ISO 6491:1998

Overview

ISO 6491:1998 specifies a spectrometric method for the determination of phosphorus content in animal feeding stuffs. The method is intended primarily for feeds with low phosphorus content (below 50 g/kg) and describes sample preparation, chemical digestion routes, color development with a molybdovanadate reagent and spectrophotometric measurement. For very high phosphorus levels the standard recommends a gravimetric approach (for example, quinoline phosphomolybdate).

Key topics and technical requirements

• Scope and applicability: Designed for animal feeding stuffs with phosphorus < 50 g/kg; particularly suited to low-phosphorus products.
• Sample preparation: Follow ISO 6498 procedures for test-sample preparation; grinding to 1 mm for solids is recommended.
• Destruction/digestion routes:
  • Dry ashing (organic feeds): sample ashed with calcium carbonate, treated with HCl/HNO3, filtered and diluted.
  • Wet oxidation (minerals/liquids): sulfuric/nitric acid digestion (Kjeldahl-flask technique) until a clear solution is obtained.
• Color development and measurement: An aliquot of the digested sample is reacted with a molybdovanadate reagent to form a yellow complex; absorbance is measured at 430 nm against a specified reference.
• Calibration and linearity: Calibration standards are prepared across a series of phosphorus concentrations (standard specifies a set of levels up to the method’s upper calibration point) and the calibration curve must be linear across the stated range. Blank tests are run in parallel.
• Expression and reporting: Results are calculated per the formula in the standard and reported to 0.1 g/kg.
• Precision: Repeatability and reproducibility are quantified in the standard - repeatability differences exceed 1 g/kg in no more than 5% of cases; reproducibility differences exceed 7 g/kg in no more than 5% of cases.

Applications and users

This ISO method is suited for:

• Feed testing laboratories performing routine phosphorus determination in animal feed.
• Quality control teams at feed manufacturers and premix producers.
• Regulatory and inspection laboratories verifying compliance with nutrient specifications.
• Agricultural research labs analyzing mineral nutrition and feed formulation.

Key practical benefits include a validated spectrometric workflow for low-phosphorus matrices, clear guidance for both ashing and wet digestion, and defined performance (precision) expectations for laboratory quality assurance.

Related standards

• ISO 6498 - Preparation of test samples (referenced by ISO 6491)
• ISO 6497 - Sampling of animal feeding stuffs
• ISO 3696 - Water for analytical laboratory use
• ISO 5725 series - Precision (repeatability and reproducibility) of measurement methods

Keywords: ISO 6491:1998, animal feeding stuffs, phosphorus determination, spectrometric method, molybdovanadate, 430 nm, feed testing, sample digestion, calibration curve.