Testing is the key to identifying problems in pools early. Let’s take a look at the different methods that can be used to test pool or spa water, and the pros and cons of each.
One of the easiest testing methods is the dip-and-read test strip. For a quick spot-check of a pool or spa’s water chemistry, test strips will do the trick. Test pads on the strip are chemically treated with reagent which reacts to the components of the water. Once the strip pads have changed color, they can be compared to color standards to determine if further testing is needed. Strips are a good way to get a snapshot of the overall state of the pool chemistry, as they will test most pool parameters. However, some limitations of this method include the fact that test strips are greatly affected by lighting conditions, the perception of the tester, and the limits of the color standards offered for comparison. You may not get the same result on repeated tests.
A step beyond the dip-and-read strip is a digital strip reader. A digital reader compares the sample color to the standard digitally, or relates the color to a concentration using a series of mathematical equations. Since lighting conditions and personal considerations are no longer a factor, it is considered to be more accurate than the dip-and-read strips. However, test strip technology, overall, has some limitations that affect the accuracy of the water test.
Measuring free chlorine
Beyond test strips, which can test most pool chemistries all at once, there are other options available. Let’s look at each parameter individually.
Orthotolidine, or OTO, is a drop test often used by consumers to test sanitation levels in pools or spas, but has become less popular with more accurate testing methods now available. There is no way to get a free chlorine reading using this test method, making it difficult to tell how much chlorine is actually available in the water.
DPD (N, N-diethyl-p-phenylenediamine) testing is also used for testing sanitizer, and it has the ability to test for free and total chlorine. DPD testing can be done as a colorimetic drop test, like OTO, and compared to a standard color block for a reading. It can also be done using a titrimetric test method, and in some cases using a photometer. The colorimetric DPD test is the most common and most cost effective method. Most DPD color blocks will measure free chlorine up to 5.0 parts per million, but might not be accurate above that residual unless the sample is diluted. Bleaching of the color in these tests can occur when chlorine is 10-15 ppm, and can lead to an incorrect reading of zero chlorine, when the residual is actually quite high. In addition, at high pH readings, the chlorine reading can also appear much lower than it actually is. Another consideration for DPD testing is that combined chlorine over 0.5 ppm can interfere and cause false high results with the free chlorine test. This could lead the user to not detect combined chlorine in the pool water. For both OTO and DPD drop tests, lighting, environment and human perception will greatly influence the readings.
DPD titration testing, known as FAS-DPD, uses a balancing reagent instead of comparing colors to known standards. A color indicator is added to the sample, and a titrating agent is added until a permanent color change, or endpoint, is reached. The amount of titrating reagent necessary for the color change indicates the concentration of sanitizer in the sample. FAS-DPD is more accurate than DPD drop testing at both high and low concentrations of chlorine. It can measure free chlorine up to 20ppm, as opposed to the 5ppm limitation of the drop test. However, be aware that in all of the DPD test methods, the presence of potassium monopersulfate can create a false high reading when testing for total chlorine.
Measuring pH is also an important piece of the water-balance puzzle. The most common way to test pH in pools and spas is the use of phenol red. This test, similar to OTO, is a colorimetric drop test that compares sample color intensity to known color block standards. Phenol red is a simple way to test pH, and is accurate from 6.8 to 8.4 on the pH scale. Outside of this range, phenol red cannot be used for accurate results. High sanitizer levels (chlorine or bromine) can cause the sample to turn dark purple when pH is above 6.6. This often is mistaken for a high pH reading, when in reality, it could be quite low. Adding a chlorine neutralizing agent will overcome this issue. pH meters are also an option. These meters are not subject to the interferences and limitations as phenol red drop testing, but must be calibrated daily and properly maintained to ensure accuracy.
To properly maintain pH, you must also test and balance total alkalinity. Titrimetric testing is the most common procedure used. An acid titrating reagent neutralizes the total alkalinity. The amount of titrant used to reach an endpoint translates to a total alkalinity reading. Halogens must be removed, usually by using sodium thiosulfate, before testing for total alkalinity. In some cases, photometers can also be used.
High levels of chlorine, as well as the presence of biguanide can result in an unexpected color change. When interference is present, the endpoint is yellow or green, as opposed to the usual red or pink color. High levels of chlorine can be negated by adding more sodium thiosulfate to the sample before testing.