Overview
ISO 2366:1974 specifies two validated spectrophotometric procedures for determining sodium (Na) content in natural and artificial cryolite: a flame emission (flame photometry) method and a flame atomic absorption (AAS) method. Both methods target the sodium resonance line at 589 nm and are applicable to cryolite materials with a molar NaF/AlF3 ratio of about 3. The standard includes sample preparation, calibration, measurement and reporting requirements.
Key topics and technical requirements
- Scope and applicability
- For natural and artificial cryolite of normal composition (NaF/AlF3 ≈ 3).
- Analytical principles
- Flame emission: atomize sample in an oxy-hydrogen (or equivalent) flame and measure emission intensity at 589 nm.
- Flame AAS: atomize in an air/acetylene flame and measure absorption of the 589 nm line from a sodium hollow‑cathode lamp.
- Sample preparation
- Test portion: dried sample (0.500 g typical) is attacked with concentrated sulphuric acid to remove HF, evaporated, then dissolved in hydrochloric acid and water. Final solutions are made to defined volumetric marks and transferred to plastic bottles.
- Apparatus: platinum dish for digestion; volumetric flasks; plastics bottles (polyethylene, PTFE, polypropylene suitable).
- Calibration and standards
- Standard sodium solutions prepared gravimetrically (using dried anhydrous sodium sulfate) to give known mg Na per mL (e.g., 1.00 mg Na per mL for a 1 g/L solution).
- Prepare a series of standard “matching” solutions and plot calibration charts (mg Na vs emission intensity or absorbance).
- Use bracketing measurements and blank tests; ensure atomization rate and flame conditions are constant.
- Instrumentation settings
- Wavelength centered at 589 nm; bandpass generally ≤ 6 nm for flame photometry.
- AAS: stable lamp current and a clear non‑luminous oxidizing flame; atomization rates typically controlled (document gives 2.5–5 mL/min guidance).
- Quality control
- Blank determinations, bracketing with solutions close to expected concentration, and replacement of water through the burner after measurements.
Applications and users
- Analytical chemistry laboratories in:
- Aluminum industry and cryolite producers to verify raw material composition.
- Metallurgical quality control where cryolite composition affects electrolysis performance.
- Research labs studying fluoride/aluminium compounds.
- Users include laboratory analysts, quality assurance managers, and instrument specialists responsible for sodium determination by flame photometry or flame AAS.
Related standards
- ISO/R 1619 - Cryolite (natural and artificial): preparation and storage of test samples (referenced for sample preparation and handling).
- The ISO 2366 methods were developed under ISO/TC 47 and approved in 1974; they have IUPAC endorsement.
Keywords: ISO 2366, cryolite analysis, sodium determination, flame emission, flame atomic absorption, spectrophotometric methods, sample preparation, calibration curve, chemical analysis.
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