Overview
ISO 1694:1976 specifies a 1,10-phenanthroline photometric method for the determination of iron content in natural and artificial cryolite (NaF/AlF3 molar ratio approximately 3 to 1.7). The procedure is applicable to products with iron content equal to or greater than 0.020 % (m/m as Fe2O3). The method is photometric/spectrophotometric and is technically identical to the earlier ISO Recommendation R 1694‑1970.
Key topics and technical requirements
- Principle: dissolution of a test portion by alkaline or acid fusion, reduction of Fe(III) with hydroxylammonium chloride, formation of the Fe(II)–1,10‑phenanthroline complex in a buffered medium and photometric measurement of the coloured complex at about 510 nm.
- Detection / applicability: suitable for cryolite and synthetic materials with NaF/AlF3 ratio ~3–1.7; lower reporting limit ~0.020 % (m/m Fe2O3).
- Reagents: analytical-grade reagents only; key reagents include 1,10‑phenanthroline (or its hydrochloride monohydrate), hydroxylammonium chloride, sodium acetate buffer and standard iron solutions (e.g., Fe2O3 based standards corresponding to 0.200 g/L and 0.010 g/L).
- Apparatus and conditions: platinum dish and lids, controlled electric furnaces for alkaline fusion (≈550 °C and 750 °C stages) and for acid fusion (≈700 °C), spectrophotometer/photoelectric absorptiometer with maximum transmission 500–520 nm, and pH meter/indicator paper for pH adjustment.
- pH control: the reaction medium must be buffered so the pH of the colour development stage is between 3.5 and 4.2; adjustment is made using sodium acetate or acetic acid.
- Calibration and blanking: prepare a calibration graph by plotting absorbance versus mg Fe2O3 per 100 mL using a series of standard colorimetric solutions; run blank tests and use compensation solutions where necessary.
- Reporting: test report should reference the method and include results, expression used, any deviations or unusual observations, and details of any optional operations performed.
Applications and users
- Routine quality control and specification testing of cryolite used in the aluminium industry (electrolysis fluxes and synthetic cryolite products).
- Analytical and mineral testing laboratories performing iron determination in fluoride-aluminium matrices.
- Research groups and industrial labs assessing raw materials, blends, or synthetic cryolite for iron contamination and process control.
Related standards
- ISO 1619 - Preparation and storage of test samples (cryolite)
- ISO 1693 - Determination of fluorine content (modified Willard‑Winter)
- ISO 2366 / ISO 2367 / ISO 2830 - Sodium and aluminium determination methods
- Other ISO methods for moisture, silica and related elemental analyses
Keywords: ISO 1694, cryolite, iron determination, 1,10‑phenanthroline, photometric method, spectrophotometric, alkaline fusion, acid fusion, Fe2O3, calibration graph.