Sodium carbonate

(endorsed 2005)

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

General description

Sodium carbonate, $\text{Na}_{2}\text{CO}_{3}$ (also known as soda ash), is a hygroscopic, greyish-white powder. It is supplied in the form of crystalline granules containing more than 99% sodium carbonate. It is soluble in water (to 250 g/L) and noncombustible. The chemical is available in different grades. Dense soda ash (specific gravity 2.15, bulk density 1000 kg/m³) is most commonly employed in the water industry, but light soda ash (specific gravity 2.53, bulk density 500 kg/m³) may also be used. Liquid soda ash is also available as a solution of various concentrations. Liquid soda ash is typically supplied as a 10 % w/v solution that has a specific gravity of 1.1 (25°C) and a pH of up to 12.5. A 1% solution has a pH of 11.3.

Appropriate materials for handling sodium carbonate include rubber linings, iron, steel, fibreglass reinforced plastic and polyethylene.

Chemistry

Sodium carbonate is found in natural deposits and is mined. It is also recovered, with other chemicals, from lake brines. However, most is produced through the Solvay process, in which ammonia and carbon dioxide are passed into a saturated sodium chloride solution, forming first ammonium hydrogen carbonate, then soluble ammonium chloride and a precipitate of sodium hydrogen carbonate (sodium bicarbonate). The precipitate is filtered off and heated to produce sodium carbonate.

Sodium carbonate produces hydroxide and bicarbonate ions in water:

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

Sodium carbonate is used together with lime to remove noncarbonate hardness (that portion of calcium and magnesium present as noncarbonate salts) as shown below:

$$\text{MgSO}_{4}+\text{Ca(OH)}_{2}\leftrightarrow\text{Mg(OH)}_{2}+\text{CaSO}_{4} \newline \text{CaSO}_{4}+\text{Na}_{2}\text{CO}_{3}\leftrightarrow\text{CaSO}_{3}+\text{Na}_{2}\text{SO}_{4}$$

The solubility of magnesium hydroxide varies with pH. A pH of 11–11.3 is usually needed to remove magnesium effectively; this will require a concentration of lime higher than the stoichiometric requirement.

The quantity of sodium carbonate needed to remove noncarbonate hardness can be estimated using the following equation:

$$\text{Na}_{2}\text{(CO)}_{3} \text{(mg/L) = 1.05 x (noncarbonate hardness removed (mg/L))}$$

Noncarbonate hardness is expressed as $\text{CaCO}_{3}$.

Typical use in Australian drinking water treatment

In drinking water treatment, sodium carbonate is used mainly as a source of alkalinity and pH adjustment. It is more expensive than lime but is generally easier to handle, because of its higher solubility. If hard water is used for making up or diluting a solution of sodium carbonate, calcium carbonate may precipitate. This reduces the strength of the solution and can produce scale in the delivery pipelines. In this situation, the service water supplied to the soda ash system needs to be softened.

Sodium carbonate is usually made up as a solution of up to 20% concentration. Concentrations of sodium carbonate used in drinking water treatment depend on the quality of the water to be treated and the purpose of the treatment (water softening, pH adjustment or alkalinity increase). Based on stoichiometry, 1 mg/L of sodium carbonate provides alkalinity equivalent to about 0.7 mg/L of hydrated lime. Typical sodium carbonate concentrations used can vary from 5 to more than 500 mg/L, and the appropriate concentration should be determined by laboratory trials.

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 carbonate (JECFA, KIWA 1994, NRC 1982):

• arsenic

• cadmium

• calcium

• chloride

• chromium

• iron

• lead

• magnesium

• mercury

• nickel

• sulfate

Residual and by-product formation in drinking water

When employed in drinking water treatment, sodium carbonate 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 residue derived from using sodium carbonate in water softening is 30–300 mg/L.

Status

Sodium carbonate 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 B201-98. 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, 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.