How Much Muriatic Acid to Add to Adjust Swimming Pool pH

pH is a measurement of the hydrogen ion concentration in water. The pH of pool water affects 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 always 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.

Controlling the pH is important not only for comfortable swimming, but also for the long life of the materials that make up the pool.

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.

What Causes Pool pH to Become Unbalanced?

Many things can affect the pH balance in a swimming pool:

• disinfectants
• source water
• evaporation
• water treatment chemicals
• airborne debris
• swimmer waste
• aeration

Common Problems Caused by a LOW pH:

• etching of the pool surface
• metal corrosion
• staining
• chlorine loss
• wrinkles in vinyl liners
• eye and skin irritation

Common Problems Caused by a HIGH pH:

• scaling (which can lead to clogged filters, clogged heater elements, and reduced circulation)
• cloudy water
• chlorine inefficiency
• eye and skin irritation

To lower the pH, the most common liquid acid used is muriatic acid, also known as hydrochloric acid.

To raise the pH, soda ash (sodium carbonate) is normally used. Sodium bicarbonate (baking soda) can be used to raise pH if the alkalinity is low, but it’s more commonly used to raise alkalinity.

Briefly, alkalinity refers to dissolved salts, like carbonates, that keep pH stable. If alkalinity is in the right range, pH and chlorine levels are easier to maintain.

To lower alkalinity, add muriatic acid. Muriatic acid lowers both pH and alkalinity.

To raise alkalinity, add sodium bicarbonate (baking soda). This is different from sodium carbonate (soda ash).

This depends on how high your pool’s 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.

For a more exact idea of how much acid to add, see the table below, or see this pool calculator that will give you an amount based on estimated pool volume and chemistry.

When you add acid, pour it into the deepest part of the pool, while the pump is running and circulating the water. Wait at least an hour before using the pool; if you added a lot of acid, wait 24 hours and test the pH before using the pool.