How Much Muriatic Acid to Add to Adjust Swimming Pool pH
What Is pH?
pH is a measurement of the hydrogen ion concentration in water. The pH of pool water impacts the comfort of swimmers as well as pool maintenance. "pH" stands for "protens hydrogen" or "power of hydrogen"; water conducts electricity and is easily ionized, so it will contain some level of hydrogen ions. The pH scale is exponential and is set up so that the smaller the pH number measured in the water (or any solution), the more acidic it is. The higher the pH, the more alkaline it is.
Why Does pH Need to Be Balanced?
Considering that the pH of teardrops from a human eye is about 7.5, having a pH as close to that number as possible will make swimming more comfortable. As a pool service tech I sometimes hear people complain that swimmers' eyes hurt because the chlorine level is too high. But in these cases, an unbalanced pH level, not a high chlorine level, is almost always the problem.
Many things can affect the pH balance in a swimming pool:
- source water
- water treatment chemicals
- airborne debris
- swimmer waste
Common problems related to LOW pH include:
- etching of the pool surface
- metal corrosion
- chlorine loss
- wrinkles in vinyl liners
- eye and skin irritation
On the other end of the spectrum, HIGH pH can cause:
- scaling (which can lead to clogged filters, clogged heater elements, and reduced circulation)
- cloudy water
- chlorine inefficiency
- eye and skin irritation
Controlling the pH is not only important for swimmer comfort, but also for the protection of the pool system and its components. Before making any changes to the pH level, be sure to test the water and make certain that the total alkalinity is at the correct level (80-120ppm). Usually, if the alkalinity is balanced, the pH won't fluctuate very much.
The most common liquid acid used to lower the pH is muriatic acid, also known as hydrochloric acid. To raise the pH, soda ash (sodium carbonate) is normally used. Sodium bicarbonate (baking soda) should be used to raise alkalinity; it can be used to raise pH only if the alkalinity is low.
How Much Acid to Add
This depends on just how high the pH is and the size of the pool. If you are using test strips, it's usually hard to determine exactly how high the pH level is; you just know it's high. A better test kit, or a DPD drop test kit, will use phenol red to give you a better idea of your true pH reading. Some better test kits include an "acid demand" test to help calculate a more precise dosage.
If you have a pH reading of 7.8 or higher, and an average size (15,000 gallons) in-ground pool, you should add 1/4 gallon (a quart) of muriatic acid, and re-test after the water has circulated for an hour. If you have a smaller pool, or an above-ground pool, you would start with one cup of acid, and then re-test and add more if needed. If you have a spa, we are talking only capfuls at a time. Very little acid is needed to adjust pH in a spa.
Make sure to always maintain the proper alkalinity level, and you will have fewer problems with pH changes.