Determination of Biuret Content in Fertilizers: Testing Methods, Standards, and Applications

Biuret is a toxic condensation by-product formed during urea manufacturing, and excess levels can cause serious phytotoxicity, germination failure, and root damage in crops. Accurate determination of biuret content in fertilizers is therefore essential for product quality control, regulatory compliance, and safe agricultural use. This article provides a comprehensive technical overview of biuret testing principles, applicable standards such as ISO 5314 and GB/T 2441.2, sampling requirements, analytical procedures, and practical applications.

Technical Overview

Biuret (NH₂-CO-NH-CO-NH₂) is an impurity generated when urea undergoes thermal condensation at elevated temperatures and pressures during prilling, granulation, or evaporation. In this reaction, two molecules of urea lose one molecule of ammonia and combine to form biuret. Because biuret is poorly utilized by plants and actively interferes with protein metabolism, it is classified as a harmful substance in urea-based fertilizers.

The phytotoxic risk of biuret is particularly high in foliar fertilization, seed-placed applications, and concentrated root-zone contact. Symptoms of biuret damage include leaf tip burn, chlorosis, inhibited germination, and reduced root development. For this reason, national and international product standards impose strict limits: the Chinese national standard for urea products (GB/T 2440 series) restricts biuret in agricultural urea to approximately 0.9% to 1.5% depending on grade, while products intended for foliar spraying are generally expected to contain less than 0.5%.

The analytical determination of biuret relies on its characteristic coordination chemistry. In an alkaline medium containing potassium sodium tartrate (Rochelle salt), biuret forms a stable violet-colored copper complex. The absorbance of this complex, measured spectrophotometrically in the 540-550 nm region, is directly proportional to the biuret concentration according to the Beer-Lambert law. This copper-complex spectrophotometric principle forms the basis of the reference methods described in ISO 5314 and GB/T 2441.2, and it remains the most widely accepted technique for routine quality control of urea and urea-based fertilizers.

Testing Samples

Urea and Urea-Based Solid Fertilizers

Prilled and granular urea, coating-free and polymer-coated urea, urea ammonium sulfate blends, slow-release and controlled-release urea products, and stabilized urea formulations containing urease or nitrification inhibitors constitute the largest sample category. These materials are typically tested against the urea product standard limits of 0.9-1.5% biuret.

Compound (NPK) and Specialty Fertilizers

Compound fertilizers, blended NPK formulations, high-nitrogen water-soluble powders, and specialty horticultural fertilizers often contain urea as the primary nitrogen source. The complex matrices of these products, particularly those containing humic substances, trace elements, or organic amendments, may require method adaptation or chromatographic separation to eliminate color and matrix interferences.

Liquid Fertilizers and Foliar Products

Urea ammonium nitrate (UAN) solutions, clear liquid fertilizers, suspension fertilizers, and foliar nutritional sprays must meet the most stringent biuret requirements because of their direct contact with plant foliage. Biuret levels in these products are frequently specified at or below 0.5%.

Raw Materials and Industrial Applications

Feed-grade urea, technical-grade urea for industrial resins and adhesives, urea-formaldehyde raw materials, and imported bulk urea consignments are also submitted for biuret verification as part of incoming material inspection and contractual acceptance testing.

Testing Items

The core testing item is the quantitative determination of biuret content, expressed as mass fraction (%) on an as-received or dry basis. Supporting determinations are frequently requested alongside biuret to provide a complete quality profile:

• Biuret content (% w/w) in urea, compound fertilizers, and liquid fertilizer products
• Total nitrogen (N) content for conformity with product grade specifications
• Moisture (free water) content, which influences biuret formation during storage
• Particle size distribution and crushing strength for granular products
• Conformity assessment against GB/T 2440, ISO 5314, or contractual specification limits

Testing Methods

Copper-Complex Spectrophotometry (Reference Method)

The internationally recognized reference method, described in ISO 5314 (Fertilizers - Determination of biuret content of urea) and the Chinese national method GB/T 2441.2, is based on the spectrophotometric measurement of the violet copper-biuret complex. The general procedure is as follows:

