ISO 4158:1978

Overview

ISO 4158:1978 specifies a gravimetric method for the determination of silicon content in ferrosilicon, ferrosilicomanganese and ferrosilicochromium alloys. The method is applicable to alloys containing 8–95% (m/m) silicon and covers principle, reagents, apparatus, sampling, procedure, expression of results, reproducibility and the required test report. The procedure uses oxidative fusion (sodium peroxide), acid decomposition and gravimetric isolation of silica, with a conversion factor to report silicon content. Note: the standard warns of hazards from perchloric acid fumes when heated.

Key topics and technical requirements

• Scope: suitable for ferro‑alloys with 8–95% Si (m/m).
• Principle: convert silicon to silicate by sodium peroxide fusion, acidify, isolate silica by double insolubilization and volatilization (HF + H2SO4), then weigh residues to calculate silicon by difference.
• Sample sizes:
  • ≤ 50% Si: 0.50 ± 0.0002 g (typical)

  • 50% Si: 0.25 ± 0.0002 g

  • Note: for < 25% Si a 1 g sample may be used.
• Reagents and materials: sodium peroxide fusion flux, perchloric acid, hydrofluoric acid, hydrochloric and sulfuric acids, silver nitrate (10 g/L) for chloride check; laboratory-grade reagents and distilled water are required.
• Apparatus: silica‑free crucibles (iron-free/graphite/nickel/zirconium), platinum capsules, PTFE or inert beakers, muffle furnace, desiccator and standard filtration equipment.
• Calculation: silicon content is calculated from weighed masses of the capsule plus impure silica and capsule plus impurities, with a conversion factor 0.4674 to convert SiO2 → Si. Blank corrections are required (masses m1..m4 specified in the method).
• Performance: stated reproducibility (95% confidence) for an experienced operator is approximately ±0.30% for Si > 50% and ±0.20% for Si ≤ 50%.

Practical applications and users

• Laboratories performing metallurgical analysis of ferroalloys.
• Quality control and incoming material inspection in steel mills, alloy producers and foundries.
• Research and process control in metallurgy and materials science where accurate silicon determination in ferrous alloys is required.
• Accredited testing labs preparing test reports in line with ISO practice.

Practical benefits include a definitive, traceable gravimetric result suitable for certification, process control and product specification compliance.

Safety and best practice

• The method involves perchloric acid and hydrofluoric acid; use appropriate fume hoods, PPE and procedures for perchloric acid heating and HF handling.
• Follow laboratory safety regulations and local hazardous‑chemical protocols.

Related standards

• ISO 3713 - Ferro‑alloys - Sampling and preparation of samples (referenced for sample preparation).