric tests. In an acidic pH, chlorine will react with potassium iodide to release iodine. Then a standard sodium thiosulfate solution is added to reduce the iodine. The reaction starts as a brown color and becomes more yellow as the titrant is added. Since yellow is a difficult color to see in a titration, starch is sometimes added near the endpoint and the sample turns blue. Unlike the FAS DPD method, this method only tests total chlorine.
There are iodometric kits for many different chlorine concentrations. To determine if the amount of chlorine used in a breakpoint chlorination, shock, or hyperchlorination is correct, a 1 drop = 1 ppm method is available. In many food processing plants, chlorine concentrations can range from 50 to 500 ppm. Here a 1 drop = 10 or 100 ppm kit might be best. Methods are also available for percent concentrations. Thus, the operator who finds that the amount of bleach being fed isn’t producing the proper residual can test the bleach solution or can test bleach being delivered from the supplier.
Test Strips
Several manufacturers make test strips that can read higher chlorine concentrations. Daycare centers in some states use strips to test up to 800 ppm chlorine. While strips aren’t usually used in commercial pools, they are easy to use and great for spot checking or troubleshooting.
While recreational water operators and inspectors usually work in lower concentrations of halogen residuals, there are methods available to reach those higher concentrations that are sometimes necessary. All test kit manufacturers have telephone hot lines. If you suspect a testing problem, a quick call can usually help select the correct remedy.
References
www.cdc.gov/healthyswimming
Tom Seechuk
Market Manager
Tom Seechuk has a B.S. in biology from Loyola College in Baltimore. He has worked in the test kit area for 28 years, 18 of which have been with LaMotte Co. He is currently a market manager for industrial and wastewater products. He has been a member of the Recreational Water Quality Committee of the APSP for more than 19 years.
© 2008 by The Association of Pool & Spa Professionals. All rights reserved. No part of this publication may be reproduced or transmitted without written permission.
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