ISO 4945:2018

Overview

ISO 4945:2018 specifies a spectrophotometric method for the determination of nitrogen in steel. It covers measurement of nitrogen mass fractions from 0.0006 % to 0.050 % in low-alloy steels and from 0.010 % to 0.050 % in high-alloy steels. The method is not applicable to samples containing silicon nitrides or with silicon content greater than 0.6 %.

Key topics and technical requirements

• Principle: Dissolve the test portion in hydrochloric acid, decompose acid‑insoluble residue by fuming with sulphuric acid (with K2SO4 and CuSO4·5H2O), make the solution alkaline and steam‑distil ammonia into dilute sulphuric acid, then form a blue coloured phenol–hypochlorite complex (with sodium hypochlorite and sodium nitroprusside) and measure absorbance at about 640 nm.
• Reagents & solutions: Includes HCl (1+1), fuming H2SO4, NaOH (strong and diluted solutions), potassium sulfate, copper(II) sulfate pentahydrate, sodium phenoxide, sodium hypochlorite, and sodium nitroprusside (sodium pentacyanonitrosylferrate(III)). Water must meet ISO 3696 grade 2.
• Apparatus: Class A volumetric glassware (ISO 648, ISO 1042), spectrophotometer (640 nm, 10 mm cells), and a steam distillation apparatus (see examples in Annex A).
• Test procedure essentials:
  • Choose test portion mass per expected nitrogen (e.g., 2.0 g for very low N; 1.0 g for higher ranges).
  • Perform a blank test in parallel.
  • Dissolution, filtration, fuming of residues, steam distillation, formation of the coloured complex, and spectrophotometric measurement.
  • Establish a calibration curve using prepared nitrogen standards.
• Limitations: Not suitable for samples with fine nitrides (requires membrane filtration if present) or silicon > 0.6 %.

Applications and users

• Who uses it: Metallurgical laboratories, steel manufacturers, quality control and inspection bodies, R&D teams, and certification labs for material compliance.
• Practical uses:
  • Routine quality control to verify nitrogen content in low‑ and high‑alloy steels.
  • Incoming material inspection and final product certification.
  • Research on alloy composition effects and failure analysis where precise low‑level nitrogen determination is required.

Related standards

• ISO 3696 - Water for analytical laboratory use
• ISO 648 and ISO 1042 - Laboratory glassware (pipettes, volumetric flasks)
• ISO 14284 - Sampling and preparation of steel and iron for chemical analysis

Keywords: ISO 4945:2018, nitrogen determination, steel, spectrophotometric method, steam distillation, phenol‑hypochlorite, sodium nitroprusside, analytical method, low alloy steels, high alloy steels.