Muriatic acid, a solution of water and hydrochloric acid, is a strong acid and a dangerous chemical, but when it's used properly and safely, it can help to keep a swimming pool's water chemistry in an ideal state.
Disinfection and pH
The job of the chlorine that's added to swimming pools is to keep the pool water clear and safe for swimmers by killing bacteria and algae. The effectiveness of chlorine as a disinfectant and an algae-killer is directly affected by the pH level of the pool water. The ideal pH level for pool water is between 7.4 and 7.6. When the water has a pH level of 7.4, chlorine is 65 percent effective as a disinfectant, but when the pH level rises to 8.0, the effectiveness of chlorine drops to 20 percent. In this case, the water is more likely to harbor bacteria and become clouded with algae.
The water's pH level rises naturally over time and with use, so it's often necessary to bring it back down to a level that optimizes the effectiveness of chlorine. Adding acid to the water works to lower the pH level.
The water's pH level should not be confused with its total alkalinity, which is a measure of the concentration of alkaline substances in the water. It is a measure of the water's buffering ability, or its ability to resist changes in pH. Therefore, although a high pH level is a bad thing, a total alkalinity level of between 80 and 120 parts per million is ideal. Adding too much acid will not only lower the pH, it will also lower the water's total alkalinity, so only add as much acid as is necessary to get the water's pH in line. Excess acid can also damage pool equipment and the pool itself.
Before adding acid, test the water's pH level to determine how much of a change you need to make. Make sure the pool pump is running and that there are no swimmers in the pool.
Fill a bucket 3/4 of the way to the top with water from the pool, then add a small amount of acid to the bucket. In general, the recommended dose of acid is 12 fluid ounces per 10,000 gallons of water, but you should not add all of the acid at one time. Always put the water in the bucket first, followed by the acid, never the other way around.
Pour the mixture slowly and carefully into the deep end of the pool.
Wait four hours, and then test the pH again.
Repeat steps 2 and 3 as necessary to get the pH down to the desired level, but never add more than the total recommended dosage. It is generally safe to swim in the pool 30 minutes after the last application of acid.
Walking the Acid
Some pool technicians recommend "walking the acid" by pouring it in little by little around the perimeter of the pool rather than dumping it in one place, claiming that this method lowers water pH without affecting total alkalinity. However, a paper published in 1995 in the Journal of the Swimming Pool and Spa Industry concluded that there is no benefit to walking the acid.
Muriatic acid can also be used to clean pool walls if they're covered with algae, dirt, minerals or chlorine stains. The process of acid washing a pool is potentially dangerous, and doing it incorrectly can damage pool walls, so it is best left to trained technicians.