Determination of Total Nitrogen in AUS 32 (Aqueous Urea Solution) by the Dumas Combustion Method

Abstract

This application note specifies a standard procedure for the determination of total nitrogen in Aqueous Urea Solution 32%(AUS 32), a NOₓ reduction agent for diesel engines, using the Sundy SDDN636 Dumas Nitrogen Analyzer. The procedure employs the Dumas combustion method, enabling the precise determination of the total nitrogen content in AUS 32. It provides reliable technical support for the quality control of automotive urea products used in SCR systems and serves as an objective and accurate analytical basis for commercial settlement of raw materials.

Application Note No.: SD-DN-001-01003

Introduction

In the Selective Catalytic Reduction (SCR) after-treatment systems of diesel vehicles, the total nitrogen content of Aqueous Urea Solution 32% (AUS 32) directly determines its NOₓ reduction capacity. AUS 32 is the critical parameter determining both de-NOₓ efficiency and compliance with emission standards.

The determination of total nitrogen via the high-temperature combustion method based on the Dumas principle involves catalytic oxidation and subsequent reduction of the sample to molecular nitrogen prior to detection. Compared to the conventional Kjeldahl method, the Dumas method eliminates the need for strong acid digestion during sample preparation, thereby significantly streamlining the analytical workflow. Equipped with automated sampling and continuous analysis systems, this method facilitates high-throughput rapid determination of batch samples. This capability markedly shortens the analytical cycle, minimizes manual intervention and chemical reagent consumption, while simultaneously enhancing data repeatability and accuracy.

Consequently, the Dumas method provides efficient and robust technical support for production process control, quality inspection, and the evaluation of exhaust emission treatment efficacy.

Fig. 1  Schematic diagram of the Dumas method

Test Principle: Dumas Combustion Method

The fundamental principle of the Dumas combustion method involves complete combustion of the sample in a high-temperature, oxygen-rich environment, ensuring the quantitative conversion of all nitrogen present in the sample into nitrogen oxides (NOₓ). These nitrogen oxides are subsequently reduced to molecular nitrogen(N₂). The concentration of N₂ is then determined by a Thermal Conductivity Detector (TCD), and the total nitrogen content of the sample is quantified against a calibration curve.

Fig. 2  Schematic diagram of Dumas nitrogen analyzer

Sample Preparation

Aqueous Urea Solution: Promptly transfer approximately 100 mL of thoroughly homogenized aqueous urea solution sample into a clean, dry polyethylene or glass container. Seal the container immediately and store until analysis. The sample must be completely dissolved and free from urea crystallization. If necessary, the sample may be warmed to a temperature not exceeding 40℃ to ensure complete dissolution.

Reference Materials

Certified Reference Materials (CRMs) shall be used, such as Ethylenediamine tetraacetic Acid (EDTA), Nicotinamide (Niacinamide), or high-purity urea with low biuret content(e.g., purified urea crystals or Analytical Reagent grade urea).

Reagents

Combustion Accelerators: Nitrogen-free carbohydrate reagents, such as sucrose and cellulose.

Reducing Agents: Copper metal and associated catalysts.

Adsorbents: Magnesium perchlorate and soda asbestos (sodium hydroxide coated on asbestos or alternative alkali substrates).

Absorbents: Magnesium oxide.

Consumables and Accessories

Consumables: Tin capsules

Ancillary tools: Pipettes, forceps

Method Parameters

Operating Procedure

Sampling and Preparation: Homogenize the AUS 32 sample by thorough shaking. Promptly transfer approximately 100 mL of AUS 32 into a clean, dry polyethylene or glass container and seal securely. Accurately weigh approximately 200mg magnesium oxide (MgO) into a tin capsule and tare the balance.

Encapsulation: Using a calibrated pipette, dispense a 100 µL aliquot of the homogenized AUS 32 directly onto the magnesium oxide bed within the capsule.Record the exact mass of the dispensed solution. Fold and crimp the tin capsule to encapsulate the wetted absorbent, ensuring the formation of a compact, droplet-shaped parcel. Load the encapsulated samples sequentially into the auto-sampler carousel.

Determination: Magnesium oxide shall be used as an adsorbent carrier during analysis to prevent volatilization loss of the liquid sample and to ensure complete combustion.

Validation Data

Calibration Substance: Ethylenediaminetetraacetic acid (EDTA), Reference Material AR2092-250 (Batch No. 1122V), certified nitrogen content 9.55% .

Calibration Points: 125mg、140mg、155mg、170mg、185mg

Verification Substance: EDTA

Verification Points: 150mg、160mg

Conclusion: The mean determined total nitrogen content of the AUS 32 test sample was 15.05%, with a standard deviation of 0.02%. The results demonstrate effective control over volatile losses in liquid matrices and confirm a high degree of analytical stability and precision for the method.

Test Results

The data were generated by linear calibration using EDTA as the calibration substance (AR2092-250, Batch 1122V; Nitrogen Content: 9.55 %), with calibration points at 125 mg, 140 mg, 155 mg, 170 mg, and 185 mg. Verification was performed using EDTA at verification points of 150 mg and 160 mg.

References

ISO22241-1 Diesel Engines – NOx Reduction Agent AUS 32 Part 1: Quality Requirements

ISO22241-2 Diesel Engines – NOx Reduction Agent AUS 32 Part 2: Test method

GB 29518 Nitrogen Oxide Reductant for Diesel Engines – AUS 32