• Sample preparation: reduce the laboratory sample using a riffle splitter or cone-and-quarter technique to obtain a representative test portion; representative sampling of bulk lots follows ISO 8358 or GB/T 6679.
• Test portion dissolution: weigh a suitable mass (typically 1-5 g, depending on the expected biuret level) to the nearest 0.0001 g, dissolve in water, and dilute to volume in a volumetric flask; filter if insoluble matter is present.
• Color development: transfer an aliquot to a volumetric flask, add alkaline potassium sodium tartrate solution to maintain copper in solution, add copper(II) sulfate solution, make to volume, and allow the violet complex to develop for the specified standing time at controlled room temperature.
• Calibration: prepare a series of standard solutions from high-purity recrystallized biuret covering the working range, treat identically, and construct a calibration curve of absorbance versus biuret mass.
• Measurement: measure the absorbance of sample and standard solutions at 550 nm against a reagent blank using matched glass or quartz cuvettes (typically 1-3 cm path length) on a calibrated UV-Vis spectrophotometer.
• Calculation: determine the biuret mass in the aliquot from the calibration curve and calculate the mass fraction, applying dilution factors and expressing results on the required basis.

High-Performance Liquid Chromatography (HPLC)

For compound fertilizers, organic-matrix products, and samples with intrinsic color that interferes with the spectrophotometric method, HPLC determination provides superior selectivity. Chromatographic separation of biuret from urea, ammonium salts, and matrix components, followed by refractive index or UV detection, is covered for compound fertilizer matrices by the Chinese national standard GB/T 22924. HPLC is also the preferred confirmatory technique for dispute resolution and arbitration testing.

Quality Control and Data Integrity

Accredited third-party laboratories performing biuret determinations apply rigorous quality assurance measures to guarantee defensible results:

• Calibration with certified or recrystallized high-purity biuret reference material, verified for purity
• Reagent blank correction and matrix-matched calibration where required
• Duplicate determinations with acceptance criteria for repeatability, typically better than 5% relative deviation
• Spike recovery verification in the range of 95-105% for complex matrices
• Method detection limits typically at or below 0.05% (w/w), with reporting limits suitable for specification-limit conformity decisions
• Full documentation of weighing, volumetric operations, instrument calibration, and environmental conditions for traceability

Applications

Biuret content determination serves critical functions across the fertilizer value chain:

• Manufacturing quality control: monitoring of prilling tower operation, granulation temperature, evaporation conditions, and storage stability to keep biuret formation below specification limits.
• Regulatory compliance and market surveillance: verification of conformity with national mandatory standards and import/export inspection requirements before products enter commercial channels.
• Trade and contract settlement: independent third-party test reports for buyers, sellers, and arbitration bodies in international urea transactions.
• Foliar and specialty fertilizer safety: qualification of liquid and water-soluble products for sensitive applications such as fertigation, foliar spraying, and seedling production.
• Agricultural advisory services: diagnosis of suspected fertilizer-related crop injury and evaluation of product suitability for specific application methods.
• Research and development: evaluation of new slow-release coatings, stabilized fertilizer formulations, and process optimization studies aimed at minimizing biuret generation.

FAQ

Q: Why is biuret content limited in urea fertilizers?

A: Biuret is not a nutrient and is poorly metabolized by plants. At elevated concentrations it disrupts protein synthesis and causes leaf burn, poor germination, and root damage, which is why product standards impose maximum permissible levels.

Q: What is the maximum allowable biuret content?

A: Under the Chinese urea product standard (GB/T 2440 series), agricultural urea is limited to approximately 0.9-1.5% biuret depending on grade, while foliar and liquid products are customarily required to remain below 0.5%. Contractual specifications may set stricter limits.

Q: Which standard methods are accepted for biuret determination?

A: The copper-complex spectrophotometric methods of ISO 5314 and GB/T 2441.2 are the primary reference methods for urea, and GB/T 22924 provides an HPLC-based approach for compound fertilizer matrices.

Q: What sample quantity is required and how long does testing take?

A: A representative laboratory sample of approximately 200-500 g is generally sufficient. Routine spectrophotometric determination is usually completed within 5-7 working days, including documentation.

Q: Can biuret be accurately measured in colored or organic compound fertilizers?

A: Yes. Where humic substances or intrinsic color interfere with the spectrophotometric method, HPLC separation or validated matrix-matched calibration with blank correction is applied to ensure accurate results.

Q: What detection capability and precision can be expected?

A: The spectrophotometric method reliably quantifies biuret from approximately 0.05% upward, with repeatability typically better than 5% relative deviation in the range of commercial specification limits.

Q: How can manufacturers reduce biuret formation?

A: Biuret generation is minimized by controlling evaporation and granulation temperatures, shortening residence time of molten urea at high temperature, maintaining low water content, and avoiding prolonged hot storage of finished product.