Determination of Nitrogen Content in Fertilizers

Abstract

The determination of nitrogen content in fertilizers was performed using the Sundy SDDN636 Dumas nitrogen analyzer, which employs high-temperature combustion with high-purity oxygen to convert nitrogen into nitrogen oxides (NO_x), followed by reduction and thermal conductivity detection. The method was validated in compliance with AOAC Official Method 993.13. Typical results for urea, potassium nitrate (with sucrose as a combustion aid), organic-inorganic compound fertilizers, and organic water-soluble fertilizers agreed well with their theoretical values, demonstrating that the SDDN636 provides accurate and reliable total nitrogen determination for fertilizer quality control.

Application Note: SD-XNS-001-007

Introduction

Nitrogen is a primary macronutrient essential for crop growth and development, directly influencing plant physiological metabolism, yield formation, and quality characteristics. Fertilizers serve as the principal means of delivering nitrogen in agricultural systems. Accurate determination of nitrogen content in fertilizers is therefore critical for evaluating product quality, verifying nutrient label accuracy, and ensuring the safety of agricultural inputs. It also provides a vital basis for precision fertilization, reducing resource waste, and mitigating agricultural nonpoint source pollution.

The Sundy SDDN636 utilizes high-purity oxygen combustion at high temperature to convert nitrogen into nitrogen oxides (NO_x). NO_x is reduced to nitrogen, which is quantitatively analyzed using a thermal conductivity detector. The SDDN636 automates the entire sample testing process—combustion, reduction, purification, and quantitative detection.

Instrument Configuration and Features

The Sundy SDDN636 is designed in strict compliance with AOAC Official Method 993.13. It incorporates systematic optimizations in continuous testing capability, automated operation, and ease of use, meeting practical laboratory requirements for high-throughput, high-efficiency analysis.

(1) High throughput and flexible adaptability

Equipped with a 36-position sample tray, expandable up to 144 positions, to meet the throughput requirements of laboratories.

(2) Large sample capacity; compatible with solids and liquids

Supports both solid and liquid samples, with a capacity of up to 1.5 g for solids and 1 mL for liquids.

(3) Intelligent self-diagnosis and fully automated, efficient analysis

The system features automatic leak testing, fault alarms, and hardware protection. The entire testing process is fully automated. Single-sample analysis takes approximately 3–4 minutes (with adjustments for different sample types).

(4) Complete combustion for reliable results

Combines high-performance oxygen-enriched combustion technology with a high-precision gas-flow control system, ensuring that test results comply with applicable standards.

(5) Vertical design; no benchtop required

Equipped with high-quality casters and positioning mechanisms at the base for easy mobility and stable placement.

Method Reference

AOAC Official Method 993.13 Nitrogen (Total) in Fertilizers, Combustion Method

Sample Preparation

Solid samples: After pulverization or grinding, pass the sample through a 0.50 mm laboratory sieve (or a 1.00 mm sieve if the sample is moist). Mix thoroughly and store in a clean, dry, sealed container.

Liquid samples: Mix thoroughly and store in a clean, dry, sealed container.

Standard Reference Materials

EDTA certified reference material (CRM), 9.55 ± 0.13% N, purity ≥99% (CAS No. 60-00-4, Alpha Resources LLC)

Reagents and Accessories

Sucrose, analytical reagent grade

Urea, analytical reagent grade

Potassium nitrate, reagent grade

Tin Foil Cups, Tin Capsules

Method Parameters

Procedure

(1) Preparation

a. Turn on the computer and the instrument.

b. Launch the testing software, connect to the instrument, enter the test program, and verify that the instrument parameters are correct.

c. Initiate heating; the analysis can begin after the instrument has maintained a constant temperature for one hour. During this period, the instrument automatically performs a leak test and purges the gas lines.

(2) Blank Determination

a. After the instrument has reached a stable temperature, perform a blank test.

b. Prepare three to five (or more) blank replicates, and initiate the analysis sequence.

c. The system generates a valid blank value for system blank subtraction.

(3) Calibration

a. Select different masses of EDTA for calibration.

b. Weigh 10 mg, 20 mg, 50 mg, 100 mg, 150 mg, and 200 mg of EDTA (N%: 0.96%–19.10%), 2–3 replicates at each mass.

c. Calibrate the instrument following the procedure outlined in the operator’s manual. The calibration curve must have an R² ≥ 0.999.

(4) Calibration Verification

a. Verify the calibration by analyzing 2–3 replicates of 15 mg and 120 mg EDTA.

b. Confirm that the results are within the acceptable tolerance range.

(5) Sample Analysis

a. Solid samples: Weigh approximately 100 mg of sample into a tin foil cup, wrap tightly, and place on the sample tray. (Adjust the sample weight according to the nitrogen content.)

b. Liquid samples: Weigh approximately 100 mg of sample into a tin capsule, seal, and place on the sample tray for testing.

c. Samples with high nitrate content should be analyzed with sucrose added as a combustion aid.

Typical Results

Table 1 Typical Results of Nitrogen Determination in Fertilizers

Summary

The experimental results demonstrate that the analytical principle of the SDDN636 nitrogen analyzer and the precision of its results fully comply with the requirements of AOAC Official Method 993.13. In addition, the instrument’s rapid testing capability and high-throughput design improve batch testing efficiency while maintaining result accuracy.

The SDDN636 Nitrogen Analyzer is an efficient and reliable solution for determining total nitrogen in fertilizers. It offers a standards-compliant technical approach for quality control in the fertilizer industry.

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