Sodium Thiosulfate (Pulp & Paper / Dechlorination Grade) Storage & Tank Compatibility

Storing Sodium Thiosulfate (Pulp & Paper / Dechlorination Grade)? Start Here

Sodium thiosulfate (Na₂S₂O₃) is a colorless, highly water-soluble inorganic salt long known to operators as "hypo" or chlorine-control. In pulp, paper and water-treatment service it acts as a mild reducing agent: it neutralizes residual chlorine, chlorine dioxide and excess oxidizing bleach, quenching the bleaching reaction and protecting fibers and downstream equipment. Shipped as crystals, powder or ready-made solution, it dissolves readily to give a near-neutral brine that is non-flammable and carries no GHS hazard classification. Because the working fluid is simply a dissolved salt, it is gentle on polyethylene: the chemistry that makes thiosulfate aggressive toward stainless steel and carbon steel has little effect on HDPE or crosslinked PE. That combination - low hazard, high solubility, and excellent polymer compatibility - makes bulk PE storage the natural fit for dechlorination and pulp-bleach quench systems.

Polyethylene (HDPE / XLPE) Compatibility with Sodium Thiosulfate

Sodium thiosulfate solution is one of the easier industrial fluids to store in polyethylene. As a fully dissolved inorganic salt, it does not swell, plasticize or chemically attack the polymer backbone, and published HDPE and LDPE resistance charts list thiosulfate (hypo) as Resistant / Excellent at both ambient and elevated test temperatures. Both HDPE and crosslinked polyethylene (XLPE) earn a Satisfactory (S) rating for ambient and warm aqueous service.

Practical guidance still applies. Specify a tank rated for the solution's specific gravity, which increases with concentration. Keep the stream near neutral; strongly acidified thiosulfate can decompose and liberate sulfur and sulfur dioxide, so design the system to avoid acid contact. Use compatible PE, PP or PVC/CPVC fittings and EPDM seals rather than carbon steel. As with any chemical, verify the tank, gaskets and fittings together against your actual concentration and temperature before commissioning.

Material compatibility at a glance

Sodium thiosulfate ships and stores well in polyethylene. Aqueous solutions are rated Satisfactory (S) against HDPE and XLPE in published polyethylene chemical-resistance charts, making PE bulk tanks, totes and lines the standard choice for dechlorination and pulp-bleach quench duty. PP and PVC/CPVC are also S. Stainless steel is only Conditional because the thiosulfate ion can drive pitting and crevice corrosion, and carbon steel is Unsuitable.

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

• Although sodium thiosulfate carries no GHS hazard classification, treat dusts and solutions as mild irritants - wear safety glasses and gloves and avoid creating airborne dust.
• Keep the solution near neutral pH. Contact with strong acids decomposes thiosulfate and can release sulfur dioxide gas and elemental sulfur; never mix with acids in storage or spill cleanup.
• Do not store or mix with strong oxidizers (chlorine, hypochlorite, nitrates, peroxides) outside the controlled dosing point - thiosulfate is a reducing agent and reacts with them.
• Provide adequate ventilation in enclosed dosing rooms and address spills promptly to prevent slick, slippery surfaces.
• Store closed and dry; the anhydrous salt is hygroscopic and will cake if left open to humid air.
• Follow the manufacturer Safety Data Sheet and local codes for storage, secondary containment and disposal.

Common questions

Can sodium thiosulfate solution be stored in an HDPE or poly tank?

Yes. Aqueous sodium thiosulfate is rated Satisfactory against HDPE and crosslinked polyethylene in standard resistance charts, so PE bulk tanks, totes and lines are the normal choice. Match the tank's specific-gravity rating to your solution concentration.

Is sodium thiosulfate flammable or hazardous to handle?

It is a non-combustible inorganic salt with no flash point and carries no GHS hazard classification. Published SDS NFPA ratings are low (Health 1, Flammability 0-1, Instability 1). Standard glasses and gloves are appropriate; avoid dust and acid contact.

Why is stainless or carbon steel less suitable than polyethylene?

The thiosulfate ion can promote pitting and crevice corrosion on stainless steel and attacks carbon steel, while it does not chemically affect polyethylene. That is why PE, PP and PVC/CPVC are preferred for thiosulfate storage and piping.

What happens if sodium thiosulfate contacts acid?

Acidification decomposes thiosulfate, releasing sulfur dioxide gas and precipitating elemental sulfur. Keep the stored solution near neutral and never combine it with strong acids in tanks, lines or spill cleanup.