Overview
ISO 3360:1976 specifies a photometric method for the determination of fluorine (F) content in phosphoric acid and sodium phosphates intended for industrial use (including foodstuffs). The procedure is applicable to products with fluorine content equal to or greater than 0.5 mg/kg. The method separates fluorine by steam distillation in a phosphoric acid medium and quantifies it as a blue-coloured complex formed with alizarin complexone and lanthanum nitrate, measured photometrically at about 600 nm.
Key topics and technical requirements
- Scope: Designed for phosphoric acid and sodium phosphate products (industrial and food-grade) with F ≥ 0.5 mg/kg.
- Separation principle: Fluorine is liberated and collected by steam distillation from a test portion in a phosphoric acid medium.
- Color development:
- Reaction with a combined reagent of alizarin complexone and lanthanum nitrate at pH 4.6 (succinic acid buffer).
- Acetone is added to improve complex stability and analytical sensitivity.
- The complex produces a blue color measured photometrically at ~600 nm.
- Reagents and standards: Includes preparation of combined colorimetric reagent, buffer (pH 4.6), sodium hydroxide/nitric acid for pH adjustments, and fluorine standard solutions (e.g., 1.00 mg F/L and dilutions).
- Apparatus: Borosilicate steam-distillation assembly (Vigreux column, Graham condenser), spectrophotometer or electro-photometer with filter near 600 nm; cuvettes of 4 or 5 cm optical path.
- Calibration and quantitation:
- Use an external calibration curve (typical standards correspond to microgram-level F in 50 mL calibration flasks).
- Test aliquots for colour development are adjusted to contain up to 20 μg F prior to photometric measurement.
- Quality control: Blank determinations, apparatus conditioning (multiple distillations for new glassware) and routine checks to ensure recovery and low blank levels.
Applications and users
- Quality control and compliance testing in:
- Phosphate fertilizer production
- Food-grade phosphates and acid manufacturers
- Industrial chemical producers using phosphoric acid or sodium phosphates
- Laboratories performing regulatory analyses, product specification testing, and batch release testing where accurate fluorine monitoring is required.
Related standards
- ISO 3361 (soluble silica), ISO 847 (sulfate titration), ISO 848 (calcium titration), ISO 849 (iron photometric), ISO 3706–3709 (related phosphate analyses). See ISO annex for full list.
Keywords: ISO 3360:1976, fluorine determination, phosphoric acid, sodium phosphates, alizarin complexone, lanthanum nitrate, photometric method, steam distillation, pH 4.6, acetone, 600 nm.