The water's pH is a measure of its total acid-alkalinity balance -- the relative proportion of acids and alkalis in the water (check out Chem4Kids: Acids & Bases for a description of acids and alkalis). Simply put, water that is either too acidic or too alkaline will cause undesirable chemical reactions. If the water is too acidic, it will corrode metal equipment, cause etching on the surface materials and cause skin irritation. If the water is too alkaline, it can cause scaling on the pool surface and plumbing equipment and can cloud the water. Additionally, both high acidity and high alkalinity alters the effectiveness of the chlorine. The chlorine won't destroy pathogens as well if the water is too alkaline, and it will dissipate much more quickly if the water is too acidic.
On the pH scale, zero indicates extreme acidity, 14 indicates extreme alkalinity and 7 indicates a neutral state. Most pool experts recommend a pool pH between 7.2 and 7.8. To raise or lower pH, a pool custodian simply adds acids or alkalis into the water. For example, adding sodium carbonate (soda ash) or sodium bicarbonate (baking soda) will generally raise the pH, and adding muriatic acid or sodium bisulfate will lower the pH.
Maintaining the proper balance of chemicals in the pool is a continual process, because any new element -- oils from a swimmer's body, a shot of chlorine, stuff that falls in the water -- shifts the water's total chemical makeup. In addition to pH, pool custodians also monitor total alkalinity, calcium hardness and total dissolved solids.
When you consider all the chemistry and machinery involved in swimming pools, it's clear that they really are remarkable pieces of technology. It takes a surprising amount of work and ingenuity to build and maintain these summertime staples.
For more information on swimming pools and related topics, check out the links on the next page.