Overview
ISO 2833:1973 specifies a modified Willard–Winter method for determination of fluorine (F) in sodium fluoride for industrial use. The method separates fluorine from a test portion by steam distillation in an acidic medium (sulfuric or perchloric acid) and determines fluorine by titration with thorium nitrate, using colorimetric indicators or a spectrophotometric end‑point. The standard was published by ISO in 1973 and is also approved by IUPAC.
Key topics and requirements
- Principle: steam distillation of fluoride from a weighed sample into a collection flask, followed by titration of the distillate.
- Distillation media: concentrated sulfuric acid (≈24 N) or perchloric acid (≈64.5% w/w) as described in the procedure.
- Titrant: thorium nitrate solution (approx. 0.067 N).
- Calibration: prepare an NaF reference solution and standardize the thorium nitrate.
- Conversion: 1 ml of the standardized thorium solution corresponds to ~0.4525 mg F (used to calculate results).
- End‑point options:
- Visual titration using sodium alizarinsulphonate and methylene blue (pH control required).
- Spectrophotometric titration using sodium alizarinsulphonate only; end‑point defined by reaching an absorbance of 0.60 at 525 nm under specified conditions.
- Reagents and apparatus specified: HCl ~0.06 N, NaOH solution (20 g/L), pH buffer (pH 2.7), Claisen distillation flask (250 ml) with Vigreux column, Graham condenser, steam generator, pH‑meter, spectrophotometer with titration accessory, burette (10 ml).
- Sampling and calculations: recommends sample mass (~0.20 g), blank titration, and expresses fluorine content as mass percent. Precision data from interlaboratory trials are reported in the standard.
Applications and users
- Who uses ISO 2833:
- Quality control and analytical laboratories in chemical manufacturing (producers of industrial sodium fluoride).
- Process control teams ensuring product specification compliance.
- Regulatory and materials testing labs verifying fluoride content for downstream industrial uses.
- Practical value:
- Reliable, standardized procedure for accurate fluorine determination in NaF raw material or finished products.
- Two end‑point options (visual and spectrophotometric) allow adaptation to available equipment and required sensitivity.
- Useful for certificate of analysis generation, supplier qualification, and batch release testing.
Related standards and notes
- ISO 2833 references sample preparation and handling procedures (related ISO methods for sampling/preparation).
- Approved by IUPAC, increasing its acceptance in international analytical practice.
- Safety note: the method uses strong acids (sulfuric, perchloric) and steam distillation-appropriate laboratory safety and fume‑hood precautions are required.
Keywords: ISO 2833, sodium fluoride, fluorine determination, Willard‑Winter method, thorium nitrate titration, spectrophotometric titration, industrial sodium fluoride.
Open preview (PDF)Sodium fluoride for industrial use — Determination of fluorine content — Modified Willard-Winter method
