Automotive urea — AdBlue® / DEF
Urea and water. Sold to four decimal places.
AUS 32 to ISO 22241 — the same molecule that goes on a field, purified until the trace metals are counted in milligrams per tonne. Because what it feeds is not a crop, it is a catalyst, and a catalyst does not recover.
The desk
The contamination that matters here would be invisible in any other urea trade.
Selective catalytic reduction works by injecting a urea solution into hot exhaust gas, where it decomposes to ammonia and reduces nitrogen oxides over a catalyst. The chemistry is straightforward. The commercial consequence is not.
The catalyst is poisoned by metals — permanently. Alkali and alkaline-earth metals block the active sites, phosphorus glazes the surface, and none of it reverses when clean fluid is used again. So the specification limits calcium, iron, sodium, potassium, aluminium and magnesium to 0.5 mg/kg each, and copper, zinc, chromium and nickel to 0.2 mg/kg. That is half a gramme of iron in a tonne of fluid.
Those numbers are not achievable by filtering a fertiliser-grade product. They require technical-grade urea made to a low-biuret specification, deionised water, and a plant with no wetted carbon steel, brass or galvanising anywhere in it. Which is why automotive urea sits on this page as a fertiliser derivative and behaves in trade like a fine chemical: the failure mode is not a discount, it is a fleet of ruined aftertreatment systems and a warranty argument that lands on whoever supplied the fluid.
The standard
ISO 22241-1, complete.
Reproduced in full because it is short, because every line of it is a rejection criterion, and because "meets ISO 22241" appears on a great many documents attached to product that does not.
| Parameter | Minimum | Maximum | Unit |
|---|---|---|---|
| Urea content | 31.8 | 33.2 | % by mass |
| Density at 20 °C | 1.0870 | 1.0930 | g/cm³ |
| Refractive index at 20 °C | 1.3814 | 1.3843 | — |
| Alkalinity as NH₃ | — | 0.2 | % |
| Biuret | — | 0.3 | % |
| Aldehydes | — | 5 | mg/kg |
| Insolubles | — | 20 | mg/kg |
| Phosphate as PO₄ | — | 0.5 | mg/kg |
| Calcium | — | 0.5 | mg/kg |
| Iron | — | 0.5 | mg/kg |
| Aluminium | — | 0.5 | mg/kg |
| Magnesium | — | 0.5 | mg/kg |
| Sodium | — | 0.5 | mg/kg |
| Potassium | — | 0.5 | mg/kg |
| Copper | — | 0.2 | mg/kg |
| Zinc | — | 0.2 | mg/kg |
| Chromium | — | 0.2 | mg/kg |
| Nickel | — | 0.2 | mg/kg |
Density and refractive index are the field test
Both are functions of urea concentration, and a handheld refractometer reads the second one in seconds. It is the only parameter on the table that can be checked at a tank, which is why it is the standard receiving check — and why "we tested it on delivery" usually means only that the water content was right.
Biuret at 0.3% is a urea-grade decision
The same ceiling as foliar-grade fertiliser urea, and it is set in the producer's reactor. A blender cannot fix biuret; it can only buy urea that already meets it.
The metals are the real test
Everything above the metals block can be got right with good process control and clean water. The metals block is a statement about the plant itself — its pipework, its tanks, its pumps and its filling line. That is what a certificate of analysis is really evidencing.
Naming
Same fluid, four names, and one of them is a trademark.
Worth being precise about, because the words carry different obligations and are used interchangeably by people who have not checked.
- AUS 32
- Aqueous Urea Solution 32 %. The neutral technical designation used by ISO 22241, and the correct term for a contract.
- AdBlue®
- A registered trademark of the German automotive industry association, licensed to producers who meet the standard and the association's own conditions. Product may be fully ISO 22241 compliant and still not be entitled to the name. Using the mark without a licence is a trademark matter, and we write AUS 32 in contracts for that reason.
- DEF
- Diesel Exhaust Fluid — the ordinary North American term, referencing the same solution, generally against ISO 22241 and the API certification programme.
- ARLA 32
- The Brazilian designation, again the same fluid to the same concentration.
If a supplier's documentation switches between these terms within one offer, that is worth a second look. It is often a sign that the paperwork has been assembled from more than one source.
Materials
What may touch it, and what quietly ruins it.
Urea solution is mildly corrosive and an excellent solvent for exactly the metals the specification prohibits. So the containment is part of the product.
| Suitable | Not suitable — and why |
|---|---|
|
Austenitic stainless steel, grades 304 and 316 High-density polyethylene Polypropylene PVDF and PTFE Titanium |
Carbon and galvanised steel — contributes iron and zinc directly Copper and brass — copper is limited at 0.2 mg/kg; a brass fitting is enough Aluminium — corrodes and contributes aluminium Zinc and zinc coatings — same problem as galvanising Ordinary mild-steel drums and IBC cages with wetted metal parts |
Storage
Temperature at both ends of the scale.
- It freezes at about −11 °C
- Which is by design: 32.5 % is close to the eutectic, so the urea and the water freeze and thaw together and the concentration is unchanged on thawing. Freezing does not spoil the fluid. It does expand it by roughly 7 %, so containers and tanks need ullage, and vehicle systems are built with heated lines for the same reason.
