Why you should not use muriatic acid to lower the pH of your pool.

First, you need to understand what Total Alkalinity is and how it affects the pH of your pool water. By definition Total Alkalinity is a measurement of water's ability for the pH to change. In other words, the higher the Total Alkalinity, the harder it is for the pH to change. It is important that you maintain the Total Alkalinity between 80-120 PPM. (This can vary somewhat depending on local and source water.)

Muriatic acid is a dilute form of hydrochloric acid which is a very strong acid. Even diluted, muriatic acid is still a very strong acid. When you add muriatic acid to your pool (even if you have diluted it more in a bucket of water), not only do you lower the pH of your water but you also lower the Total Alkalinity of the water. With the Total Alkalinity lowered, it is now easier for the pH to rise again (this can be due to many reasons), so if you add more muriatic acid to lower the pH, you also lower the Total Alkalinity even more, thereby making it even easier for the pH to go up again. As you can see, it is a vicious circle you are in.

How do you lower the pH without lowering the Total Alkalinity? You should use a dry acid (pH lower, pH minus, etc.) mixed into a bucket of water first, then poured around the pool. Dry acid is a much milder acid than muriatic acid and when diluted in water will have almost no effect on the Total Alkalinity of the water while lowering the pH. This will allow you to maintain the Total Alkalinity level, which will make it more difficult for the pH to change, thereby reducing your pH fluctuation.

Why does the dry acid not reduce the Total Alkalinity like muriatic acid does? Because for the Total Alkalinity to go down, you must get the pH below 5.5 in the water. Adding muriatic acid (because it is so strong) lowers the pH of the water. When you pour it to below 5.5, the chemical reaction that lowers Total Alkalinity now takes place. Adding a dry acid (a much milder acid) especially when diluted in a bucket of water does not lower the pH in that area to below 5.5. Therefore the chemical reaction that lowers Total Alkalinity does not take place.

Now, when your pool professional tells you that it is cheaper in most cases to use a dry acid than to use muriatic acid, you will understand.

Note:If your source water (water you add to pool) has a very high Total Alkalinity, you may want to use muriatic acid to lower both the pH and the Total Alkalinity at the same time.