Download Pool Tip #17: pH (PDF format, 37KB)

pH is the log of the reciprocal of the concentration of hydrogen ions in a volume of water, or simply a measure of the acidity or alkalinity of the pool water. As the number of hydrogen ions in the solution increases, the pH decreases and the water becomes more acidic. Conversely, as the number of hydrogen ions in the solution decreases, the pH increases and the water becomes more basic.

pH is measured on a logarithmic scale from 0 (acids) to 14 (base), with 7.0 being neutral. Since pH is the negative base 10 logarithm of the hydrogen ion concentration of water, it can be expressed, for example as 0.0000001, which can be written as 1.0 x 10 – 7 , which is the same as saying the pool water has a pH of 7.0.

Keeping pool water within ideal pH ranges increases bather comfort, and prevents damage to the pool, its components and equipment. Ideal pH levels depend on the other four components of water balance (total alkalinity, calcium hardness, water temperature and total dissolved solids), but typically range from 7.2 to 7.8.

Low pH levels cause chlorine to dissipate rapidly. Equipment corrodes, and pool surface materials etch or crack.

At high pH levels, less hypochlorous acid (HOCl) forms and chlorine becomes less effective. ORP levels plummet, algae growth may increase, the water clouds, scaling occurs and circulation pipes calcify, and filter runs (the time between backwashing filters) shorten.

As you’ve probably noticed, pH levels fluctuate constantly. This problem is magnified if total alkalinity levels are permitted to drop.

Pool chemicals, rain, air pollution, the fresh make–up water added to the pool, plaster and other pool surface materials and equipment the water comes in contact with, and waste products introduced into the pool by swimmers, all cause the pH to change. Some products like muriatic acid, cyanuric acid, sodium bisulfate, and trichlor tablets are acidic and cause the pH to drop. Others, like dichloro-s-triazinetrione or carbon dioxide are somewhat neutral. While other chemicals like sodium hypochlorite or sodium carbonate are alkaline.

To keep pH in the proper range, the pool operator must test frequently using a color comparator test kit, colorimeter, test strips or pH meter, and adjust as necessary.

Several chemicals are sold for the purpose of adjusting pH in pool water. To raise pH, sodium carbonate (soda ash), sodium hydroxide (caustic soda), and sodium sesquicarbonate are recommended. Commercial grade hydrochloric acid (muriatic acid), boric acid, sodium bisulfate (dry acid), and carbon dioxide (CO₂) are most often used to lower the pH of pool water.