A calcium hardness test that stays purple may have an unclear endpoint or metal interference. It does not tell you to add calcium to the pool. First confirm the test method, then repeat using the manufacturer’s troubleshooting instructions.
The specific correction below applies to Taylor’s calcium hardness titration. Other brands may use different reagents and colors. Keep a total hardness test, a calcium hardness test, and a purple pH test separate.
Know which color change counts
In Taylor’s calcium test, the treated sample changes from red toward blue as titrant is added. Mixed red and blue can look purple during the transition. That intermediate appearance is not automatically the endpoint.
Taylor recommends one additional drop to confirm the final color is stable. If that drop causes no further change, exclude that confirmation drop from the count. Keep swirling as instructed so each addition mixes before you judge it.
Use a neutral background and good light. A colored wall, tinted glasses, or dim equipment room makes subtle transitions harder to judge. Comparing phone pictures adds another layer of uncertain color reproduction.
Recognize the interference problem
Taylor identifies a persistent purple endpoint as possible interference from metal ions, commonly copper or iron. These may come from source water, equipment, or previous products. The endpoint alone does not measure the metal concentration.
Small purple dots are a different observation from the whole sample staying purple. Taylor describes these dots as magnesium hydroxide and says they do not interfere with the endpoint. Record what you see rather than treating both observations as the same fault.
Repeat Taylor’s test with the specified correction
- Start again with a fresh sample at the volume specified by the kit.
- Add five or six drops of the calcium hardness titrant before the buffer and indicator.
- Continue the normal instructions for that sample size.
- Count the remaining titrant drops needed for the stable endpoint.
- Add the initial titrant drops to the later titrant drops before applying the drop factor.
The correction changes the order inside the test sample. It is not a treatment for the swimming pool. Do not pour test reagents into the pool or substitute a pool chemical for a kit reagent.
Keep the count in two parts
Suppose your specified method uses 10 ppm per drop. In a hypothetical retest, you add six titrant drops first and then 22 more to reach the endpoint. The result is 28 × 10, or 280 ppm as calcium carbonate.
Counting only the final 22 drops would report 220 ppm. Including a later confirmation drop that caused no change would also distort the result. Write the initial count down before continuing so neither mistake slips into the calculation.
Use the factor for the actual sample size. Do not switch to a smaller sample while keeping the previous multiplier. Any dilution must follow the manufacturer’s specified procedure and calculation.
If the second test is still unclear
Check reagent identities, expiry, sample volume, and the order of addition. Repeat with clean equipment and the correct supplies. Do not keep adding uncounted drops until you happen to like the color.
A manufacturer approved standard can help check technique and reagent performance. A second method or a careful professional retest may also help. Share the kit model, sample size, full drop count, and recent pool products with whoever assists.
Pause a hardness correction until you have a defensible reading. Our testing guide covers the broader checks. If two reports differ, use the comparison process before acting on either one.