- Heat is what actually degrades it
- Urea hydrolyses slowly to ammonia, and the rate rises sharply with temperature. Stored below about 25 °C the fluid holds specification for roughly a year; held in the thirties, that shortens considerably, and in direct sun on a quay it shortens again. Shelf life is a storage-condition statement, not a fixed number, and any supplier quoting one without a temperature is quoting half of it.
- Sealed, and away from light
- Opaque or shaded containment, kept closed. Ammonia loss changes the concentration and takes the fluid out of the density and refractive-index window — which is the one thing the receiving check would have caught.
- Dedicated equipment
- Tanks, pumps, hoses and nozzles used for this and nothing else, and labelled. The cost of dedicating them is trivial against the cost of one contaminated delivery into a fleet.
- What we do about it
- Manufacturing date on every consignment, not just a batch number; storage-condition requirements written into the contract; and the fluid re-tested on arrival where the transit conditions warrant it rather than only at despatch.
Scope
Where our obligation ends.
- What we supply
- AUS 32 to ISO 22241 in bulk road and ISO tank, IBC and drum, and the technical-grade low-biuret urea that blenders make it from.
- What we do not do
- We do not blend, we do not operate filling lines, and we do not run dispensing infrastructure. We are not a licensee of the AdBlue® trademark and we do not represent product as licensed under it.
- What we will not do
- Supply fertiliser-grade urea solution against an ISO 22241 requirement, or accept a certificate of analysis that does not carry the full metals block. A partial certificate on this product is not a partial answer; it is the absence of one.
- Where our responsibility stops
- At the agreed delivery point, against a specification tested on despatch and, where agreed, on arrival. Contamination introduced by receiving equipment is outside our control — which is why we would rather discuss your tanks and hoses before the first delivery than after a claim.
- Onward handling
- We will tell you plainly if your receiving arrangement will take the fluid out of specification. That conversation costs a sale occasionally. It costs less than the alternative.
How to specify
Six lines and we can price it.
The third one is the one buyers most often leave to chance, and the one that decides whether the fluid arrives in specification.
Standard and edition
ISO 22241 with the part and the year. And whether you require a licensed trademark or simply a compliant AUS 32.
Packing
Bulk road tanker, ISO tank, IBC or drum — with the container material stated and confirmed dedicated.
Your receiving arrangement
Tank material, pump, hose and nozzle, and whether they are dedicated. Tell us even if it feels like our concern rather than yours — it is where most contamination happens.
Volume and cadence
Litres or tonnes, and the delivery frequency. It determines whether shelf life is a live constraint or an irrelevance.
Climate and storage
Ambient range at the storage site, whether the tank is shaded or heated, and the expected residence time.
Testing requirement
Certificate per batch, and whether you want independent verification on arrival as well as on despatch.
Inspection
A full certificate, or none.
Independent verification by an agency appointed for the consignment — SGS, Bureau Veritas or Intertek — instructed jointly. On this product the analysis is the whole quality system, and the metals block is the part that is expensive to run and therefore the part most often quietly omitted.
- Sampling
- Into clean HDPE or fluoropolymer sample containers — never metal, and never a container that has held anything else. On a product with 0.2 mg/kg limits, the sample bottle is a plausible source of the result.
- Full parameter set
- Every line of the table above, per batch. Metals by ICP-MS or ICP-OES with the detection limit stated — a metal reported as "not detected" without a stated limit is not a result.
- Receiving check
- Refractive index or density at the tank on every delivery. It is quick, it needs no laboratory, and it catches dilution, ammonia loss and the wrong product in the compartment.
- Batch traceability
- Manufacturing date and batch number on the certificate and on the container, reconciled to the delivery. Without a date, shelf life is unmanageable.
- Container condition
- Seals verified intact, container material confirmed, and dedication evidenced. Photographed at loading and at delivery.
- Documentation per consignment
- Full certificate of analysis against ISO 22241-1 including the complete metals block, manufacturing date and batch number, safety data sheet in the destination language, container dedication statement, delivery note with the receiving-check result recorded, and the inspection report.
Straight answer
What we will tell you before you ask.
- That a certificate without the metals block proves nothing
- Concentration, density and biuret are the easy part. The metals are what the specification is actually for, and they are the lines most often missing from a document offered as compliant.
- That AdBlue® is a trademark and AUS 32 is the product
- Compliant fluid is not automatically entitled to the mark. We contract on AUS 32 to ISO 22241 and say separately whether a licence applies.
- That freezing is harmless and heat is not
- It thaws back to the same concentration; that is why 32.5 % was chosen. Storage in the thirties is what shortens shelf life, so a shelf life quoted without a temperature is half a statement.
- That your hose can fail the specification
- A brass fitting, a galvanised tank or a shared drum is enough at these limits. We will ask about your receiving equipment before the first delivery, and we would rather have that conversation than the other one.
- That we will not send fertiliser urea against this spec
- It cannot be filtered up to standard. If low-biuret technical urea is not available for a window, we will say the window is not available rather than substitute.
Enquiries
Every enquiry is answered by the desk that handles it.
Send the product, quantity, delivery basis, destination and timing. We revert with availability, an indication and the documentation that accompanies it. Specifications are released once we know who we are speaking with.