ISO 6654:1991

Overview

ISO 6654:1991 specifies a spectrometric method for the determination of urea content in animal feeding stuffs. The standard defines the urea content, references sample preparation according to ISO 6498:1983, and describes the principle, reagents, apparatus, sampling, test-sample preparation, procedure, result expression and test reporting. The method uses a color‑developing reagent and spectrometric measurement to quantify urea for quality control and regulatory verification.

Key Topics

• Principle: Suspension of a representative test portion in water with decolorant, filtration, addition of dimethyl-4-aminobenzaldehyde (4-DMAB) and measurement of absorbance at 420 nm.
• Reagents highlighted in the standard:
  • Activated charcoal (non-adsorbing of urea)
  • Dimethyl-4-aminobenzaldehyde (4-DMAB) solution
  • Carrez I and Carrez II clarification reagents
  • Urea standard solution (1 g/L)
• Apparatus: Common laboratory equipment including a rotary shaker, spectrometer capable of 420 nm measurements with 10 mm cuvettes, volumetric flasks and test tubes.
• Sampling and sample prep: Sampling is to follow future ISO guidance; sample preparation uses ISO 6498. The procedure describes taking a weighed test portion (typical guidance in the standard) and using charcoal and Carrez reagents to clarify the extract before analysis.
• Calibration and measurement: Preparation of a calibration curve from urea standard solutions and measurement of sample absorbance relative to a blank. The standard specifies preparing a series of standard concentrations and measuring absorbance at 420 nm to derive urea concentration.
• Reporting: Results expressed as mass fraction (percentage) of urea in the sample, with requirements for reporting method details and any deviations or incidents affecting results.

Applications

This ISO method is applicable for laboratories and quality teams involved in:

• Feed quality control: Routine screening of mixed feeds, concentrates and other animal feedingstuffs for urea content.
• Regulatory compliance: Verifying urea levels to meet safety, labelling or regulatory limits.
• Ingredient verification: Confirming urea additions or inadvertent contamination in feed formulations.

Practical benefits include reproducible spectrometric measurement, a clear calibration approach, and defined reagents and apparatus to support consistent inter-laboratory results.

Related Standards

• ISO 6498:1983 - Preparation of samples of animal feeding stuffs for analysis (normative reference in ISO 6654:1991).

For implementation, laboratories should ensure reagents are of analytical quality, use distilled or equivalent water, and document all procedural details and any deviations in the test report to ensure traceability and auditability of results.