When a pool's pH level is too high, the pool is vulnerable to problems with microorganism and algae growth, as well as swimmer discomfort. Adding an acidic amendment to the water will pull pH down to an acceptable level, but other factors than just acidity come into play.
Why pH Matters
Water's pH level is a measure of the water's relative acidity or basicity. When pH is 7.0, the water is neutral, neither acidic or basic. When pH is less than 7, the water is acidic, and when pH is greater than 7, the water is basic.
The pH level of swimming pool water matters because, if the pH is too high, the chlorine that is commonly used as a disinfectant in the water does not work effectively. At a pH level of 7.0, for example, chlorine is 73 percent effective, but at a pH of 8.0, it is only 21 percent effective. High pH can also cause cloudy water, skin and eye irritation, and build up of mineral scale on surfaces. In general, the ideal pH range for pool water is between 7.2 and 7.8.
Pool water pH tends to drift upward naturally over time and with use of the pool, but it may also rise significantly when the pool's total alkalinity level is too high or when too much pH-increasing chemical is added to the pool.
Chemicals to Lower pH
One of the most commonly used pH-reducing chemicals is muriatic acid, which is a liquid solution of hydrochloric acid. Another common pH reducer is sodium bisulfate, which is available in a granular form. Less commonly used acids include sulfuric acid and nitric acid.
Before you add acid to your pool to lower pH, use a water test kit to check pH, total alkalinity and borate levels, all of which will influence how much acid you'll need to add, as will the type of pH-reducing chemical you're using and the size of your pool. Water alkalinity and borates act as buffers to changes in pH, so the higher these levels, the more acid you'll need to add to lower pH.
After you've gathered all of this information, use a water-chemistry calculator to determine how much acid you need to add.
Prepare the acid by pouring it into a plastic bucket of pool water, and then, while the pool's pump is running and there are no swimmers in the pool, carefully pour the acid solution into the deep end of the pool.
Add acid a little at a time, no more than about a quart per 10,000 gallons of water, and retest the pH level after about 4 hours. If pH is still too high, add more acid, up to the recommended total dosage, and retest again in 4 more hours.
When pH is once again in the recommended range, retest the pool's total alkalinity and adjust it as necessary.