Overview
ISO 4694:1987 specifies an ion-selective electrode method for the determination of fluorine (fluoride) content in iron ores. The standard applies to natural iron ores, concentrates and agglomerates (including sinter products) with fluorine in the range 0.005 to 1.0 % (m/m). It defines the principle, reagents, apparatus, sampling and sample preparation, detailed procedure, expression of results, precision criteria and test report requirements. The document includes three informative annexes (flowsheet for acceptance, precision derivation, and graphical precision data).
Key topics and technical requirements
- Principle: Fusion of a test portion in sodium hydroxide, dissolution in water and hydrochloric acid, filtration and potentiometric determination using a fluoride ion-selective electrode in the presence of a sodium citrate buffer.
- Scope: Validated for 0.005–1.0 % fluorine in ores and concentrates (international interlaboratory testing covered 0.005–0.214 %).
- Reagents: Analytical-grade sodium hydroxide (fusion), hydrochloric acid (fluorine-free), sodium citrate buffer, and a series of fluoride standard solutions (Solutions A–E) prepared and stored in plastic bottles.
- Apparatus: Silver (or nickel) crucibles for fusion, fluoride ion-selective electrode, calomel reference electrode, high-impedance pH/millivoltmeter, magnetic stirrer, plastics volumetric flasks and beakers.
- Sampling & sample prep: Use laboratory samples prepared per ISO 3081/3082/3083 and predried test samples (dried at 105 ± 2 °C per ISO 7764). Test-portion mass is selected according to expected fluorine content (table provided in the standard).
- Calibration & measurement: Prepare calibration solutions from standard solutions, measure electrode potentials, plot a semi-log calibration curve (mV vs log[fluoride]) and read test solution concentration. Results are calculated from measured fluoride concentration and the mass of the test portion, reported to five decimal places and rounded to three.
- Quality control & precision: Each run must include analysis of a certified reference material of the same ore type. The standard provides repeatability and between-laboratory tolerance equations (derived from an international trial) and an acceptance flowsheet (Annex A).
Applications and users
- Routine chemical analysis in mineral and iron ore laboratories
- Quality control for iron ore producers, concentrators and sinter plants
- Regulatory and compliance testing where fluorine content affects product specification or process behavior
- Research and development in extractive metallurgy and environmental monitoring of ores
Related standards
- ISO 3081, 3082, 3083 (sampling and sample preparation for iron ores)
- ISO 7764 (preparation of predried test samples)
- ISO 648, ISO 1042 (laboratory glassware)
- ISO 5725 (precision of measurement methods - referenced for statistical treatment)
Keywords: ISO 4694:1987, iron ores, fluorine determination, fluoride ion-selective electrode, sodium hydroxide fusion, fluoride analysis, ore quality control.