Sodium hypochlorite

(endorsed 2005)

Sodium hypochlorite is used as a disinfectant and oxidant in the treatment of drinking water. It provides available chlorine in a liquid form, with less risk than storing and handling chlorine gas.

General description

Sodium hypochlorite, $\text{NaOCl}$, or liquid bleach, is a strong oxidising agent that is usually stored and used in solution. It has a disagreeable, sweetish odour and a pale greenish colour. Sodium hypochlorite solution releases vapours that cause corrosion in the presence of moisture.

Sodium hypochlorite is usually supplied as 10–13% w/v available chlorine. More concentrated solutions are not practical because of the instability of sodium hypochlorite, which forms chlorate and chlorite (both of which are of potential concern to health) as solution strength increases. Other factors that affect the stability of sodium hypochlorite are temperature, period of storage, impurities and exposure to light. The oxidising capability of 1 L of sodium hypochlorite (12.5% strength) is equivalent to the oxidising capability of 125 g of chlorine gas. Sodium hypochlorite is generated by combining chlorine and sodium hydroxide.

Suitable materials for storing and handling sodium hypochlorite include ceramics, glass, fibreglass reinforced plastic, polyethylene and polyvinyl chloride, and rubber or plastic linings.

Chemistry

Sodium hypochlorite is generated by combining chlorine and sodium hydroxide.

Sodium hypochlorite hydrolyses in water forming hypochlorous acid (HOCl), which partially dissociates to hypochlorite ion ($\text{OCl}^{-}$):

$$\text{NaOCl}\rightarrow\text{Na}^{+}+\text{OCl}^{-} \newline \text{OCl}^{-}+\text{H}_{2}\text{O}\leftrightarrow\text{HOCl}+\text{OH}^{-}$$

The relative distribution of hypochlorous acid and hypochlorite ion resulting from these reactions depends on pH and temperature. At 25°C, hypochlorous acid is the predominant species between pH 1 and pH 7.5, and hypochlorite ion predominates at pH values greater than 7.5. Oxidation reactions and the disinfecting properties of chlorine tend to be more effective at low pH values, because of the predominance of hypochlorous acid, which is a stronger oxidant.

Sodium hypochlorite is a base, which will raise the pH of water, whereas chlorine gas produces an acidic reaction that lowers the pH of the solution. The extent of the pH change will depend on the alkalinity of the water.

Typical use in Australian drinking water treatment

In drinking water treatment, sodium hypochlorite is used as a disinfectant. Sodium hypochlorite solution is more expensive than chlorine, but its use is becoming more widespread, because of concerns about the safe transport and handling of hazardous gaseous chlorine in pressurised tanks.

Sodium hypochlorite can be added at various points of the treatment process:

• for oxidation of organics or metals

• for disinfection purposes

• to maintain a chlorine residual in the distribution system (pre-coagulation, intermediate or postfiltration chlorination).

Concentrations can range from 1 to 5 mg/L (as available chlorine), although 2–3 mg/L is typical. The selection of the appropriate chlorine dose should take into account the amount of disinfection by-products formed, and the required C.t (concentration × contact time) and chlorine residual; the World Health Organization (WHO) recommends 0.5 mg/L for 30 minutes. A free chlorine residual of more than 0.2 mg/L throughout the distribution system is preferred. In some systems, rechlorination is employed within the distribution system, with chlorine added after water has left the treatment plant, to boost chlorine residuals.

Superchlorination (10–50 mg/L as available chlorine) may be used to disinfect or clean tanks or pipelines. It can also be used to temporarily treat taste and odour issues caused by high ammonia levels. The process is usually followed by dechlorination, to chemically remove excess chlorine.

Knowledge of the breakpoint phenomenon (whereby chlorine applied in sufficient doses will oxidise ammonia and eliminate chloramines, resulting in the formation of a free chlorine residual) is necessary when dealing with water containing ammonia.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in sodium hypochlorite:

• chlorate

• iron

• manganese

• mercury

• nickel

Residual and by-product formation in drinking water

When employed in drinking water treatment, sodium hypochlorite should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

The use of a disinfectant such as chlorine results in the formation of free chlorine and combined chlorine residuals and disinfection by-products. By-products include trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), haloketones, chloral hydrate and chloropicrine. Although many specific chlorine disinfection by-products have been identified, a significant percentage of the total organic halogens have yet to be identified.

Many factors affect the distribution of disinfection by-product species, including pH, temperature and levels of total organic carbon (TOC), bromide and chlorine. The THMs (chloroform, bromodichloromethane, dibromochloromethane, bromoform) are the best known chlorination by-products. Chlorinated THM, HAA and HAN species are generally present in higher concentrations than brominated species; however, brominated species predominate in high-bromide waters.

Status

Sodium hypochlorite was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B300-99. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC.

Connell GF (1996). The Chlorination/Chloramination Handbook. Water Disinfection Series, American Water Works Association. Denver, Colorado.

White GC (1992). Handbook of chlorination and alternative disinfectants, 3rd edition. Van Nostrand Reinhold, New York.