Sulfurous Acid Storage & Tank Compatibility

Storing Sulfurous Acid? Start Here

Sulfurous acid (_H2SO3) is the corrosive, weakly acidic solution formed when sulfur dioxide dissolves in water. It exists only in aqueous form, releasing pungent sulfur dioxide gas as it warms or decomposes, and is valued across industry as a reducing agent, bleaching agent, dechlorinating chemical, and preservative. In water treatment it neutralizes residual chlorine; in pulp, paper, and food processing it bleaches and inhibits microbial growth. Because the acid is corrosive to skin, eyes, and respiratory tissue and readily attacks metals, containment material selection is critical. Polyethylene tanks resist the dilute aqueous acid and the dissolved sulfur dioxide that drives its behavior, making cross-linked and high-density polyethylene the preferred storage choice. This page summarizes the verified identity, hazard ratings, physical properties, and honest material-compatibility guidance for safely storing and handling sulfurous acid.

Polyethylene (HDPE & XLPE) Compatibility with Sulfurous Acid

Polyethylene is well suited to sulfurous acid in the dilute aqueous concentrations encountered in water treatment, pulp and paper, and food applications. Both high-density polyethylene (HDPE) and cross-linked polyethylene (XLPE) are rated suitable against the aqueous acid and against the dissolved sulfur dioxide that defines its chemistry, which is why they are the standard tank materials for this service. The acid is a reducing agent rather than a strong oxidizer, so it does not embrittle polyethylene the way concentrated nitric or chromic acid can. As with any acid service, confirm the specific concentration, the operating temperature, and the required design margin against the resin maker's current chemical-resistance chart before finalizing a tank. Keep the tank vented appropriately, since warming the solution liberates sulfur dioxide gas, and select gaskets and fittings of compatible thermoplastics or properly graded elastomers.

Material compatibility at a glance

Sulfurous acid is a corrosive, reducing acid that aggressively attacks common metals while leaving polyethylene and most engineering thermoplastics intact. For bulk and day-tank storage, cross-linked or high-density polyethylene is the industry-standard wetted material, supported by PP, PVC/CPVC, and PVDF for piping and fittings. Elastomer seals should be selected by grade for the specific concentration and temperature. Carbon steel and stainless steel are unsuitable because the acid corrodes them and can liberate hydrogen on contact with metal.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• Corrosive: causes severe skin burns and serious eye damage (H314, H318). Wear chemical splash goggles, a face shield, acid-resistant gloves, and protective clothing.
• Harmful if inhaled (H332): the solution releases pungent sulfur dioxide gas. Use only with adequate ventilation or local exhaust and avoid breathing vapor or mist.
• Keep away from metals; contact can corrode containers and evolve flammable hydrogen gas, and closed containers may build pressure or rupture when heated.
• Store in clearly labeled, vented polyethylene or other compatible containers away from heat, oxidizers, and strong bases.
• Have eyewash and safety shower immediately accessible; in case of contact, flush with water for at least 15 minutes and seek medical attention.
• Do not mix with oxidizers or acids that could drive sudden decomposition or release of sulfur dioxide.

Common questions

What kind of tank should I use to store sulfurous acid?

A cross-linked (XLPE) or high-density polyethylene (HDPE) tank is the standard choice. Polyethylene resists dilute sulfurous acid and the dissolved sulfur dioxide that drives its chemistry, while metal tanks corrode. Always size the tank for proper venting and confirm concentration and temperature against the resin maker's chart.

Why can't I store sulfurous acid in a steel or stainless tank?

Sulfurous acid is corrosive to most metals. It attacks carbon steel and can pit stainless steel, and contact with metal can release flammable hydrogen gas. Polyethylene and compatible thermoplastics avoid these problems, which is why they are used for containment.

Is sulfurous acid flammable?

No. Sulfurous acid is non-combustible and does not burn. However, when heated it decomposes and releases corrosive and toxic sulfur dioxide fumes, and contact with metals can generate flammable hydrogen gas, so it is not hazard-free.

Does sulfurous acid need a vented tank?

Yes. Because warming the solution liberates sulfur dioxide gas, the storage tank should be properly vented to relieve pressure and prevent gas buildup. Confirm vent sizing and any scrubbing requirements with your engineer based on volume and temperature.