ISO 4941:1994

Overview

ISO 4941:1994 specifies a validated analytical procedure - the thiocyanate spectrophotometric method - for the determination of molybdenum (Mo) content in steel and iron. This second edition (1994) replaces the 1978 version and is applicable to Mo contents from 0.005 % (m/m) to 0.125 % (m/m). The method produces a coloured molybdenum–thiocyanate complex that is extracted and measured spectrophotometrically at about 470 nm.

Key topics and technical requirements

• Scope and limits
  • Applicable Mo range: 0.005–0.125 % (m/m). The upper limit was reduced from the earlier edition because precision criteria were not met at higher concentrations.
• Analytical principle
  • Dissolution of the test portion (acid digestion and carbide oxidation), formation of a molybdenum–thiocyanate coloured complex in the presence of iron(II) and/or copper(II) and extraction into butyl acetate.
  • Spectrophotometric measurement at ~470 nm using 1 cm or 2 cm optical path cells (use 2 cm for Mo <0.025 % and 1 cm for Mo >0.025 %).
• Reagents and apparatus
  • Specified reagent grades (concentrated acids, ammonium thiocyanate, L-ascorbic acid, copper(II) solution, tin(II) copper chloride) and class A volumetric glassware.
  • Required apparatus includes a separating funnel, spectrophotometer, and matched optical cells.
• Calibration and calculation
  • Use calibration graph of absorbance versus Mo concentration (μg/mL). The standard gives equations and a formula to convert measured absorbance into % Mo.
• Interferences and timing
  • Vanadium (V) and tungsten (W) interfere if V/Mo > 16 or W/Mo > 8. Higher ratios may be tolerated only if the measurement is performed quickly after extraction (within ~30 minutes).
• Precision
  • Precision data (repeatability and reproducibility) are provided following ISO 5725 methodology; example values: at 0.01 % Mo r ≈ 0.0008, R ≈ 0.0023.

Applications and users

• Practical uses:
  • Quality control and chemical analysis in steelworks and iron foundries.
  • Materials testing laboratories, certification bodies and third‑party analytical services verifying Mo alloying levels.
  • Research and development in metallurgy and process optimization where trace Mo quantification is required.
• Primary users:
  • Metallurgical analysts, analytical chemists, laboratory managers in the steel industry, and conformity assessment laboratories.

Related standards

• ISO 5725 - precision of test methods (repeatability/reproducibility)
• ISO 3696 - water for analytical laboratory use
• ISO 377-2 - sampling of wrought steels
• ISO 4800, ISO 1042, ISO 648 - laboratory glassware specifications

Keywords: ISO 4941, molybdenum determination, thiocyanate spectrophotometric method, steel analysis, Mo content, spectrophotometry, analytical standard, vanadium interference, tungsten interference.