Swimming Pool Blog

Common Questions about Water Testing


Common Questions about Water Testing
by Rob Cox, October 30, 2017


Taylor test kits are known for their accuracy and ease of use, but also for their Pool & Spa Water Chemistry booklet included in their complete water test kits, shown here. It's a 64 page booklet that covers every aspect of water chemistry and water testing, with treatment tables, FAQ's, conversions and tons of other stuff for the pool chemistry enthusiast.

Since many of you may not own a Taylor test kit, and others may not make it all the way to page 45, I've paraphrased below, a particularly useful section of the Taylor Testing & Treatment Guide - with the kind permission from the folks at Taylor.


Chapter 7: Common Questions about Water Testing


1. DPD Test for Chlorine is Bleaching Out, or Not

If the chlorine level in the water is over 10 ppm, the DPD reagents may partially or totally 'bleach-out', resulting in a false-low or zero chlorine reading. First, retest the water using 10 drops of DPD #1 and #2 reagents. High chlorine levels should give a reading higher than the first reading. Second, rinse out the test cell, and add 10 drops each of DPD #1 and #2 reagents, THEN add just a few drops of pool water, and see if the sample turns pink, indicating a high chlorine level. To determine an approximate chlorine level, dilute the test sample with bottled water, half and half, and multiply the reading by 2 to obtain the full chlorine level.

2. My Water Sample turns Cloudy when I add DPD #1?

The test will still work, even if your chlorine test sample turns cloudy after adding DPD #1. This may happen with pool water that has a high level of hardness. The cloudiness should not affect your test results and should disappear when you add DPD #2 reagent. To avoid cloudiness entirely, add the sample water to a test cell already containing 5 drops each of DPD #1 and DPD #2.

3. Why are Free and Total Chlorine readings the same, even with chloramines in the water?

A small amount of DPD #3 was probably left in your test vial from a previous test. Even a trace amount of DPD #3 will interfere with a Free chlorine test, making it actually a Total chlorine test. Clean your test vial completely after each use by rinsing several times, and shaking to dry.

4. Can I use a Bromine comparator to test a Chlorine pool?

Yes, DPD and OTO undergo the same reaction with bromine as with chlorine, but the color intensities produced are different. To convert from bromine to chlorine, simply divide the value on the bromine comparator by 2.25. Likewise, if a chlorine comparator scale is used to test for bromine, simply multiply by 2.25 to determine ppm of bromine. Note: Taylor comparators have dual sanitizer values listed on a single vial, so conversions are not needed.

5. Why do I get a blue/purple color when testing for pH?

High chlorine chlorines, over 10 ppm can interfere with phenol red or pH indicator solution. You can pull a sample and place under a bright light, or bring to a boil to reduce chlorine level. If your Taylor test kit has the green capped Total Alkalinity test bottles, add 1-2 drops of reagent #7 Sodium Thiosulfate, to remove the chlorine from the pH test sample.

6. Total Alkalinity sample turns blue, but endpoint color is Yellow, instead of Red

Your test is producing a yellow endpoint due to high chlorine levels in the water. Add an extra drop of #7 reagent to remove the chlorine interference.

7. Can Sample Size be Increased to Improve Titration Test Accuracy?

Yes, for Titration tests like the Total Alkalinity and Calcium Hardness tests, where you count the number of drops and multiply by 10 ppm, you could double the water sample size, from 25 ml to 50 ml, and then each drop would represent 5 ppm, instead of the normal 10 ppm. Likewise, if you reduced the sample size from 25 ml to 12.5 ml, each drop in a titration test would count as 20 ppm.

8. Can I Use Other Manufacturer's Reagents with a Taylor Test Kit?

In most cases no, each manufacturer makes test reagents in different concentrations and drop orifice size, made to match their particular comparator color scale. The only exception to this is with DPD test reagents, which can be substituted, but other test reagents may produce inaccurate results when used with different comparators.

9. Very High Total Chlorine reading after shocking with Non-Chlorine Shock?

Non-chlorine shock, or MPS reacts with DPD #3 when used to perform the total chlorine test, the same as chlorine, and can produce false high total chlorine readings, giving you the impression that chloramines are very high. Allow the MPS level to subside over several hours and test again, or use Taylor DeOx reagent R-0867 to remove the interference of MPS and DPD #3 reagents.

10. Calcium Hardness sample turns blue, but endpoint color is Purple, instead of Blue?

There are metal ions interfering with the test, most likely copper ions from algaecide use, copper pipes or heat exchanger. To avoid the metal ion interference, add 5 drops of the titrant #12 to the water sample Before adding the buffer and indicator solution, then proceed normally with the test. Remember to include the 5 drops of #12 added when calculating total number of titrant drops used.

11. What is the Shelf Life of Taylor Reagents?

There is not a single answer because storage conditions play a major role in determining the freshness of any test reagent. As with other perishables, Taylor test reagents should be stored in climate controlled areas. Liquid reagents are made to remain fresh for at least one year after manufacturing, and in proper conditions may for two seasons, or 2 years. Store reagents at a consistent temperature, between 40-80 degrees F, out of direct sunlight, and with all caps tightly sealed.


Taylor test kits have been a Poolcenter favorite product for many years, the K-2005 and K-2006 test kits are trusted by pool service people, pool stores and health inspectors, and allow the home pool owner to run the full battery of tests on any pool or spa, for complete home water analysis. And if you don't have the complete Taylor test kit, you can still own the complete 64 pg. Taylor Testing & Treatment Guide, Taylor #2004B.


- Rob