Fluorosilicic Acid Storage & Tank Compatibility

Storing Fluorosilicic Acid? Start Here

Fluorosilicic acid (also called hydrofluosilicic or hexafluorosilicic acid, _H2SiF6) is a strong, corrosive inorganic acid sold only as a clear aqueous solution, most commonly in the 20 to 35 percent range. It is the dominant fluoride source for municipal drinking-water fluoridation and is also used in metal finishing, electroplating, and as a feedstock for fluorosilicate salts. The solution behaves like a powerful mineral acid with the added hazard of fluoride chemistry: it etches glass and silica, attacks metals, and on heating evolves hydrogen fluoride and silicon tetrafluoride. Because of this dual aggression, water utilities and chemical handlers store and dose it almost exclusively from polyethylene tanks. The acid is miscible with water in all proportions and dissolves with the release of heat, so dilution and transfer must be controlled. Specific gravity rises with concentration, an important factor when sizing tank walls.

Why Polyethylene Tanks Suit Fluorosilicic Acid

HDPE and crosslinked polyethylene (XLPE) are the industry-standard tank materials for fluorosilicic acid. Polyethylene is essentially inert to the aqueous acid: it does not contain the silica or reactive metal that fluoride attacks, so it resists the solution across the commercial concentration range at ambient temperature. This is why nearly every water-treatment plant doses fluorosilicic acid from polyethylene day tanks and bulk tanks. Specify a tank rated for the full specific gravity of your grade (up to roughly 1.46 for 35 percent material), keep the solution at ambient temperature to avoid evolving hydrogen fluoride fumes, and use vented fittings of PVC, CPVC, or polypropylene with FKM or EPDM gaskets. Provide acid-resistant secondary containment. Polyethylene is suitable, but the dissolved fluoride still demands HF-aware handling, ventilation, and personal protective equipment.

Material compatibility at a glance

Fluorosilicic acid is shipped, stored, and metered almost universally in polyethylene because HDPE and XLPE resist the aqueous acid where metals, glass, and concrete fail. Pair HDPE or crosslinked polyethylene tanks with PVC, CPVC, or polypropylene piping and FKM or EPDM seals. Avoid all stainless and carbon steel, glass, and unlined concrete: the fluoride content etches silica-based materials and corrodes metals while releasing hydrogen fluoride fumes.

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

• Treat skin or eye contact as a hydrofluoric-acid-type exposure: fluoride penetrates tissue and can cause deep, delayed burns. Flush immediately with water and seek medical care; calcium gluconate gel may be indicated per your safety plan.
• Causes severe skin burns and eye damage (H314 / H318); wear chemical goggles, face shield, acid-resistant gloves, and apron.
• Harmful if inhaled and may cause respiratory irritation (H332 / H335); handle with adequate ventilation or local exhaust to control fumes.
• Corrosive to many metals (H290) and reacts with silica and glass; never store in glass, metal, or unlined concrete.
• On heating or contact with strong bases the acid evolves toxic hydrogen fluoride and silicon tetrafluoride; keep cool and segregated from alkalis.
• Harmful to aquatic life (H402 / H412); prevent releases to drains and waterways and provide containment.

Common questions

Can I store fluorosilicic acid in a polyethylene tank?

Yes. HDPE and XLPE polyethylene are the standard tank materials for fluorosilicic acid and are used throughout the water-treatment industry. Choose a tank rated for the specific gravity of your concentration and pair it with PVC, CPVC, or polypropylene fittings and FKM or EPDM seals.

Why can't fluorosilicic acid be stored in metal or glass?

The dissolved fluoride attacks silica and reactive metals. It etches and dissolves glass, corrodes carbon steel and stainless steel while liberating hydrogen, and degrades unlined concrete. Polyethylene contains none of these vulnerable materials, which is why it is preferred.

What concentration is fluorosilicic acid usually supplied at?

It is almost always an aqueous solution, most commonly in the 20 to 35 percent range, with water-fluoridation grade frequently near 23 to 25 percent. Specific gravity increases with concentration, so confirm your grade before sizing a tank.

Is fluorosilicic acid flammable?

No. The aqueous solution is non-combustible and carries an NFPA flammability rating of 0. The principal hazards are corrosivity and fluoride toxicity, not fire, although heating can drive off corrosive hydrogen fluoride and silicon tetrafluoride fumes.