The pH scale measures how acidic or alkaline a substance is, ranging from 0 to 14, with 7 being neutral. In pools and spas, water chemistry is carefully controlled, typically maintaining a slightly alkaline pH between 7.4 and 7.6 for swimmer comfort and proper equipment function. Chlorine compounds are added as sanitizers to destroy pathogens. The effect of chlorine on the water’s pH is not uniform; whether it increases or decreases pH depends entirely on the specific chemical formulation used.
Why Chlorine’s Effect Depends on the Type
The difference in pH impact stems from the carrier compound bound to the chlorine molecule. When dissolved in water, all chlorine sanitizers release hypochlorous acid (HOCl) and hypochlorite ions (\(\text{OCl}^-\)), which are the active disinfecting agents. However, the remaining byproduct dictates the subsequent shift in the water’s balance. Chlorine products are broadly categorized as either stabilized or unstabilized.
Unstabilized chlorine, such as liquid or granular forms, contains no cyanuric acid (CYA) and typically has a high pH. Stabilized chlorine, often in tablet form, includes CYA in its structure. CYA acts as a protective shield against ultraviolet light degradation but also introduces an acidic component. The intrinsic pH of the chemical and the nature of the released byproducts determine the immediate and long-term effect on the pool’s pH level.
Sanitizers That Raise pH
Unstabilized chlorine compounds are highly alkaline. Sodium Hypochlorite, commonly known as liquid chlorine or bleach, is manufactured by dissolving chlorine gas in sodium hydroxide, a strong base. This results in a solution with an initial pH typically ranging between 11 and 13. When introduced into the pool, the high concentration of hydroxyl ions (\(\text{OH}^-\)) causes an immediate upward shift in the water’s pH and total alkalinity.
Calcium Hypochlorite (cal hypo), a granular or tablet form of unstabilized chlorine, is also alkaline, with a pH usually between 10 and 12 when dissolved. When cal hypo dissociates, it releases sanitizing chlorine and calcium hydroxide (\(\text{Ca}(\text{OH})_2\)). This calcium hydroxide byproduct is highly basic and causes a noticeable rise in the water’s pH. The use of these alkaline sanitizers necessitates the regular addition of a pH reducer to maintain balance.
Sanitizers That Lower pH
Stabilized chlorine products, which contain Cyanuric Acid (CYA), are acidic and cause the water’s pH to drop over time. Trichloroisocyanuric acid, or Trichlor, is a common tablet or stick form of stabilized chlorine with a very low pH, typically around 3.0. When Trichlor dissolves, it releases hypochlorous acid and CYA, which acts as a weak acid. The highly acidic nature of the product and the continuous release of acidic byproducts drastically lower the pool’s pH and alkalinity.
Sodium Dichloro-s-triazinetrione, or Dichlor, is a granular stabilized chlorine compound often used for shocking. Dichlor is less acidic than Trichlor, with a pH ranging from 6.5 to 7.0, which is close to neutral. However, it is still slightly acidic and continuously adds CYA to the water, contributing to a long-term downward pressure on the pH level. For every part per million of chlorine added by Dichlor, approximately 0.9 ppm of CYA is also introduced.
Maintaining pH Stability
Managing the pH shifts caused by chlorine requires the use of separate chemical compounds designed for balancing water chemistry. If the pH is consistently rising due to alkaline sanitizers, an acid must be introduced to counteract the effect. Common chemicals for lowering pH include muriatic acid (hydrochloric acid) or sodium bisulfate, often sold as a dry acid. These chemicals introduce hydrogen ions (\(\text{H}^+\)) into the water, which reduces alkalinity and lowers the pH.
Conversely, if stabilized chlorine causes a drop in pH, an alkaline substance is needed to raise it back into the target range. Sodium carbonate, known as soda ash, is used to quickly increase pH. Sodium bicarbonate, or baking soda, can also be used, but it primarily targets total alkalinity (TA), which buffers against rapid pH fluctuations. Maintaining the TA level, typically between 80 and 120 ppm, helps prevent the pH from rapidly swinging after chemical additions.