Are Dilutions Necessary When Testing A Pool or Spa?
Water quality factors can, and often do, exceed the testing ranges provided in manufacturers' test kits. Chlorine, bromine, pH, total alkalinity, calcium hardness, and cyanuric acid can all exceed the ranges within Taylor's popular Complete™ Series test kits: K-2005, K-2005C, K-2006, and K-2006C.
There are three different types of tests employed in these kits. Colorimetric tests for chlorine and bromine using liquid DPD reagents, and pH using phenol red, may be thought of as one group. Titrimetric tests for chlorine and bromine using DPD powder and FAS-DPD reagent, along with tests for total alkalinity and calcium hardness, can be considered a second group. The turbidimetric test for cyanuric acid stands alone with its own issue.
In the colorimetric tests for chlorine, bromine, and pH, the most frequent problem encountered is a level above or below the range provided by the color standards. In a DPD test for chlorine or bromine, any value that exceeds the highest color standard on the comparator cannot be reported accurately. A dilution is required. On our 2000 Series™ comparator we have markings at 1.8 mL, 4.5 mL, and 9 mL on the small comparator tube. A test is normally performed using a 9 mL water sample. However, if the reading is off-scale, an approximate value can be obtained by retesting with a 4.5 mL sample and adding chlorine- or bromine-free water to the 9 mL mark (this is called a 1:1 dilution). Cap and invert the comparator twice to allow for complete mixing of the water and sample. Next, add the reagents. Because you have used only half the amount of sample water in this dilution, you must multiply the result by 2. If the value is still off-scale, thoroughly rinse and fill the sample cell to the 1.8 mL mark with sample water, and add chlorine- or bromine-free water to the 9 mL mark (this is a 1:4 dilution). To obtain the approximate value you must multiply the result by 5.
When testing pH, a dilution is not the answer since the introduction of different water, likely with a different pH, will produce inaccurate results. The pH range provided in all of our Complete Series test kits is 7.0 to 8.0. If the pH level is higher than 8.0 or lower than 7.0, the Acid or Base Demand reagents provided with the kits are used to determine the proper amount of treatment product to obtain the desired pH. Tables D, E, and F in the Pool & Spa Water Chemistry booklet are used along with the amount of Acid or Base Demand reagent dispensed in the test to determine the amount of treatment product to bring the pH of your water to the proper level. Introducing smaller doses over time instead of applying the entire portion all at once will prevent damage to surfaces and equipment.
The drop-count method used in the FAS-DPD titration for chlorine and bromine, and the total alkalinity and calcium hardness titrations share the same kind of test limitations. Using more than 50 drops of titrant will create testing error too great to provide a reliable test result. In most cases you will use a 25 mL sample; however, when high total alkalinity and/or calcium hardness levels are anticipated, a smaller 10 mL sample will be required. Rinse and fill the large comparator tube to the 10 mL mark with the water to be tested. Add the reagents as directed in the instructions. The drop equivalence for FAS-DPD when using a 25 mL sample is 0.2 ppm. When using a 10 mL sample, the drop equivalence becomes 0.5 ppm. For total alkalinity and calcium hardness, the equivalence of 10 ppm per drop for a 25 mL sample becomes 25 ppm per drop for a 10 mL sample.
A turbidimetric test is employed to determine cyanuric acid (CYA) levels in an outside pool or spa. When added to a sample, the test reagent reacts with the CYA present to form a precipitate in an amount proportional to the CYA level. This treated sample is then slowly dispensed into the small comparator tube until the black dot on the bottom of the tube just disappears when viewed from the top. The volume of sample is compared to a scale on the side of the tube to determine the level of CYA in the water. The scale used in marking the comparator tube is logarithmic, which to you means the closer you get to 100 ppm of CYA the less treated sample is needed to make the black dot disappear. Unfortunately, there is very little distance from the 100 ppm mark to the bottom of the comparator tube, making CYA levels over 100 ppm impossible to read. For CYA levels at or above 100 ppm, do a dilution to obtain a more accurate reading. In a container, mix one part sample water with one part CYA-free water (tap or bottled water will do). Retest and multiply the result by 2. If the result is still too high, do another dilution, this time using one part sample water to two parts CYA-free water, and multiply the result by 3. For a 1:4 dilution, the multiplier is 5. Once the reading is within the scale, you are finished.