Overview
EN ISO 5378:1994 specifies a spectrophotometric method for determining nitrogen content in starches and derived products through the Kjeldahl method. This international standard, developed by CEN in alignment with ISO 5378:1978, provides a precise procedure suitable for nitrogen contents below 0.025% (m/m). It involves the destruction of organic matter using sulphuric acid and a compound catalyst, followed by alkalization, distillation of liberated ammonia, and measurement via spectrophotometry using Nessler reagent.
This method is critical for accurate nitrogen analysis in food and agricultural products where protein and amino acid nitrogen content is essential and serves as a foundation for quality control and regulatory compliance in starch-related industries.
Key Topics
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Scope and Application
Designed for starches and related products with nitrogen content below 0.025% (m/m), focusing on protein and amino acid nitrogen while excluding nitrate, nitrite, and other nitrogen forms. For higher nitrogen contents, ISO 3188 is referenced.
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Principle
Organic nitrogen compounds are digested by concentrated sulphuric acid in the presence of a catalyst. Ammonia produced is distilled and trapped in sulphuric acid, then reacted with Nessler reagent to form an ammonium salt measured spectrophotometrically at 430 nm.
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Reagents and Equipment
- Concentrated sulphuric acid and sodium hydroxide solutions
- Compound catalyst (potassium sulphate and copper(II) sulphate)
- Nessler reagent (mercuroiodide-potassium iodide-potassium hydroxide solution)
- Kjeldahl digestion and distillation apparatus
- Spectrophotometer adjustable to 430 nm wavelength and appropriate cells
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Analytical Procedure
- Sample preparation, including grinding and sieving as needed
- Digestion of the sample with sulphuric acid and catalyst
- Alkalization and steam distillation of ammonia
- Ammonia absorption into sulphuric acid
- Addition of Nessler reagent and spectrophotometric measurement
- Use of calibration curves from known ammonium salt standards
- Blank and check tests to verify accuracy
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Calculation and Expression of Results
Nitrogen content is calculated as a percentage of sample mass using absorbance data and calibration standards. The method requires duplicate determinations to ensure reliability.
Applications
EN ISO 5378:1994 provides a reliable, standardized approach for nitrogen determination essential to maintaining product quality, safety, and integrity in starch-related industries.
Related Standards
- ISO 1871 – General directions for nitrogen determination using the Kjeldahl method
- ISO 3188 – Determination of nitrogen content in starches and derivatives by the Kjeldahl method using a titrimetric approach for samples with nitrogen content greater than 0.025%
- ISO 1227/Add. 2 – Vocabulary related to starch and derived products
- ISO 1042 – Specifications for volumetric laboratory glassware such as flasks and pipettes
- ISO 565 – Specifications for sieves used in sample preparation
- ISO 648 – Specifications for laboratory pipettes
These complementary standards ensure comprehensive analytical consistency and support broad application of Kjeldahl nitrogen determination techniques in food and agricultural product testing.
Keywords: EN ISO 5378, Kjeldahl method, nitrogen determination, starch analysis, spectrophotometric method, Nessler reagent, sulphuric acid digestion, ammonia distillation, food quality control, starch derivatives, nitrogen content standard.