The saline water passes through an electrolytic cell(s) utilising titanium electrodes coated with precious metals which provide high selectivity towards chlorine production. A DC current at the electrodes results in a number of electrolytic reactions taking place at the anode and cathode. For more information on the major chemical reactions select from the menu located on the right.
In a sodium chloride solution, chlorine gas will be created at the anode (positive electrode) by the oxidation of the chloride ion Cl-. At the cathode (negative electrode), water is hydrolysed producing hydroxyl Ions and hydrogen: 1. Anode: |
Chemical reactions These gases in turn will react with the water to give hypochlorous acid and a hypochlorite ion, in proportions which depend on the pH of the water. 3. In Neutral to Acidic Conditions: Cl2 + H20 -----> Cl- + HCIO +H+ The hydrolysis of chlorine into hypochlorous acid, reaction 3, occurs rapidly and is complete in pH values ranging from 7 to 9. The partial dissociation of hypochlorous acid, reaction 4, to a great extent is prevented by a lower pH. The undissociated form is desirable, as it is a better disinfectant than the hypochlorite ion. These chlorine derived compounds are the primary disinfectants produced by the electrolytic process. A number of other competing reactions also take place in the electrolytic cell. |
The amount of chlorine produced is directly proportional to the amperage drawn at each electrolytic cell. The amount of electric current required to generate one mole of chlorine, (Cl2, -70.906g), or its equivalent in chlorine compounds after hydrolysis, is calculated as follows: a) One mole chlorine Cl2 requires 2 faradays of electrons flowing. These calculations assume a 100% cell efficiency. In reality a number of competing reactions take place. Cell losses, varying electrolyte conductivity, impurities etc., reduce cell efficiencies to in the region of 70% to 90%. A number of wide ranging variables such as water chemistry, cell design, ohmic losses and other factors ultimately dictate the chlorine production in each particular application. |
As can be seen by reaction 2 on the previous page, hydrogen gas is also produced at the cathode. Hydrogen is generated at the rate of 18.656 moles per 1000 Amps/hr. I mole of H2 = 2.0158g. 18.656 moles H2= 37.606764 g Volumetric measurements of H2 can be calculated by: V= Vo 273+t Po / 273 P Where Assuming an ambient temperature of 25°C and atmospheric pressure, Hydrogen air mixtures can be hazardous in a wide range of hydrogen/air ratios: In water, hydrogen oxygen mixtures do not ignite. However care must be taken in ensuring that hydrogen levels do not build up in air or piping to flammable levels. Accordingly, the SAL plant design and installation incorporates the following design factors: a) Cell and plumbing design to avoid the accumulation of gas build. |
Water containing salt in concentrations of 5,500 to 6,500ppm is mildly corrosive, but as all chlorine is made from salt, conventionally chlorinated water is corrosive too. Whereas saline water has no harmful effects at all, impurities in chemical chlorine can damage pool surfaces. Most leading brands of filters and pumps are not affected by mildly saline water, and carry warranties to that effect. Modern corrosive resistant materials such as UPVC should be used in plumbing, as is now customary with any water treatment systems. Sea water has substantially higher salinity levels, and in such cases, the usual special precautions need to be taken against corrosivity. |