A swimming pool owner has many tasks that need to be performed to keep a swimming pool safe and sanitary for swimming. Two of the most important jobs for pool owners in keeping their pools sanitized relate to chlorine and pH levels. Though swimming pool chlorine and pH levels are separate issues, they're also dependent on each other in certain ways. For example, when a pool pH test kit produces purple colors, the problem really lies with the pool's chlorine levels.
Though different pool test kits vary in just how their equipment samples chlorine or tests pH, they all share certain similarities, too. For one, almost all pool pH test kits use the same reagents to create colors that will reveal a swimming pool's pH. Also, one color that means the same thing among all pool pH test kits is purple. If a pool pH test results in a purple color, pool chlorine levels are too high and are interfering with the test's reagents.
A swimming pool pH test that turns purple isn't usable to measure pool pH, and a new test sample will have to be taken. Once you have a new pool pH test sample, all you need to do to prevent it from turning purple again is to add thiosulfate reagent to the test sample. Thiosulfate, or sodium thiosulfate, effectively neutralizes chlorine. Adding a drop of thiosulfate to a pool pH test sample will allow the pool's true pH to easily be determined.
A swimming pool's pH and chlorine levels go hand in hand. And incorrect swimming pool pH will negatively affect pool chlorine levels, just as too-low or too-high levels of pool chlorine will negatively affect pool pH. After neutralizing the chlorine in a pool pH test sample and after determining pool pH, you should always test your pool's chlorine levels. A purple swimming pool pH test indicates that chlorine levels in the pool may need to be lowered.
To prevent purple swimming pool pH test results in the future, you should always try to maintain your pool's chlorine within recommended ranges. In swimming pools, chlorine levels should be between 1 and 3 parts per million (ppm), with 2 ppm being ideal. Swimming pool pH should also be maintained between 7 and 8, though 7.4 to 7.6 is ideal. You'll make your swimming pool water sanitation tasks much easier when you consistently maintain proper pool pH and chlorine levels.
You can raise swimming pool pH by adding soda ash, or you can lower it by adding muriatic acid. Pool chlorine levels are raised by adding more chlorine. About 1.5 pounds of soda ash are required in 20,000 gallons of water to raise pool pH above 7. About 3/4 pound of calcium hypochlorite chlorine is required in 20,000 gallons to raise a pool's chlorine to 2 ppm. Swimming pool chlorine levels drop naturally, but you can also lower them by using thiosulfate according to directions.
- PoolCenter.com: Water Testing FAQs - When I Test for pH...Purple Color
- Swimming@Home: Swimming Pool Owner's Guide to Water Chemistry - pH Explained
- Recreonics: Basic Swimming Pool Water Chemistry
- FINS: Beginner FAQ: Water Treatment - Chlorine
- CleanPoolAndSpa: Adjusting Your Swimming Pool pH Levels
- PoolAndSpa.com: Pool Water Chemistry - Technical Details
- CSGNetwork.com: Pool Chemical Conditions Calculator
- TroubleFreePool: Recommended Pool Chemicals
Tony Guerra served more than 20 years in the U.S. Navy. He also spent seven years as an airline operations manager. Guerra is a former realtor, real-estate salesperson, associate broker and real-estate education instructor. He holds a master's degree in management and a bachelor's degree in interdisciplinary studies.