What is AUS 32?
The ISO 22241 specification for automotive urea solution — 32.5% high-purity urea in demineralised water, sold as AdBlue or diesel exhaust fluid. Fertilizer-grade urea cannot be substituted because its additives poison the SCR catalyst.
Key points
- 32.5% technical urea in demineralised water, per ISO 22241.
- The concentration is chosen for the lowest freezing point.
- Fertilizer urea destroys the catalyst — it is not a substitute.
At a glance
| Standard | ISO 22241 |
|---|---|
| Concentration | 32.5% urea by mass |
| Water | Demineralised |
| Freezing point | About −11 °C |
| Sold as | AdBlue, diesel exhaust fluid (DEF) |
| Handling | Stainless steel or approved plastics only |
The specification
AUS 32 — Aqueous Urea Solution 32.5% — is defined by ISO 22241 and sold under brand names including AdBlue and, in North America, diesel exhaust fluid. It is 32.5% urea by mass in demineralised water, a concentration chosen because it has the lowest freezing point of any urea solution, around −11 °C.
The standard covers far more than concentration: it caps alkalinity, biuret, aldehydes, insolubles and a list of metals in the low parts per million.
What it does
AUS 32 is injected into the exhaust stream of diesel engines fitted with selective catalytic reduction. Heat decomposes the urea to ammonia, which reacts with nitrogen oxides over the catalyst to produce nitrogen and water.
That is why purity limits are so tight. The controlled elements are catalyst poisons, and contamination degrades conversion efficiency permanently rather than temporarily.
Why fertilizer urea will not do
Agricultural urea carries anti-caking agents, formaldehyde-based conditioners and higher metal content. Dissolving it to 32.5% produces a solution that looks correct and destroys the catalyst.
So AUS 32 requires technical-grade urea produced to the standard, made up with demineralised water, and handled in dedicated equipment. Stainless steel or approved plastics only — ordinary carbon steel contaminates the solution.
Frequently asked questions
- What is the difference between AUS 32 and AdBlue?
- None chemically. AUS 32 is the ISO 22241 specification; AdBlue is a registered trademark for a product meeting it. Diesel exhaust fluid is the North American term for the same thing.
- Can fertilizer urea be used to make AdBlue?
- No. Agricultural urea contains anti-caking agents and higher metal content that poison the SCR catalyst. AUS 32 requires technical-grade urea and demineralised water.
- Why is AUS 32 exactly 32.5%?
- Because that concentration has the lowest freezing point of any aqueous urea solution, roughly −11 °C, which maximises usability in cold climates.
Related terms
An impurity formed when urea is overheated during production. It matters because high biuret damages foliage in leaf-applied fertilizer — agricultural urea is normally capped at 1% maximum, with tighter limits for foliar use.
The laboratory document stating the measured composition of a specific parcel — purity, contaminants, moisture. It refers to the lot actually shipped, unlike a typical or datasheet analysis which describes what a plant usually produces.
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