OFF 32

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Container from the North American market
Left: gas pump for refueling with AUS 32; right: diesel fuel

AUS 32 ( aqueous urea solution ) is a 32 percent aqueous urea solution standardized in accordance with ISO 22241 . It is used for exhaust gas aftertreatment in diesel engines in road and rail vehicles. There it enables a reduction of the emitted nitrogen oxides (NO x ) by means of selective catalytic reduction (SCR) by up to 90 percent. The standard itself defines and speaks of the liquid as a NOx reduction agent AUS 32 and abbreviates it with AUS 32 .

The liquid is traded worldwide under various names, in North America for example also under Diesel exhaust fluid , in Brazil also under ARLA 32 (Portuguese: Agente Redutor Liquido de Óxido de Nitrogênio Automotivo ) or as HWL (urea-water solution). The protected brand AdBlue of the German Association of the Automotive Industry has achieved importance .

For the technical application and mode of action of the urea solution, see SCR for diesel engines .

Physical Properties

The liquid has the following physical properties, among others:

property value unit
Urea content 31.8-33.2 Wt%
Start of crystallization −11.5 ° C
Refractive index 1.3817-1.3843 -
Thermal conductivity 0.57 W / (m K)
Specific heat capacity 3.51 J / (g K)
Enthalpy of fusion 270 J / g
Density at 20 ° C 1.087-1.093 g / cm³
Dynamic viscosity 1.4 mPa · s

The concentration of 32% was chosen because it is a eutectic solution, so that in the case of freezing the urea concentration in the liquid and solid phase does not differ. This means that liquid urea solution can be taken from a partially frozen AdBlue tank without changing the urea concentration in the tank. In addition, the eutectic solution has the lowest freezing point of all mixing ratios.

The aqueous solution does not pose any particular risk in terms of European chemicals law. According to transport law, it is also not a dangerous good. Skin contact should be avoided; Any residues can be washed off with water.


The urea solution is produced worldwide and used for exhaust gas aftertreatment. The standard describes detailed requirements for production, quality assurance and logistics. For example, there must be no significant contamination when decanting into smaller containers.

There are urea solutions in different container sizes for bulk buyers and retailers. It is also offered at fuel pumps for refueling road and rail vehicles .

Individual evidence

  1. a b Preview of ISO 22241-1 , accessed 2018-04-03.
  2. ^ Robert Bosch GmbH: Kraftfahrtechnisches Taschenbuch. 28th edition, Springer Vieweg, Wiesbaden 2014, ISBN 978-3-658-03800-7 , p. 719.