Seeing Colours

All questions and answers on this page have been used with permission from Taylor Technologies, Inc.

Generally speaking, colour-matching tests and drop-count titrations give straightforward answers. However, sometimes you may encounter situations that are a bit puzzling. Here, we answer some questions we've heard on our technical support line.
  1. My test results indicate there is no chlorine in the pool, but I know there is. Why does this happen?

    If the chlorine is high, usually over 10 ppm, the DPD reagents may partially or totally bleach out, resulting in a false-low or zero chlorine reading. First, retest using 10 drops each of DPD #1 and #2. High chlorine should give a reading higher than the first reading. Second, rinse out the test cell, add 10 drops each of DPD #1 and #2 THEN add a few drops of the water to be tested. If you see pink, the chlorine is high. To determine an approximate chlorine concentration, dilute the sample and retest. For a 1:1 dilution multiply the result by 2; for a 1:3 dilution multiply by 4, etc.

  2. Since a drop-count test is all about seeing a colour change, how can I be certain I've reached the true endpoint of a reaction?

    A sample treated with indicator will show a mix of two colours until the titration's colour change is complete. (For instance you will see both red- and blue-coloured water in your test cell midway through a hardness test as shown below.) When you think you've reached the endpoint, add one more drop of titrant to be certain the colour change is permanent. If the colour remains unchanged, do not count this last drop.

  3. Why do I sometimes get a purplish blue colour instead of a yellow-to-red colour when using phenol red indicator?

    A high chlorine level (usually over 10 ppm) is interfering with the test. Wait until the sanitizer level is normal and retest to assure an accurate reading.

  4. When I add my total alkalinity reagent I get a yellow endpoint instead of a red endpoint. Has the indicator gone bad?

    Possibly. Outdated total alkalinity indicator can cause this problem as can excess chlorine in the sample. If in doubt about the potency of the reagent, replace it. When a high level of chlorine is present, add an extra drop of thiosulfate reagent to the sample to remove the chlorine interference then follow the test procedure as written. If your water sample contains a biguanide (PHMB) sanitizer like Baquacil® the alkalinity titration will properly go from green to purple instead of the usual green to red.

  5. I got a purple endpoint instead of a blue endpoint when I did a hardness test. What happened?

    There are metals interfering with the test, most likely ions from algaecides, pipes, or even the source water itself. To avoid this problem, add five or six drops of titrant to the sample before adding the buffer and indicator, then proceed normally with the test. Always include the number of drops of titrant added at the beginning of the test when counting the total number of drops required to reach the endpoint.