Overcoming Interferences in Water Tests
Wednesday, October 31, 2018
How confident are you in the accuracy of your water tests?
An overdose of chlorine or bromine can produce readings that are at times wildly off the charts. Such errors not only foul up sanitation tests, but also make it difficult to gauge levels of pH and total alkalinity — and they affect both the drop-test and color-matching methods.
Of course, excessively high sanitation isn’t the only culprit. Many other things can interfere with test results.
Because too much chlorine or bromine is the most common cause of inaccurate results, we’re going to first look at how to adjust your water samples to account for those discrepancies. Then we’ll examine some of the other factors that corrupt water tests.
If chlorine or bromine is greater than 10 to 15 parts per million, DPD reagents may be partially or totally bleached out. This can play a nasty trick: You’ll incorrectly think that there is no chlorine or bromine in the pool when in fact there is more than enough.
The fix: If there is no color development — no pink or red — and you are sure there is chlorine and bromine in the sample, dilute and retest to determine the correct concentration. Halve your sample size. For example, if the required sample size is 10mL, take it down to 5mL, then add 5mL of water with zero sanitizer, such as bottled water. Then follow the instructions exactly as written. Compare the resulting color against the chart. The chart should have a color that matches your water sample, along with a value. Double the value the color represents. That’s your correct reading.
•Another possible problem is that a small amount of DPD No. 3 was left in the test cell. A trace amount can make the total chlorine value read less than the free chlorine value.
The fix: Be sure to thoroughly rinse out the test cell after each use.
The great thing about FAS-DPD tests —commonly called a drop test — is that they can read up to 20 ppm of sanitizer. However, if the sanitizer is greater than 20 ppm, DPD reagents may partially or totally bleach out, resulting in a false low- or zero-sanitizer reading.
The fix: You can perform a dilution, just as described above. In a DPD test, a typical sample size is 25mL. You would halve to 12.5mL, fill the remaining with bottled water, then multiply by the dilution factor — in this case, 2.
Alternatively, you can dilute your sample with bottled water and simply add as many dippers of indicator powder as necessary to hold a pink color. Then add drops of titrant until the sample goes from pink to colorless. Multiply the number of drops by 0.2. For a 1-to-1 dilution, multiply the result by 2; for a 1:3, multiply by 4, etc.
•Monopersulfate (non-chlorine oxidizer) can corrupt total- and combined-chlorine readings. You’ll know this is the case because of a quick reaction to DPD No. 3, resulting in an abnormally high number (a false high.) This also applies to the color-matching method.
The fix: Use a deox reagent to remove any monopersulfate and test again.
When the level of sanitizer is greater than 15 ppm, it can affect the organic dye used in the indicator, resulting in a purple sample, instead of the normal yellow to red.
The fix: Dump out the water, rinse the bottle thoroughly and get a new sample. Then add one drop of thiosulfate. This will neutralize any excess sanitizer in the sample. Next, add 5 drops of phenol red indicator, mix your sample and match the color.
Note: Be sure to add only one drop of thiosulfate. More than one drop will change the pH of your sample, and you’ll get a false high reading.
Total Alkalinity test (drop-test method)
For accurate results, you want the water to turn green to red when testing for total alkalinity. If it turns blue to yellow, you’ve got a problem. Again, the likely cause is excess sanitizer.
The fix: Obtain a new sample. Instead of adding one or two drops, as per the instructions, go ahead and add four to five drops. This will eliminate excess halogen without compromising the sample. Then you’ll get the right color spectrum — again, green to red.
•An old or compromised indicator can produce unreliable results. Be sure to check the expiration date on the label prior to testing.
The fix: Replace with fresh reagents and retest.
•Poly(hexamethylene) biguanide, also known as PHMB, is used primarily for microbiological control. When present it can produce a curious reading. The water turns blue to yellow.
The fix: Actually, this isn’t a problem, and no correction is needed. This color change is accurate.
•When cyanuric acid is more than 30 ppm, it titrates at total alkalinity, producing a false high reading.
The fix: Adjust by subtracting 1/3 of the CYA reading from the tested TA reading. This correction produces a carbonate alkalinity value.
Calcium hardness test (drop test)
Nothing affects CH test results other than metal ions from algaecides, pipes or the water itself. The usual suspects are copper, iron and/or manganese. The presence of these metals will reveal themselves when the sample turns purple or stays red instead of the blue end point.
The fix: Obtain a new sample. Add 5 or 6 drops of titrant before adding the buffer and indicator. Proceed as normal with the test. Add the initial 5 or 6 drops of the titrant to the total drop count, and multiply this number by the drop equivalence to obtain the correct value.
Copper/iron test (color-matching)
High levels of iron can interfere with copper test readings and vice versa.
The fix: If you’re getting questionable readings, dilute the sample and retest to determine the concentration of these metals. For a 1-to-1 dilution, multiply the result by 2; for a 1:3 dilution, multiply by 4, etc. Alternatively, you can use a digital reader calibrated to detect copper and iron individually.
Conclusion: Don’t let bizarre chemical readings throw you for a loop. By following the instructions above, you’ll be able to determine exactly what’s going on in the pool in order to bring water back into balance.