Chlorine, as a disinfectant, is neither purely acidic nor purely alkaline; its chemical form in water is highly dependent on the water’s pH level. The term “chlorine” refers to several forms, including elemental gas, salts, and solutions, all exhibiting varying pH characteristics when mixed with water. While raw commercial products, like household bleach, are often highly alkaline, the final diluted solutions used in pools or drinking water are typically managed to be near neutral or slightly acidic. This complexity arises because chlorine immediately reacts with water to establish a dynamic chemical balance between two main species.
The Chemical Reality of Chlorine in Water
The sanitizing action of chlorine results from a rapid and reversible chemical reaction. When chlorine compounds, such as sodium hypochlorite or calcium hypochlorite, are introduced to water, they immediately form hypochlorous acid (\(\text{HOCl}\)) and the hypochlorite ion (\(\text{OCl}^-\)). This reaction creates an equilibrium between these two species, collectively known as “free available chlorine.” The overall \(\text{pH}\) of the solution dictates the ratio of these two forms.
Hypochlorous acid (\(\text{HOCl}\)) is a weak acid that readily releases a hydrogen ion (\(\text{H}^+\)), which lowers the \(\text{pH}\). Conversely, the hypochlorite ion (\(\text{OCl}^-\)) acts as a base, accepting a hydrogen ion and raising the \(\text{pH}\). The acidic or alkaline nature of chlorinated water depends entirely on which form is dominant.
At a neutral \(\text{pH}\) of 7.5, the concentrations of \(\text{HOCl}\) and \(\text{OCl}^-\) are roughly equal, each contributing about 50% to the total free available chlorine. If the \(\text{pH}\) drops below 7.5, the equilibrium shifts, favoring the acidic \(\text{HOCl}\) species. If the \(\text{pH}\) rises above 7.5, the alkaline \(\text{OCl}^-\) ion becomes the dominant form.
How pH Controls Chlorine’s Sanitizing Power
The distribution of \(\text{HOCl}\) and \(\text{OCl}^-\) has direct consequences for disinfection effectiveness. Hypochlorous acid (\(\text{HOCl}\)) is significantly more potent at killing pathogens than the hypochlorite ion (\(\text{OCl}^-\)). This is because \(\text{HOCl}\) carries no electrical charge, allowing it to easily penetrate the negatively charged cell walls of microorganisms.
The \(\text{OCl}^-\) ion carries a negative charge, causing it to be repelled by the negative charge of a microbe’s cell wall. Studies show that \(\text{HOCl}\) can be 20 to 300 times more effective as a germicide than its alkaline counterpart. This difference in efficiency means a high \(\text{pH}\) solution with low \(\text{HOCl}\) concentration is a much weaker disinfectant than a low \(\text{pH}\) solution with the same total chlorine amount.
For this reason, maintaining a specific \(\text{pH}\) range is necessary in applications like swimming pools and public water systems to ensure effective sanitation. If the water becomes too alkaline (high \(\text{pH}\)), the majority of the chlorine converts to the less effective \(\text{OCl}^-\) form, drastically reducing sanitizing power. For instance, at a \(\text{pH}\) of 8.0, only about 20% of the chlorine is in the highly effective \(\text{HOCl}\) form, compared to 50% at \(\text{pH}\) 7.5.
Practical Examples of Acidic and Alkaline Chlorine
Common household bleach, a solution of sodium hypochlorite (\(\text{NaOCl}\)), is highly alkaline, with a \(\text{pH}\) typically ranging between 11 and 13.5. This high alkalinity is intentionally maintained during manufacturing by adding sodium hydroxide to ensure the product remains stable and retains its strength. The dominant species in this concentrated, alkaline state is the hypochlorite ion (\(\text{OCl}^-\)).
When this sodium hypochlorite is diluted for use in swimming pools or municipal water treatment, the goal is to adjust the final solution to a slightly acidic or neutral range. Pool operators often aim for a \(\text{pH}\) between 7.2 and 7.8. This range maximizes the presence of the powerful \(\text{HOCl}\) species for effective disinfection while remaining comfortable for swimmers.
Other Acidic Chlorine Forms
Chlorine gas dissolved in water reacts to form hypochlorous acid and hydrochloric acid, creating a strongly acidic solution. Hypochlorous acid solutions produced by electrolysis are also available. These are naturally slightly acidic, typically falling between \(\text{pH}\) 5 and 7.