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Seeing Colors

Generally speaking, color-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. The most reliable method to determine an approximate chlorine concentration is to 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 color 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 colors until the titration's color change is complete. For instance, you will see both red- and blue-colored water in your test cell midway through a hardness test as shown below. This is called a “transitional” color. When you think you've reached the endpoint, add one more drop of titrant to be certain the color change is permanent. If the color remains unchanged, do not count this last drop. 


The sample starts out colorless (1) then turns a distinct color with addition of an indicator (2). As titrant is added, the sample shows a mixture of two colors (3). Finally, the color changes completely at the titration's endpoint (4).


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

A high sanitizer level (usually over 10 ppm) is interfering. Retest, this time adding only 1 drop Thiosulfate N/10 (R-0007) to the sample first if using a 2000 Series™ comparator. For a Residential Series™ comparator, add 1 drop of R-0007 to a 50 mL sample; then add that treated sample to the comparator.


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 Outdated total alkalinity indicator can cause this problem as can excess chlorine in the sample.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 (e.g., Baquacil®), the alkalinity titration will probably go from green to purple instead of the usual green to red.


A total alkalinity titration should go from green to red as seen above.


If biguanide is present in the sample, the color change will properly go from green to purple.


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 from copper-based algaecides, metal 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.

A hardness titration should go from red to blue as shown here.