Soap makers buy it by the bag as lye. Facilities managers buy it by the drum as caustic soda for drain lines and water treatment. Cleaning brands dilute it into oven cleaner and hair-relaxer formulas. All three are shipping the same molecule, and sodium hydroxide dangerous goods classification treats them very differently depending on two things: whether the product is solid or dissolved, and, if dissolved, how strong.
This guide sets out both entries precisely — UN1823 for the solid, UN1824 for the solution — where the packing group boundaries actually sit, and why one packaging material is flatly off-limits regardless of concentration.
Why one chemical has two UN numbers
Sodium hydroxide is corrosive by the same Class 8 test as any other entry in the class: it destroys living skin tissue on contact, or corrodes steel and aluminium at a measurable rate. What changes between the solid and the solution is how much of that corrosivity is present in a given package, and that is a dilution question, not an identity question.
A sealed bag of anhydrous pellets carries the full corrosive strength of the chemical with nothing to weaken it, which is why UN1823 never drops out of packing group II. A litre of drain cleaner at 2% sodium hydroxide is a different hazard profile entirely — weak enough, in many cases, to fall outside Class 8 altogether. Sodium hydroxide dangerous goods classification has to account for both ends of that range under one chemical name, which is exactly why the regulations split it into two entries rather than one.
UN1824 and UN1823 at a glance
| Field | UN1823 (solid) | UN1824 (solution) |
|---|---|---|
| Proper shipping name | Sodium hydroxide, solid | Sodium hydroxide solution |
| Hazard class | 8 | 8 |
| Packing group | II, fixed | II or III, or not regulated |
| Label | Class 8 corrosive | Class 8 corrosive |
| Special provisions | IB8, IP2, IP4, N34, T3, TP33 | B2, IB2, N34, T7, TP2 |
| ERG guide | 154 | 154 |
| Marine pollutant | No | No |
| Common form | Lye beads, soap-making lye, crystal drain cleaner | Caustic soda solution, liquid drain cleaner, oven cleaner |
For how packing groups are set across every hazard class, see our guide to hazardous goods categories and packing groups I, II and III. UN1823's fixed packing group II is the simpler of the two determinations here; UN1824 is where the actual classification work happens.
Sodium hydroxide dangerous goods classification by concentration
For the solution, packing group is a direct function of the sodium hydroxide percentage measured against the Class 8 corrosivity criteria in 49 CFR 173.137. There is no separate lab test to commission for a typical SKU — the percentage on the SDS is the input the determination needs.
| Sodium hydroxide | Packing group | Typical product |
|---|---|---|
| At or below 5% | Not regulated | Dilute ready-to-use cleaning solution |
| Above 5%, up to 20% | III | Concentrated liquid drain cleaner |
| Above 20% | II | Industrial caustic soda solution, 50% bulk grade |
Most consumer-facing drain cleaner concentrates sit comfortably above the 5% threshold and below 20%, which lands them in packing group III. Industrial and water-treatment grade caustic — commonly sold at or near 50% — is well into packing group II territory. The genuinely easy mistake is at the low end: a "gentle" reformulation that drops available sodium hydroxide to 4% can move a product out of Class 8 entirely, and a seller who keeps declaring it as packing group III is over-declaring a product that no longer needs a hazmat process at all.
The one packaging material that is never allowed
Both UN1823 and UN1824 carry special provision N34, which prohibits aluminium construction in any part of a packaging or tank that is normally in contact with the material. This is not a corrosion-resistance recommendation — it is a listed regulatory requirement, and the chemistry behind it is straightforward: sodium hydroxide reacts with aluminium to produce hydrogen gas, and hydrogen becomes flammable at concentrations as low as roughly 4% in air. In a sealed or poorly vented container, that reaction can build pressure and an explosive atmosphere without any external heat source at all.
N34 is not boilerplate. It exists because sodium hydroxide and aluminium generate hydrogen on contact, and a sealed tank gives that gas nowhere to go. The packaging material is part of the classification, not a detail left to the shipper's judgement afterwards.
Classify7 ruleset team
When the packaging material actually mattered
On 7 April 2026, a cargo tank hauling roughly 1,100 gallons of sodium hydroxide solution exploded at an industrial site in Mooresville, North Carolina, killing the truck's driver and injuring a facility employee. The driver had reported a leak around twenty minutes before the explosion, and site camera footage reportedly showed vapour coming from underneath the vehicle beforehand. The National Transportation Safety Board opened an investigation and confirmed the vehicle involved was an aluminium cargo tank.
The NTSB's inquiry into the precise cause was still open at the time of writing, and nothing here should be read as a finding the agency has not made. What the incident does illustrate, independent of its ultimate cause, is exactly the failure mode special provision N34 is written to prevent: sodium hydroxide and aluminium do not belong in sustained contact, and a transport chemistry that many shippers treat as a paperwork formality is, in a large enough tank, a genuine physical hazard. It is a useful reminder that a Class 8 special provision is not administrative padding — it is describing a reaction that will happen whether or not the shipping paper mentions it.
Where the carriers actually diverge
Regulatory eligibility is one question; what a given service will actually carry is another.
- UPS and FedEx accept properly classified, packaged and labelled UN1823 and UN1824 on ground and, within limits, air services, subject to their own dangerous goods guides and quantity limits.
- USPS accepts both under Publication 52's hazardous materials rules, with the same noncompliance fee exposure on undeclared packages that applies across their hazmat programme.
- Amazon FBA treats corrosive solids and liquids as a reviewed category rather than an automatic acceptance; inbound shipments of drain cleaner or bulk lye are commonly held for hazmat review before they reach a fulfilment centre.
Our carrier-by-carrier hazmat comparison sets these positions out side by side, and the Amazon FBA hazmat review guide covers what a corrosive submission actually needs to clear. For the federal penalty framework behind a missed declaration, PHMSA's hazmat programme sets out the civil penalty structure that applies regardless of carrier.
- Declaring a reformulated, lower-strength solution at its old packing group instead of re-checking against the current SDS.
- Treating UN1823's fixed packing group II as if it could vary the way UN1824's does.
- Using or approving aluminium-lined packaging, valves or tank fittings for either entry.
- Assuming a small hobbyist bag of lye is below the threshold for any hazmat process without checking the actual pack size limits.
- Skipping the declaration on a mailed bottle of concentrated drain cleaner and assuming a small parcel will not be checked.
Checking your own SKU
The determination needs two things you almost certainly already have: the physical state, and, for a solution, the sodium hydroxide percentage from the SDS or certificate of analysis. Send them to POST /api/classify alongside the product description and you get the correct UN number back with the concentration-derived packing group, the N34 packaging flag, and the per-carrier position for the service you plan to use.
For the wider corrosives picture, see hazardous goods categories and packing groups, or browse the encoded Class 8 entries to see what else shares the class. Solid and dissolved sodium hydroxide sit inside the same regulatory framework as every other corrosive — sodium hydroxide dangerous goods classification is a concentration and packaging-material check once you have both figures in hand.
