pH is the term used to refer to the degree of activity of an acid or base (alkali) in the water. It is the most important chemical factor to be maintained in swimming pools. pH is measured on a scale from O to 14 with 7 being neutral. Pool water pH is best when kept in the range of 7.2 to 7.8.
A value of 7 to 14 is considered basic with 14 being the greatest base activity. Another word for basic is alkaline; however, this is not to be confused with total alkalinity. pH and total alkalinity are not the same but can be influenced by each other.
A pH value between 0 and 7 is considered acidic with 0 being the greatest acid activity and getting weaker as it approaches a value of 7. When pH remains below 7.2, the water is considered to be corrosive. This means etching of plaster and metals in equipment such as heat exchangers will result. In addition, it is more difficult to keep chlorine in the pool because while more effective as a sanitizer at the low pH, chlorine is also much less stable resulting in the consumption of larger quantities of chlorine than would be used at normal pH levels.
Maintaining the pH higher than 7.8 will increase the tendency to form scale or cloudy water. Calcium, the major component in scale, is a relatively unstable mineral and when the pH is high, the calcium is not as soluble and it will have a greater tendency to precipitate or "fall out" of solution resulting in cloudiness or scale. High pH will also reduce chlorine effectiveness resulting in the need to maintain higher chlorine levels to achieve maximum sanitization. If the pH is low, a pH increaser is added to raise the pH. If the pH is high, pH Down is used. pH Down comes in two forms: liquid acid or dry acid.
Changes in the pH of pool water can be caused by many factors but one of the most significant cause is the sanitizer used. Since the sanitizer is the most frequently added chemical in pools, it can have a powerful impact on pH and overall water quality.
Of the sanitizers typically used in pools, chlorine is the most common. Chlorine comes in a variety of forms and varies widely in pH. For example, most tableted forms of chlorine have a very low pH and will tend to lower pH over time, while liquid chlorine is very high in pH and will tend to raise pH values. Salt is also very alkaline and will require frequent additions of acid to maintain the proper pH and overall water balance.
Changes in pH due to sanitizers or other factors can be minimized and controlled by the proper maintenance of the next chemical factor, total alkalinity.