A saltwater chlorinator is a popular system used in swimming pools to generate chlorine through a process known as electrolysis. This electrochemical reaction is driven by a redox (reduction-oxidation) reaction. A saltwater chlorinator operates by passing an electric current through a solution of saltwater (sodium chloride, NaCl) that is in contact with electrodes in the chlorinator unit. This electrical energy causes a redox reaction to occur in the water. To understand how this works, it’s essential to break down the redox reaction into its reduction and oxidation components:

1. Oxidation (Anode Reaction): At the anode (positive electrode), chloride ions (Cl⁻) from the salt in the water are oxidised, losing electrons to form chlorine gas (Cl₂). The chlorine gas produced at the anode is unstable in its molecular form, so it quickly dissolves in the water, where it forms hypochlorous acid (HOCl), the active sanitiser.
2. Reduction (Cathode Reaction): At the cathode (negative electrode), water molecules are reduced, gaining electrons to produce hydroxide ions (OH⁻). This reaction generates hydroxide ions, which contribute to the overall alkaline environment of the pool water.

In summary, the process involves the oxidation of chloride ions to chlorine gas and the reduction of water molecules to form hydroxide ions. The chlorine produced, in the form of hypochlorous acid (HOCl), is an effective disinfectant, killing bacteria and oxidising organic contaminants in the pool water.

The pH of pool water significantly affects the efficiency of the saltwater chlorinator and the quality of chlorine produced. The pH scale measures the acidity or alkalinity of water, with values ranging from 0 (very acidic) to 14 (very alkaline), and a pH of 7 being neutral.
Chlorine exists in two forms depending on the pH of the water:

• Hypochlorous acid (HOCl): The most effective form of chlorine as a disinfectant. HOCl is formed when the pH is below 7.5, making it more efficient at killing bacteria and algae.
• Hypochlorite ion (OCl⁻): A less effective form of chlorine that occurs when the pH is above 7.5. It is still a disinfectant, but not as potent as HOCl.

At a pH around 7.2-7.6, a balance exists between hypochlorous acid and hypochlorite ions, ensuring optimal chlorine sanitisation. If the pH rises too high (above 7.6), the water becomes more alkaline, and the chlorine’s effectiveness is reduced due to the higher concentration of hypochlorite ions. If the pH drops too low (below 7.2), the chlorine becomes too acidic, which can lead to the production of more chloramine.

While a saltwater chlorinator produces chlorine, it also generates hydroxide ions (OH⁻) as a byproduct of the electrolysis process. These hydroxide ions raise the pH of the pool water, making it more alkaline. Over time, the continuous generation of chlorine and hydroxide ions will cause the pH of the pool to gradually rise, which may lead to scaling issues or metal staining.