Sodium bicarbonate

(endorsed 2005)

Sodium bicarbonate is used to correct pH, control corrosion, soften water for coagulation and prevent post-precipitation.

General description

Sodium bicarbonate, $\text{Na}_{2}\text{HCO}_{3}$ (also known as baking soda, bicarbonate of soda, sodium acid carbonate or sodium hydrogen carbonate), is in the form of a white powder or crystalline lumps, and has a slightly alkaline taste. It is soluble in water (96 g/L at 20°C) and stable in dry air, but slowly decomposes in moist air. Its specific gravity is 2.159 at 20°C, with a bulk density of 1000 kg/m3. Sodium bicarbonate is available in several grades, but is usually supplied as > 99% sodium bicarbonate. A 10 g/L solution has a pH of 8.4. The chemical decomposes with heat (> 50°C) and reacts with acid to release carbon dioxide.

Suitable storage materials for sodium bicarbonate include rubber linings and stainless steel.

Chemistry

Sodium bicarbonate is most economically produced by bubbling carbon dioxide gas through a solution of purified sodium carbonate; the bicarbonate precipitates out and can be collected and dried. Sodium bicarbonate is also an intermediate product in the Solvay process for making sodium carbonate.

Sodium bicarbonate provides bicarbonate alkalinity without significantly changing the pH of the water:

$$\text{NaHCO}_{3}\leftrightarrow\text{Na}^{+}+\text{H}_{2}\text{O}_{3}^{-}$$

It can further break down to carbon dioxide in the presence of acid:

$$\text{HCO}_{3}^{-}+\text{H}^{+}\leftrightarrow\text{CO}_{2}+\text{H}_{2}\text{O}$$

Typical use in Australian drinking water treatment

In drinking water treatment, sodium bicarbonate is used to correct pH, control corrosion, soften water for coagulation and prevent post-precipitation. It is used as a source of alkalinity for the treatment of waters with low alkalinity, but is more expensive than soda ash or lime. When it is used to improve coagulation, additional alkalinity or pH adjustment is often required.

The concentration of sodium bicarbonate required depends on the alkalinity and pH of the raw water and the targets for the treated water. Jar testing should be used to determine requirements.

Sodium bicarbonate can increase alkalinity with little increase in pH. It imparts a change of 0.60 g/L $\text{CaCO}_{3}$ alkalinity per mg/L as $\text{NaHCO}_{3}$.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water will vary depending on the manufacturing process. The following chemical contaminants may be present in this product (JECFA):

• ammonium

• arsenic

• chloride

• iron

Residual and by-product formation in drinking water

When employed in drinking water treatment, sodium bicarbonate 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.

Sodium, alkalinity, carbonate and carbon dioxide are the only significant residues that are expected to occur from sodium bicarbonate, but none of these is likely to become a problem at normal doses.

Status

Sodium bicarbonate 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

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/

Singer PC and Reckhow DA (1999). Chemical oxidation. 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, 12.1–12.51.