ISO 5378:1978

Overview

ISO 5378:1978 specifies a spectrophotometric Kjeldahl method for determining nitrogen content in starches and derived products. The method is intended for samples with a presumed nitrogen content below 0.025% (m/m). It covers digestion (mineralization) with sulfuric acid and a compound catalyst, alkalization, distillation of released ammonia, collection in sulphuric acid, and spectrophotometric measurement of the resulting ammonium salt after addition of Nessler reagent (measurement at 430 nm).

Key topics and requirements

• Scope: Applicable to starches, flours and derivatives with N < 0.025% (m/m); for higher nitrogen contents see ISO 3188.
• Principle: Acid digestion → alkalization → steam distillation → absorb ammonia in sulphuric acid → react with Nessler reagent → measure absorbance at 430 nm.
• Reagents (examples given):
  • Concentrated sulphuric acid (approx. 96%),
  • 30% (m/m) sodium hydroxide solution,
  • Compound catalyst (e.g., K2SO4 + CuSO4 as per ISO 1871),
  • Ammonium salts for calibration (sulfate/oxalate/chloride),
  • Nessler reagent (prepared from mercuric iodide, potassium iodide and KOH solution; stored in brown glass).
• Apparatus: Kjeldahl flask (500–800 mL), distillation/steam apparatus with graduated receiver and trap, spectrophotometer adjustable to 430 nm, volumetric glassware, analytical balance.
• Procedure highlights:
  • Typical sample mass: 2–5 g (weighed to 0.001 g),
  • Digest with H2SO4 and catalyst until clear, distill into 25 mL sulphuric acid solution, collect aliquot (commonly 25–50 mL) for color development,
  • Prepare calibration curve with at least five standard concentrations (document notes linearity between 0 and ~240 µg N per 50 mL),
  • Run blanks and reference checks; perform two determinations and report the arithmetic mean.
• Results: Express nitrogen as mass percent using the calibration-derived micrograms of N in the aliquot, corrected for blank.

Applications and users

• Quality control and routine analysis in:
  • Starch and food ingredient manufacturers,
  • Food testing laboratories evaluating protein-related nitrogen content,
  • Regulatory and compliance testing where low-level nitrogen determination is required,
  • Research labs studying amino-nitrogen or residual nitrogenous additives in starch products.
• Useful where sensitive, low-level nitrogen measurement is needed and photometric detection is preferred over titration.

Related standards

• ISO 1871 - general Kjeldahl digestion guidelines (compound catalyst),
• ISO 3188 - Kjeldahl titrimetric method for higher N contents,
• ISO 1227 (Add.2), ISO 1042, ISO 648, ISO 565 - referenced for vocabulary and glassware/specifications.

Keywords: ISO 5378, Kjeldahl method, spectrophotometric method, nitrogen determination, starch analysis, Nessler reagent, ammonia distillation, food quality testing.