ISO 1975:1973

Overview

ISO 1975:1973 specifies a spectrophotometric method for the determination of silicon (Si) in magnesium and magnesium alloys using the reduced silicomolybdic complex. First published in 1973, the method is applicable to alloys with silicon contents between 0.01% and 0.6% (m/m). It defines the principle, reagents, apparatus, sampling/test portions, calibration procedure, measurement at about 810 nm, result expression and test reporting. The procedure does not apply to alloys containing rare earths or thorium.

Key topics and technical requirements

• Principle: attack of sample by sulphuric acid and bromine, complexation and formation of an oxidized silicomolybdic complex followed by selective reduction to a blue complex measured spectrophotometrically (~810 nm).
• Applicable silicon range: 0.01–0.6%; two calibration curves are prepared to account for magnesium matrix effects.
• Reagents (examples given in the standard):
  • Bromine water, sulphuric acid (approx. 10 N and 1 N), potassium fluoride (50 g/L), boric acid (saturated ≈ 40 g/L)
  • Molybdic solutions (e.g., sodium molybdate 195 g/L or ammonium molybdate 140 g/L)
  • Tartaric acid (200 g/L), ascorbic acid (20 g/L)
  • Silicon standard solutions (0.100 g/L and 0.010 g/L)
• Apparatus and precautions:
  • Spectrophotometer (calibrated cells, optical path typically 1 cm), pH meter with glass electrode, and clean laboratory glassware; platinum containers for fusing silica standards.
  • pH control during reagent preparation and colour development (pH ranges specified depending on molybdate reagent).
• Procedure elements:
  • Test portion sizes: typically 1 g (for 0.01–0.05% Si) or 0.5 g (for 0.05–0.6% Si)
  • Preparation of matched standards and calibration curves, timed colour development, blank and check tests to confirm no unwanted reduction of Mo(VI).
• Results: silicon content calculated from calibration curves with blank correction and expressed as mass percent; the report must include method reference, results, sample details and any unusual features.

Applications

• Routine chemical analysis and quality control of magnesium and magnesium alloys in foundries, metallurgy laboratories, and material testing facilities.
• Verification of silicon content for alloy specification compliance, process control, failure analysis and research on magnesium-based materials.

Who should use this standard

• Metallurgical chemists and quality-control analysts working with light metals
• Test laboratories performing compositional analysis of magnesium alloys
• Materials engineers and R&D teams requiring standardized silicon determination methods

Related standards

• ISO 1975:1973 replaces the earlier ISO Recommendation R 1975-1971 (Light metals technical committee). Sampling of magnesium alloys is noted as a separate/ future subject in the standard.