The acidity/alkalinity of pool water, as measured by its pH, is one of the most important characteristics to regulate. If the pH is low – meaning the water is acidic – swimmers can suffer itching skin, rashes, burning eyes and dry hair. The pool and all its components can suffer, too, because acidic water corrodes tiles, metal accessories, and machinery in the water circulation and purification system. Additionally, free chlorine levels become difficult to maintain in acidic water. Despite all this, it's usually still safe to swim in the pool. It just might not be comfortable.
Pool Water Should Be Slightly Alkaline
The term pH stands for the "power of hydrogen," and it's measured on a scale that extends from 0 to 14. When the pH is higher than 7 – which is neutral – the water is alkaline, meaning it has an excess of negative hydroxide ions. Water with a pH below 7 is acidic, which means it has a surplus of positive hydrogen ions. The pH scale is logarithmic, so water with a pH of 6 is 10 times more acidic than water with a pH of 7, and water with a pH of 5 is a hundred times more acidic.
Pool water should, ideally, be slightly alkaline, with a pH between 7.2 and 7.6. When the pH falls below 7, the water starts to become corrosive. Imagine the effect of getting battery acid on your skin to appreciate what it would be like to swim in extremely acidic water. However, there's too much water in a pool for it ever to approach being comparable to battery acid. In fact, it's rare for pool water to even become as acidic as acid rain, which has a pH of about 4. Even mildly acidic water can be uncomfortable for swimmers, however.
Negative Effects of Acidic Water
Pool water may never get acidic enough to cause skin burns, but it can dissolve skin and hair oils, causing itchiness and dryness. In extreme cases, acidic water also can cause burning eyes. The severity of these problems increases the longer you stay in the water. It's worth noting that high levels of chloramines also can hurt the eyes, so if you have burning eyes after swimming, it isn't necessarily an indication that the pH is too low. You may need to shock the pool.
Parts of the pool that are submerged suffer the most from the destructive effects of acidic water. Pool tiles may be etched and stained, and metal components may corrode. This corrosion releases metal ions into the water that can also cause stains, but they usually don't cause health problems. Corrosive acidic water may also damage the internal mechanisms of the pool circulation pump and filtration system.
How Did the Water Become Acidic?
Normal rainwater has a pH between 5 and 5.5, so it's common for pool water to be acidic after a heavy rainstorm. This effect may be exacerbated by runoff flowing into the pool after passing over the pool decking and by organic matter swept into the pool by the runoff. Dissolved bodily fluids can also make water acidic, so you may notice a lower-than-normal pH reading after a party. Finally, you may have made the pool acidic yourself by adding too much muriatic or dry acid in an effort to regulate the pH.
How to Raise pH
Pool maintenance pros use one of two closely related chemicals to raise pH. The first is sodium bicarbonate -- commonly known as baking soda -- and the other is sodium carbonate -- or soda ash. Both come as powders and are available in large bags at any pool supply outlet.
Before deciding which of these chemicals to use, check the pH to determine how much you need to raise it to between 7.2 and 7.6. That number, along with the volume of the pool, will tell you how much to add to the water. Because both chemicals are powders, you can sprinkle them onto the surface of the water, and both will quickly dissolve. It's best to sprinkle far from the skimmers to prevent them from being sucked through the pool circulation system.
When you test the pH, You should also check the total alkalinity of the pool water, which should be between 80 and 120 ppm. If it is in this range or above, use soda ash to raise the pH. Baking soda has a greater effect on total alkalinity and is the better choice when pH and total alkalinity are both low.