Sodium hydroxide

(endorsed 2005)

Sodium hydroxide is a commonly used alkali suitable for pH adjustment, water softening and corrosion control. It requires only a simple dosing system but needs care in handling.

General description

Sodium hydroxide, NaOH (also known as caustic soda), is a white, deliquescent solid. It absorbs water and carbon dioxide from the air. The chemical is supplied as flake or pearl solids, or liquid (usually 30% or 46–50%). It has a specific gravity of 1.33 (at 30%) and 1.48 (at 46%). Liquid solutions of sodium hydroxide can freeze in cold climates, depending on concentration. Climate considerations are relevant for any caustic soda concentrations above 30%, because such solutions can freeze at temperatures above 0°C.

Appropriate handling materials for sodium hydroxide include rubber linings and steel, stainless steel, polyvinyl chloride, polypropylene, fibreglass-reinforced plastic.

Chemistry

Sodium hydroxide is commonly produced by the electrolytic dissociation of sodium chloride, with chlorine gas as a by-product.

For pH and alkalinity adjustment, caustic soda simply produces hydroxide ions in water:

$$\text{NaOH}\leftrightarrow\text{Na}^{+}+\text{OH}^{-}$$

The chemical reactions of sodium hydroxide–soda softening are as follows:

$$\begin{array}{ll} \text{CO}_{2}+\text{2NaOH}\leftrightarrow\text{Na}_{2}\text{CO}_{3}+\text{H}_{2}\text{O} & \text{(1)} \\ \text{Ca(HCO}_{3}\text{)}_{2}+\text{2NaOH}\leftrightarrow\text{CaCO}_{3}+\text{Na}_{2}\text{CO}_{3}+\text{2H}_{2}\text{O} & \text{(2)} \\ \text{Mg(HCO}_{3}\text{)}_{2}+\text{4NaOH}\leftrightarrow\text{Mg(OH)}_{2}+\text{2Na}_{2}\text{CO}_{3}+\text{2H}_{2}\text{O} & \text{(3)} \\ \text{MgSO}_{4}+\text{2NaOH}\leftrightarrow\text{Mg(OH)}_{2}+\text{Na}_{2}\text{SO}_{4} & \text{(4)} \\ \text{CaSO}_{4}+\text{Na}_{2}\text{CO}_{3}\leftrightarrow\text{CaCO}_{3}+\text{Na}_{2}\text{SO}_{4} & \text{(5)} \\ \end{array}$$

Sodium carbonate produced from equation (1) precipitates calcium noncarbonate hardness, as shown in equation (5). Sodium hydroxide can be used in combination with lime, depending on the amount of calcium noncarbonate to be removed.

Typical use in Australian drinking water treatment

In drinking water treatment, sodium hydroxide is often used instead of powdered alkalis such as lime or soda ash, because the systems for adding sodium hydroxide are less complicated and require less maintenance. The chemical can also be used in place of lime to soften water by removing carbonate and noncarbonate hardness. Sodium hydroxide can also partially or fully substitute for the soda ash requirement.

Sodium hydroxide is used to raise pH and to convert excess carbon dioxide to alkaline species. Typical concentrations used are 2–100 mg/L (as caustic soda), but higher concentrations may be required with waters of poor quality.

Sodium hydroxide imparts a change of 1.55 mg/L calcium carbonate ($\text{CaCO}_{3}$) alkalinity per mg/L as NaOH. Control of pH is difficult when sodium hydroxide is added to poorly buffered water.

For concentrations up to about 30%, caustic soda freezes at below 0°C. At 40% concentration, caustic soda will freeze at 15°C, dropping back to around 5°C at 46%. Concentrations above 50% freeze at 12°C or higher. In cold climates it may be necessary to dilute caustic solutions or heat caustic storage and delivery facilities. Softened water should be used for dilution to minimise scaling.

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 this product (JECFA, KIWA 1994, NRC 1982):

• arsenic

• iron

• cadmium

• lead

• chloride

• mercury

• chromium

• nickel

Residual and by-product formation in drinking water

When employed in drinking water treatment, sodium hydroxide 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 amount of sodium added to water when sodium hydroxide is used to adjust pH is generally insignificant.

Status

Sodium hydroxide 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 B501-98. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Benefi eld LD and Morgan JM (1999). Chemical Precipitation. In: Water Quality and Treatment, A Handbook of Community Water Supplies, Letterman RD (ed), American Water Works Association, 5th edition. McGraw-Hill Professional, New York, 10.1–10.60.

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.

JECFA (Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Joint Expert Committee on Food Additives). Compendium of Food Additive Specifications. FAO Food and Nutrition Papers 52 (two volumes). Available at https://www.who.int/foodsafety/publications/jecfa/en/

KIWA (1994) Guideline quality of materials and chemicals for drinking water supplies. Inspectorate of Public Health and Environmental Planning, Publication 94-01. Rijswijk, The Netherlands.

Lewis RJ (1993). Hawley’s Condensed Chemical Dictionary, 12th edition. Van Nostrand Reinhold, New York.

NRC (National Research Council) (1982). Water Chemicals Codex. Committee on Water Treatment Chemicals, Food and Nutrition Board, Assembly of Life Sciences, NRC, Washington, DC.