Updated January 2018. Service techs can use three methods to measure chlorine residuals in pools:
DPD (diethyl-p-phenylene diamine): This chemical reagent reacts with free, or active, chlorine. Sample water is combined with the DPD reagent in a test vial. Between 1 and 3 ppm (industry-recommended chlorine levels), the sample will turn varying shades of red.
One note of caution when doing DPD tests for chlorine: With high chlorine residuals (10 ppm and up), the reagent will bleach out, and the test actually will resemble low chlorine. At this point, a tech is likely to add even more chemicals to the pool, though the levels are too high already.
If you suspect this is an issue with a pool you’re testing, simply dilute the test sample by 50 percent with tap water and re-test. Now, a reading of 3 ppm is actually 6 ppm.
OTO (ortholtolodine): This has fallen out of favor recently due to its inability to distinguish free chlorine from total chlorine, but it still has advocates. To test for total chlorine, add the specified amount of OTO to the sample of water in the test cell. Cap the test cell, and invert it to mix (do not use your finger to cover the top of the test cell). A color change (from light yellow to a deep orange) will result. Compare the color of the test sample to the color comparator provided with the kit.
FAS-DPD: This variation of the traditional DPD method allows users to measure free and combined chlorine levels as low as 0.2 ppm — the maximum allowable level for combined chlorine, according to most health authorities and APSP — and as high as 20 ppm. By contrast, color comparators with the standard DPD test generally allow readings at the low end of 0- and 0.5 ppm and at the high end of 5- or maybe 10 ppm.
In the FAS-DPD titration test, buffered DPD indicator powder is added to a water sample and reacts with chlorine to produce the pink color characteristic of the standard DPD test. Ferrous ammonium sulfate solution (FAS) is added drop by drop until the pink color completely and permanently disappears, signaling the reaction’s endpoint.
To get the reading, the number of drops used to cause this color change is multiplied by the appropriate factor for the size of the water sample (supplied by the manufacturer).
The distinct change from bright pink to no color at all eliminates the need for color matching. This means when testing samples with high sanitizer levels, the user doesn’t have to distinguish between relatively close gradations of color or worry that any color has been bleached out. It’s also helpful to the 6- to 8 percent of the population with red-green deficiencies in their color vision.
The second part of the test determines the amount of combined chlorine present (that is, mono-, di- or trichloramines) by the number of drops needed to again turn the sample from bright pink to colorless.
The cost per test is a little higher and the procedure takes longer than other tests. But many keep it on hand for problem pools or if they are “color challenged.” Look for FAS-DPD in combo kits and as a stand-alone test.
If the pool or spa you’re testing uses bromine, you can still test residuals with OTO, DPD, or FAS-DPD. Follow the same procedures you would when measuring chlorine and multiply the results by a factor of 2.25.
If you’re measuring for chlorine residuals, you’ll likely be tracking the stabilizer levels too. Industry-recommended levels are between 30 and 50 ppm for cyanuric acid.
To test CYA levels, a reagent called melamine is used. The melamine will cause the water to become more or less cloudy (the aforementioned turbidity test). Low CYA will produce small particles that give the water a hazy appearance. Higher concentrations produce far more particles and turn the water very cloudy. The turbidity is then measured against a comparator chart depicting the relative visibility of a dot in the test vial, thereby indicating the corresponding CYA reading in parts per million.
Because chlorine only works effectively in certain pH ranges (low 7s to low 8s), it’s just as important to regularly monitor the water’s pH level.
The reagent for testing pH is phenol red — or phenolsulfonephthalein. This is an organic dye that comes in both liquid and tablet form. In a small sample of water, five drops, or one tablet, are added. The resulting colors — yellow (low), red (middle) and purple (high) — will be accurate in a pH range of 6.8 to 8.4.
Two other reagents also are used in pH testing: bromythol blue, with a range of 6.0-7.4, and cresol red, with a range of 7.2-8.8. Phenol red remains the most popular, though, because it closely reflects the pH levels recommended for pools.
Again, be aware that a high sanitizer residual (more than 10 ppm for chlorine, 20 ppm for bromine) can create false pH readings. At these high levels, the sanitizer reacts with the phenol red, resulting in false colors. Many test kits include a special neutralizing reagent that, when added prior to testing, ensures accurate results.
If your tests show that the pH needs to be raised or lowered, another test should be done to determine exactly how much adjuster should be added. To lower pH, perform an acid demand test; to raise it, perform a base demand test.
The number of drops required to cause a color change in the water sample correlates to a chart that prescribes the amount of muriatic acid (to lower pH) or soda ash (to raise it) needed to move the pH.
Total alkalinity (TA) is the water’s ability to neutralize acid, also known as the system’s “pH buffer.” To test for TA, two reagents are required. First, neutralize the chlorine in the sample; most test kits include a chlorine neutralizer. Then add the end-point indicator — that is, the chemical that changes colors during the test. The second reagent, titrant, is an acid used to trigger the end-point reaction. It’s important to perform this test carefully. To properly mix the titrant, the test vial should be gently swirled after each drop is added.
When the titrant is added and just exceeds the water’s ability to neutralize it (it’s an acid, remember), the end-point indicator will change the water color. The color change happens fast, so pay close attention. Count the number of drops needed to generate an end-point reaction. This determines the concentration of the water’s TA.
According to APSP’s Basic Pool & Spa Technology Manual, Third Edition, the ideal TA level for pool water is 80- to 120 ppm as calcium carbonate. But the proper range will vary with the type of sanitizer used.
Testing for calcium hardness is particularly important if you use calcium hypochlorite as the sanitizer, or if the source water is high in calcium. If this is the case, initially test both pool and tap water, then test continually for 30 days. After that, remain current on the water’s calcium hardness.
To measure calcium hardness, use a titration method. First, add a special pH buffer to the water sample. This raises the sample’s pH to approximately 10, the level at which the test is most accurate. Next, an organic dye is added that turns red when it reacts with calcium. Finally, add the titrant: EDTA, also known as ethylenediamine tetraacetic acid.
Add it one drop at a time. When the titrant has combined with all the calcium, it will turn the water blue. The number of drops required to turn the sample blue correlates to hardness in ppm.
While regularly testing chlorine and pH makes sense, certain tests, such as those for metal, only need to be performed periodically, or if the tech senses trouble and wants to do a little investigation.
There is always going to be some sort of metal in pool water, but if you think the levels may be high — if you see staining, for instance — performing a few metal tests would be in order.
Copper and iron are the two most likely culprits. Copper can find its way into pools through copper-based algaecides as well as the corrosion of copper heat exchangers and heat sinks. Iron is usually introduced from source water — particularly well water.
If you think there’s high metal content (as little as 1 ppm metal can cause staining), it’s best to consider a metal kit.
Typically, kits are colorimetric tests that require two or more reagents. Bicinchoninic acid is popular when testing for copper. It produces shades of purple in the water sample when that metal is present. The common reagent for detecting iron is phena-troline, which produces shades of orange and red.