By Joe Sweazy
Gone are the days of the ’flash test’ when a pool operator would walk by the pool and drop a couple of drops of Orthotolidene (OTO) onto the surface looking for the yellow indication that there was chlorine in the pool. These days, educated operators recognize the need to take the time and analyze the water using more professional methods.
Unfortunately, most of us are not able to wheel out a $10,000 spectrophotometer to poolside to get the most precise and accurate test results possible. There are, however, great test kits available that are practical for poolside testing, but we must understand what it takes to get truly accurate test results with these methods.
This article covers some of the factors to be cognizant of, including proper sample collection, prompt analysis after collection, knowing the limitations of your kit (especially at high FAC concentrations) and seeking help from manufacturer instructions or customer service when results simply don’t make sense.
One of the most overlooked factors in getting accurate results from any method is the collection of a water sample. Often times a water sample needs to be taken from the pool or hot tub and that sample taken to where the testing will take place.
In order to get a representative pool sample, there are several things to consider. First, the sample should be clean and free from any residual from washing, previous tests, etc.
To make sure this happens, rinse the sample vessel several times with the water that is going to be tested. Then, if the water is to be transported any distance, make sure there is no or very little air in the bottle.
This can be done by placing the bottle under the surface of the water, shaking the bottle to get all of the air bubbles out and then sealing the bottle while it is still underwater. Be sure not to siphon the water off of the surface where chemistry change may be occurring.
Water chemistry in that small sample can change in 30 minutes or less, so the sample should be tested as quickly as possible after being drawn. Factors that affect this sample include temperature change, sunlight, aeration from the vessel being handled and carbon dioxide on the inside of the bottle. Without a representative water sample, there is no test that can be truly accurate.
Whether bringing a sample to the testing equipment or going directly to the pool to test, you need to understand the limitations of the test that you are using. You may think that you are getting accurate test results, when in reality you have exceeded the range of the test kit you are using.
Perhaps there is an interference that is causing your method to be inaccurate and you fail to recognize it. These are other factors of which you should be aware.
When using a test kit for measuring chlorine or bromine, there is a good chance that the indicator used to measure that sanitizer level is diethyl-p-phenylene diamine, better known as DPD. There are many different DPD kits available, but all of them have interferences that can cause a false reading.
High levels of halogen sanitizer can bleach the DPD indicator. Levels of just under 20 ppm (mg/L) of chlorine (or 25 ppm of bromine) can cause the DPD indicator to start to fade, get lighter or bleach out completely.
This can lead you to believe that you have little or no chlorine in the water, when in fact there is more than necessary already in the pool. It may be detectable if you know what to look for when performing a test.
DPD reacts instantly to free chlorine. Therefore, if you see color as soon as the DPD indicator is added to the water being tested, free chlorine is probably present.
If that color dissipates or vanishes completely, chances are that there is some bleaching taking place and a dilution and retest should be performed or a secondary test with a higher range used to determine the sanitizer level. For example, a test strip that uses the FACTS method (Free Available Chlorine Test with Syringaldazine) does not bleach out at levels of 20 to 25 ppm of chlorine and many are capable of reading up to 20 ppm before any dilution is necessary.
High sanitizer levels may also affect test results for pH and alkalinity. The pH results may appear to be higher than they actually are due to the chlorine oxidizing the phenol red indicator to a reddish purple color. This color may appear to match up to the higher end of the pH scale, but in reality is a false high-test result.
Total alkalinity colors may also be affected and endpoint of the titration may be more difficult to determine. Test strips, like DPD test kits, are affected this way as well. Therefore, when chlorine levels register at or above the top of the scale, it is a good idea to wait until the level drops into the acceptable range before making any adjustments to pH and alkalinity.
There is another common interference that happens with the DPD indicator. Combined chlorine levels of around 0.5 ppm or higher can oftentimes react with the DPD free chlorine indicator making the user believe that the free available chlorine levels are higher than they actually are. In this case, it may be necessary to add more free chlorine to react with the combined chlorine, which inaccurately tested as free available chlorine.
This interference may be detected when adding the DPD indicator to the water sample being tested. Just like the bleaching issue discussed above, it is easy to observe that this interference is occurring by watching closely when adding the free chlorine indicator.
If no free chlorine is present, you do not get an immediate color change from the DPD. If after some time, color does appear, it is probably not free available chlorine, but combined chlorine instead.
Again, FACTS test strips do not have this interference issue and can be used to double check the test result if questionable. Otherwise, one is left to assume that all of the chlorine is combined chlorine and the level of shock required may be difficult to determine.
You need to understand the limitations of the test that you are using. You may think that you are getting accurate test results, when in reality you have exceeded the range of the test kit you are using.
Test kit range
Another factor that is often taken for granted is the range of the test kits. The cyanuric acid tests are a perfect example of where one might need to be concerned.
Most test kits measure cyanuric acid up to about 100 ppm. But, what happens when the actual level is well above 100 ppm? You guessed it; it still reads around 100 ppm.
Unless you know that you need to dilute the water, re-test and then multiply the result by the dilution factor, you may not know that your cyanuric acid level is well above the recommended level. For this reason, a direct measurement using a meter or test strip that indicate much higher levels should be used to double check the readings when there is the question; is the water at the high end of the range or over the range of my test?
It is also important to understand the test kit you are using was likely the most accurate the day it shipped from the manufacturer. Storage conditions and careful usage are critical to maintaining the accuracy of the test equipment over the entire expected shelf life.
In order to preserve the integrity of your test kit, it should be stored in a cool, dry place away from direct sunlight. Heat, humidity and sunlight often degrade the reagents in test kits and test strips causing them to react differently. If you are getting questionable test results or having trouble interpreting colors of the tests, it may be that your reagents or strips are old or contaminated and should be replaced.
Following directions and seeking assistance
Make sure to follow the directions of the manufacturer precisely. Don’t assume that because you have used one test, you have used them all. Many manufacturers have directions for use that are slightly or perhaps completely different and any deviation from the intended directions can cause inconsistencies and inaccurate test results. Reviewing all of the information supplied by the manufacturer oftentimes will help you to identify other potential accuracy issues.
Anytime you get test results that leave you baffled, remember that an expert is often just a phone call away. Many manufacturers appreciate hearing when their testing equipment is giving questionable results. They may help you understand your test results, while you help them identify an opportunity for improvement.
About the author
Joe Sweazy is Technical Sales and Services Manager for HACH Company/ETS Business Unit, manufacturer of AquaChek pool and spa test strips and water quality products. These products are intended to simplify analysis with reliable, accurate results. Sweazy has published more than a dozen articles on pool and spa water chemistry and has presented numerous seminars at conferences of the Association for Pool and Spa Professionals (APSP).