Calcium hypochlorite, often referred to as “cal hypo,” is a granular or tablet-based chemical widely used as an oxidizer and sanitizer in pool and spa applications. Its primary function is to introduce chlorine into the water to kill bacteria and contaminants, ensuring water safety and clarity. The use of cal hypo directly impacts the water’s chemical balance, specifically its pH level. Maintaining the correct pH is important for swimmer comfort, equipment longevity, and the overall effectiveness of the sanitizing process.
The Chemical Mechanism of pH Increase
Calcium hypochlorite raises pH. When cal hypo dissolves in water, it undergoes hydrolysis, releasing both the sanitizing agent and the pH-elevating component.
The hypochlorite ion (\(\text{ClO}^{-}\)) reacts with water (\(\text{H}_2\text{O}\)) to form hypochlorous acid (\(\text{HClO}\)) and hydroxide ions (\(\text{OH}^{-}\)). Hypochlorous acid disinfects the water, while the hydroxide ions are the source of the pH increase. The addition of hydroxide ions directly raises the water’s alkalinity.
Calcium hypochlorite itself has a high pH, typically ranging between 10.8 and 11.8 in concentrated form. The production of hydroxide ions drives the water’s pH upward, which affects chlorine efficiency.
If the water’s pH climbs too high, the effective hypochlorous acid is converted into the less effective hypochlorite ion. This reduces the sanitizer’s ability to kill pathogens and necessitates careful monitoring of the water’s pH.
Impact on Water Hardness
The other major component of calcium hypochlorite is calcium, which has a lasting impact on water chemistry. Continuous use introduces calcium ions (\(\text{Ca}^{2+}\)) into the water system. These ions accumulate over time, increasing the Calcium Hardness (CH) level of the water.
High calcium hardness, combined with high pH, can lead to scaling. Scaling occurs when dissolved minerals precipitate out of the water and form solid deposits on surfaces. These deposits often appear as white or grayish crusts on pool walls, piping, and heating elements.
The potential for scaling is governed by the Langelier Saturation Index (LSI), a calculated value that predicts whether water is corrosive, balanced, or scale-forming. A positive LSI indicates the water is saturated with calcium carbonate, making scaling likely. Users must maintain other water balance factors to keep the LSI within an acceptable range.
If calcium hardness levels become excessively high, the only effective way to reduce them is by partially draining and replacing the water with fresh, low-calcium water. This distinguishes cal hypo from other chlorine sanitizers that do not contribute calcium.
Strategies for pH Stabilization
Because calcium hypochlorite consistently raises the pH and Total Alkalinity (TA), active management is necessary to maintain proper water balance. TA acts as a buffer, preventing sudden swings in pH, and should be adjusted before correcting the pH level. The ideal range for TA is between 80 and 120 parts per million (ppm).
To counteract the basic nature of cal hypo, acidic compounds are routinely added to the water. The most common chemicals used to lower both pH and TA are muriatic acid (hydrochloric acid) and sodium bisulfate (a dry acid).
Muriatic acid is a highly effective liquid but requires careful handling due to its corrosive nature. Sodium bisulfate is often preferred because it is a granular product that is generally safer to handle and store than liquid acid.
Both chemicals work by adding hydrogen ions (\(\text{H}^{+}\)) to the water, which neutralizes the excess hydroxide ions (\(\text{OH}^{-}\)) introduced by the cal hypo. Regular testing and small, incremental additions of these acids are necessary to stabilize the water chemistry and ensure effective sanitization.