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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. To determine an approximate chlorine concentration, perform a 1:1 dilution (4.5 mL of sample water + 4.5 mL of distilled or non-chlorinated water). Follow the normal instructions for the test. Match color to a standard and multiply the associated value by 2 for the chlorine level. You may also use this dilution procedure for high bromine levels.

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 treated sample will show a mixture of the original color and the endpoint color as the titrant is added until the endpoint is reached. (For instance you will see both pink- and blue-colored water in your sample midway through a hardness test.) Add one more drop of titrant after the endpoint to be certain the color change was 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 >10 ppm, is interfering. Dump and rinse the test cell and refill. Add one drop of R-0007 (Sodium Thiosulfate N/10) to the sample. Swirl to mix. Then follow the normal instructions.

4. When I add my total alkalinity reagent the sample turns blue. Then I get a yellow endpoint instead of a red endpoint. Have the reagents gone bad?

No. Your test is producing a yellow endpoint due to an excess of chlorine in the sample. Add one extra drop of thiosulfate reagent to the sample to remove the chlorine interference when retesting.


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


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

There are metal ions interfering with the test, most likely from copper 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 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.