AUS 32 in Winter: What to Do If Urea Solution Freezes

The first frost below −11 °C, and a truck dashboard shows an SCR warning while the mechanic reports "ice" in the tank. The usual reaction is to drain the fluid and refill. The most expensive mistake is thawing the tank with hot water or "diluting it so it won't freeze". Freezing is a predictable property of the solution, not a sign of a bad batch. What really ruins the fluid is contamination, the wrong containers and overheating.

A 32.5% urea solution (international name AUS 32, Aqueous Urea Solution 32.5%, also called diesel exhaust fluid or DEF) is required by diesel engines with selective catalytic reduction (SCR) to meet Euro 4 and higher emission standards. It is injected into the exhaust and converts nitrogen oxides into nitrogen and water. Consumption is 2–6% of diesel fuel volume (Wikipedia, Diesel exhaust fluid). For a fleet of 20 trucks that means hundreds of litres a month, and every winter raises the same questions.

In short: the solution freezes at −11 °C and is fit for use after thawing. Never dilute it with water or add antifreeze or alcohol. Store it in closed polyethylene or stainless steel containers, indoors, out of direct sunlight and below +30 °C. A refractometer checks a batch in 1 minute.

Why the solution freezes at exactly −11 °C

The 32.5% concentration is deliberate. It is the eutectic point of the urea–water system: the mixture with the lowest possible crystallisation temperature. When it freezes, water and urea crystallise together, so the concentration in the ice and in the remaining liquid stays the same. After thawing you have the same 32.5% solution.

Any other concentration freezes earlier. A solution diluted with water starts crystallising at a higher temperature, while a more concentrated one drops urea out as sediment when cooled.

Numbers 🔬

• 32.5% urea and 67.5% deionised water: the composition (Wikipedia).
• −11 °C: the freezing point (Wikipedia).
• 31.8–33.2%: permitted urea content under ISO 22241-1:2019.
• 1.087–1.093 g/cm³: density at 20 °C; 1.3814–1.3843: refractive index under ISO 22241-1.
• ~60 °C: the temperature at which urea in solution starts decomposing rapidly (Wikipedia).

Myth: "frozen fluid must be thrown away"

It doesn't. If the container stayed closed and nothing got into the fluid, after natural thawing it meets the same specification as before the frost. Modern SCR systems heat the tank and lines precisely because vehicle manufacturers treat freezing as a normal operating condition.

What actually matters when it freezes:

• Ice takes up more volume than liquid. Don't fill canisters or IBCs to the brim, or the ice will deform them.
• Thaw naturally only. A warm room, not hot water, a gas torch or a heating element on the container. Local overheating above 60 °C starts urea decomposition into ammonia and biuret.
• Don't open containers in frost unless necessary. Condensation and dust getting inside are the main source of contamination in winter.

Diagnosis: is it the frost or the fluid?

SCR faults in winter are often blamed on the cold when the real cause is poor-quality fluid. This table helps tell the two apart.

How to store a fleet's supply in winter: 6 steps

1. Keep the fluid in a heated or insulated room

The optimal range is −11 to +25 °C: the supply doesn't freeze and urea decomposition is minimal. ISO 22241-3 advises against prolonged storage above +30 °C.

2. Choose compatible containers and equipment

Polyethylene, polypropylene, stainless steel. See the table below for details.

3. Always close containers

An open canister in a warehouse collects dust, moisture and fuel vapours. Any contamination leads straight to a clogged injector.

4. Use dedicated equipment

Funnels, hoses and nozzles that have touched diesel, oil or antifreeze are not suitable for urea solution. Even traces of hydrocarbons harm the catalyst.

5. Keep batch records

Record the delivery date, batch number and quality certificate number. If a vehicle has a problem, you will quickly find out which batch it was filled from.

6. Check every new batch with a refractometer

A handheld refractometer with an AUS 32 scale shows concentration in 1 minute. It is the fastest way to catch dilution with water at goods receipt.

How much to keep in stock for winter: a fleet calculation

In winter, deliveries are more often disrupted by weather and power cuts, so plan stock for 3–4 weeks of operation. The formula is simple: diesel consumption × urea solution share (2–6% depending on the engine and duty cycle).

Find your fleet's exact share from actual data: divide the solution volume filled in a month by the diesel volume. The figure is stable and lets you plan purchases without emergency deliveries in frost.

Tanks, pumps and seals: what is compatible

Urea and the ammonia it releases attack a number of metals. Corrosion products pass into the solution and damage the SCR system.

Source: ISO 22241-3, Wikipedia.

How to check batch quality: the certificate and real market data

ISO 22241-1 limits not only concentration but also impurities that damage the catalyst in tiny amounts. These are the key limits to look for on a quality certificate.

Why this matters in Ukraine. In an independent test in 2024, 3 of 5 solution samples from filling stations failed ISO 22241: sodium exceeded the limit more than 5 times and copper up to 3.7 times (expertise.in.ua, 2024). In a repeat test in 2025, one sample contained 34 mg/kg of aldehydes against a limit of 5 (expertise.in.ua, 2025). Sodium and metals appear when ordinary water is used instead of deionised water; aldehydes appear when technical rather than purified urea is used.

What to ask your supplier for: a quality certificate for every batch with ISO 22241-1 parameters and compliance with DSTU ISO 22241. Nitrogen Oxide Reductant AUS 32 by SVK is supplied with a quality certificate for each batch, in 20 l canisters and 1,000 l IBCs.

What not to do in winter

• Don't dilute with water "so it won't freeze". Diluted solution freezes earlier and triggers a reagent quality fault.
• Don't add antifreeze, alcohol, diesel or additives. Any impurity damages the injector and catalyst.
• Don't thaw with open flame, heating elements or hot water. Overheating above 60 °C destroys urea.
• Don't decant into containers that held other liquids or into metal canisters.
• Don't store in the sun in summer. What protects your supply in winter, a closed container in the shade, works all year round.

⚠️ Limitations. This advice covers storing and handling the fluid. How the tank heater, sensors and injector work depends on the vehicle model, and SCR diagnostics are a job for the service centre. If a fault persists after the engine warms up, the cause may be a faulty heater rather than the fluid.

Pre-frost checklist

• Stock moved indoors to a room at −11 to +25 °C.
• Containers are polyethylene or stainless steel, closed, not filled to the brim.
• Separate funnels and hoses labelled for urea solution only.
• A refractometer is in the warehouse; each new batch is checked on receipt.
• Batch quality certificates collected, batch numbers logged.
• Drivers reminded: never top up with water or thaw the tank with fire.

Freezing is the least of urea solution's winter problems when the fluid is good quality and stored correctly. Replacing an injector or catalyst because of diluted or contaminated reagent costs far more. To get a quote on AUS 32 for your fleet or check packaging options, visit the SVK automotive chemicals page or send a request. We reply within 24 hours.

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