Sulfuric acid

(endorsed 2005)

Sulfuric acid is used to correct pH in coagulation, water softening, corrosion control, prevention of post-precipitation and activation of silica.

General description

Sulfuric acid ($\text{H}_{2}\text{SO}_{4}$) is a strongly corrosive, dense, oily liquid. It is colourless to dark brown, depending on purity, and is miscible with water. Sulfuric acid is generally available in concentrations of 28.5–98.5%, with corresponding specific gravity of 1.2–1.85 at 20°C. The acid is very reactive and dissolves most metals; the concentrated acid oxidises, dehydrates or sulfonates most organic compounds, often causing charring.

Sulfuric acid is highly corrosive to most metals and alloys, and is corrosive to mild steel at concentrations below 90%. It can be stored in fibreglass-reinforced plastic with acid resistant resins, polyethylene, porcelain, glass and rubber linings.

Chemistry

Sulfuric acid is usually produced using the Contact process: sulfur dioxide is catalytically converted to sulfur trioxide, which is then dissolved in sulfuric acid and water.

Sulfuric acid disassociates in water to produce a strong acid:

$$\text{H}_{2}\text{SO}_{4}\leftrightarrow\text{2H}^{+}+\text{SO}_{4}^{2-}$$

Sulfuric acid is added to lime–soda softened waters to prevent post-precipitation of calcium carbonate and magnesium hydroxides in filters or in water distribution systems. These water usually have pH values of approximately 10.4, and are supersaturated with calcium carbonate ($\text{CaCO}_{3}$) and magnesium hydroxide ($\text{Mg(OH)}_{2}$). Sulfuric acid is therefore used to reduce excessive pH values and alkalinities as follows:

$$\text{H}_{2}\text{SO}_{4}+\text{2CaCO}_{3}\text{(s)}\leftrightarrow\text{Ca(HCO}_{3}\text{)}_{2}+\text{CaSO}_{4} \newline \text{H}_{2}\text{SO}_{4}+\text{Ca(OH)}_{2}\leftrightarrow\text{CaSO}_{4}+\text{2H}_{2}\text{O}$$

Sulfuric acid is used to fortify hydrolysing metal salts (aluminium and iron). The typical acid-fortified alum product, also called acidulated alum or acid alum, contains 5–20% (weight basis) of sulfuric acid. For a given amount of metal ion added to the water, strong acid-fortified products react with more alkalinity and depress the pH to a greater extent than nonfortified metal salt solutions.

Typical use in Australian drinking water treatment

In drinking water treatment, sulfuric acid is used to correct pH in coagulation, water softening, corrosion control, prevention of post-precipitation and activation of silica.

Handling and adding concentrated sulfuric acid to water requires extreme caution, because it can cause severe burns and eye damage. Also, sulfuric acid has an exothermic reaction with water that may cause violent splattering. Careful design is required in dilution systems for sulfuric acid, because the significant heating that may occur could damage pipework.

Concentrations of sulfuric acid required vary widely, depending on the alkalinity of the water and the pH required. Low concentrations (1–30 mg/L) are usually adequate to adjust pH for coagulation; higher doses may be required for water softening.

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):

• antimony

• arsenic

• cadmium

• chloride

• chromium

• copper

• fluoride

• iron

• lead

• manganese

• mercury

• selenium

• sulfate

• sulfur dioxide

• zinc

Residual and by-product formation in drinking water

When employed in drinking water treatment, sulfuric acid 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.

Status

Sulfuric acid 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

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

Schock MR (1999). Internal Corrosion and Deposition Control. In: A Handbook of Community Water Supplies, Letterman RD (ed), American Water Works Association, 5th edition. McGraw-Hill Professional, 17.1–17,109.